- Email: [email protected]
Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin
Sedimentary Geology, 62 (1989) 5-25
5
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin T H O M A S A I G N E R 1 and G E R H A R D H. B A C H M A N N
2
1 KSEPL (Shell Research), Volrnerlaan 6, 2288 GD Rijswijk (The Netherlands) 2 PREUSSAG AG, ErdiJl und Erdgas, Karl-Wiechert-Allee 4, 3000 Hannover 61 (F.R.G.)
Received June 2, 1988; revised version accepted November 14, 1988
Abstract Aigner, T. and Bachmann, G.H., 1989. Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin. Sediment. Geol., 62: 5-25. The dynamics of evaporite accumulation are studied in the Gipskeuper (Triassic) of the southwest German epicontinental basin based on a hierarchical, three-level stratigraphic analysis: (1) Among the various stratification types, the following are most important: (a) thin carbonate intervals with marine faunas; (b) massive sulfates with selenite, indicating very shallow subaqueous gypsum precipitation; (c) thin-bedded gypsarenites with a variety of sedimentary structures suggesting clastic evaporite accumulation during high-energy events ("gypsum tempestites"); (d) laminated gypsum with spectacular tepee structures, testifying inter- to supratidal settings; (e) green-grey and reddish claystones with nodular gypsum indicating evaporitic playa mudflats. (2) Facies sequences occur in several varieties and commonly form 1-4 m thick transgressive/regressive cycles with shallowing-upwards trends. The long recognized carbonate "marker beds" represent short-term trans- or ingressions into the almost flat Keuper Basin. Highly disturbed tepee horizons form widely correlatable regressive peaks. These sequences form evaporite equivalents to the ubiquitous shallowing-upward cycles known from carbonate systems. (3) The basin-fill consists of a small-scale cycles being vertically stacked upon each other to form the basic building block of the layer-cake architecture of the Gipskeuper succession. The stacking of small-scale cycles is superimposed onto a larger-scale, regressive cycle, which records a general shift from restricted marginal marine to hypersaline marine to the more continental red bed deposition of the overlying Keuper. This hierarchy of cycles probably reflects various orders of eustatic sealevel fluctuations. The Gipskeuper thus exemplifies dynamics of evaporite accumulation and allows us to understand some of the processes that produce a "layer-cake" stratigraphy. Its cycle hierarchy is a common motif in the entire southwest German Basin fill, and appears typical for epicontinental basins.
Introduction T h e depositional processes that lead to the acc u m u l a t i o n of evaporite sequences are still less u n d e r s t o o d t h a n those i n c a r b o n a t e a n d siliciclastic rocks, although significant insights have b e e n gained d u r i n g the past years (e.g. Kendall, 1984; W a r r e n a n d Kendall, 1985; Schreiber, 1978, 1986; Logan, 1987). This study aims to c o n t r i b u t e to the u n d e r s t a n d i n g of the d y n a m i c s of evaporite accumulations. A variety of s e d i m e n t a r y fabrics a n d 0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
sequences, some of which have n o t b e e n described from evaporite successions, are d o c u m e n t e d from a relatively thin U p p e r Triassic evaporite-to-red b e d sequence of the southwest G e r m a n G i p s k e u p e r F o r m a t i o n . Based o n a three-level analysis of (1) the stratification types, (2) the facies cycles, a n d (3) the stacking a n d hierarchy of cycles, a n att e m p t is m a d e to r e c o n s t r u c t the " d y n a m i c stratigr a p h y " (cf. Aigner, 1985) of this sequence (Fig. 1). The G i p s k e u p e r offers favourable c o n d i t i o n s for such a study, because it is well exposed in
DYNAMIC STRATIGRAPHY
J
J
0,na ,. I ,ac,e
II
]
as,n 0,nam,. ]
Fig. 1. General approach of this study: process-orientedanalysis of three hierarchical levels of sedimentary sequences. First step analyses individual stratification types, second step looks at arrangement of strata into facies sequences, and third step analyses packaging of cycleswithin the entire basin infill.
quarries, and a well-documented lithostratigraphy as a base for process-oriented studies has been established (Frank, 1930; Bachmann, 1974; Brunner and Wurm, 1983). Our studies focussed on the quarries east of Heilbronn in Northern Wiirttemberg (Fig. 2). This initial work concentrates on major sedimentologic/stratigraphic features in the lower part of the Gipskeuper Formation (Grundgips, Bochinger, Dunkelrote Mergel Members). Further investigations are planned for the future.
General setting The Triassic in the epicontinental German Basin is characterized by a tripartite subdivision into (1) the continental Buntsandstein, (2) the shallowmarine Musclielkalk, and (3) the marginal-marine to continental Keuper. The whole succession shows a well-developed "layer-cake" stratigraphy. Most units, be it only very thin, can be readily correlated over several hundred kilometers, and many have traditionally been used as lithostratigraphic
marker beds. The "layer-cake" stratigraphy suggests an extremely gentle depositional relief in this intracontinental basin, with slight changes in baselevel affecting huge areas instantaneously. The basin-fill therefore represents a particularly sensitive record of environmental changes. Within the Middle Keuper, evaporites (mostly anhydrite and gypsum, but also halite in some areas) are widespread throughout the Central European Basin (Fig 2a). In the southwest German subbasin (Fig. 2b), the Gipskeuper Formation records the transition from marginal marine to continental red bed conditions. While the basal Gipskeuper includes some 15-20 m of massive evaporite rocks, its upper part is dominated by red, greenish, greyish and varicoloured claystones, marlstones, and some sandstone and thin dolomitic intervals. Dependent on the type of exposure, the sulfates occur either as gypsum or anhydrite. The Gipskeuper is overlain by the Schilfsandstein, interpreted as a delta compley by Wurster (1964). For a schematic stratigraphic overview, see Fig. 2c.
0
l l i l l l
50 km
• gypsumquarry Berlin
)
y//
300 krn
':Stuttgart Fm. andstein M
100 -Anatina-B. ~
Engelhofer~ :
"(o
u_
Platte Mittlerer ~---~ horizont
~^Gips
50-
Strassburg/
EstherienSchichten
2
}l
__~_^ ~ ^
Bleigtanz Bo,nk
~ Ounkelrot e = ~ c ~ ^ Mergel Bochinger - - Bank -=-~:~-"~ Bochinger H.
~
^
/x~'~l~teO /
~
f
r : O,(Jbingen
Neckar
O
'" /
/
~"~
lll'lt
Grundgips-
B.o.-6 ~ S c h i c h t e n
dolomit lettenkeuperFm C
~1 b
/-
Fig. 2. (a) Simplified paleogeographic sketch of Central European Keuper Basin (after Ziegler, 1982). (b) Isopach map of the Gipskeuper Formation in the southwest German subbasin, after Geyer and Gwinner (1986). A - A ' = profile of Fig. 14. Quarries: 1 = Obersontheim, 2 = Talheim, 3 = Lorenzenzimmern, 4 = Raibach. (c) Generalised stratigraphic column of the Gipskeuper in southwest Germany. The studied interval comprises "Grundgipsschichten" to "Dunkelrote Mergel" members.
Stratification and facies types
Carbonate interoals Description. Carbonates occur within either the claystones or the evaporites as units up to several d m in thickness. Most of the carbonates are regionally widespread and are used as marker beds in the existing lithostratigraphic framework. For instance, the basal " G r e n z d o l o m i t " (cf. Fig. 2c) is correlatable as far as northern G e r m a n y over a distance of more than 500 km (Huth, 1956; Beutler and Schtiler, 1979; Duchrow, 1984). Although carbonate beds vary significantly in thickness, composition and internal structures, they show some general, recurring patterns. Each
has a sharp base, often erosional, partly scoured, and in some cases it m a y even be burrowed (Fig. 3a). The beds are dolomitic and vary texturally between mudstones (dolomicrites), and shelly, pelletoidal or oolitic wacke-, pack-, or grainstones. Some beds contain some quartz and phosphatic grains. Carbonate intraclasts m a y be very abundant as well as black pebbles and reworked selenitic clasts (Fig. 3b). Intervals with clasts can display imbrication, while shelly layers m a y be graded (Fig. 3c) and carbonate sands tend to be cross-bedded. Some clasts were found encrusted with stromatolites (Bachmann and Gwinner, 1971; Fig. 3d). Algae m a y also have contributed to the fine, crinkly lamination seen in some dolomicrites.
Fig. 3. Stratification and facies in the carbonate intervals. (a) Note sharp erosional base (arrow) with possible burrows and several wave ripple horizons at the top, each overgrown by selenite crystals. Lense cap diameter 5 cm. (b) Imbrication of selenite clasts within carbonate layer. Bank gamma, Raibach quarry. (c) Composite carbonate layer with black pebbles in lower and graded shell bed in upper part. Grenzdolomit, Obersontheim quarry. (d) Stromatolites on carbonate clast. Grenzdolomit, Hessental.
The carbonate units can display internal erosion surfaces, which m a y be lined with selenite crystals that appear to have grown in situ on the surface. Bed tops are often wave-rippled (Fig. 3a). The shelly fauna in the carbonate units is dominated by the mass occurrence of the bivalve Costadoria goldfussi, and some minute gastropods. Locally, the bivalves Myophoria transversa, and Unionites occur in addition ( H a g d o r n and Simon, 1985).
Interpretation. Our interpretation is based on the following main arguments: (1) The sharp lithological and textural contrast between evaporites, or claystones, and the
carbonate layers, together with evidence of considerable reworking (intraclasts) indicate a major and sudden event that temporarily changed environmental conditions. Sedimentary fabrics suggest shallow subaqueous (subtidal) deposition (crossbedding, grading, ripples, imbrication). Occasionally subaerial exposure m a y have occurred (black pebbles). (2) The sudden and episodic occurrence of a restricted marine fauna in the carbonate layers calls for episodic improvement of the ecological conditions, p r o b a b l y as during phases of marine influx (Bachmann, 1974). However, the dominance of one bivalve species, and its mass occur-
rence does not indicate a fully marine but rather a somewhat stressed, marginal marine (and at times perhaps also brackish) setting. Although the shell material is reworked, it is unlikely that the fauna was transported from marine areas into the Gipskeuper setting. (3) The extremely widespread occurrence of the carbonate intervals excludes local causes but calls for at least regional, probably basin-wide events. Possible causes include: (a) climatic changes with more freshwater runoff and mixing, allowing a restricted fauna to develop, or (b) marine ingressions. The latter appears more likely (see also von Freyberg, 1965; Richter, 1985). Kozur (1973) found faunal affinities to the western Mediterranean, indicating short-term ingressions from the south into the German Basin. The carbonate intervals can thus be interpreted as marine ingressions interrupting the more restricted evaporitic conditions with the supply of marine waters during relative rises in sealevel. In contrast, some dolomites in the upper part of the Gipskeuper (Estherienschichten) represent playadolomites (Richter, 1985). These are entirely lacking in fauna.
Massive sulfates with selenite
Fig. 4. Stratification in the massive sulfates. Selenite with marked vertical textures; note preserved gypsum crystal tops. Obersontheim quarry.
vertical textures (Fig. 4). Both preserved and truncated selenite prisms are observed. Fossils are rare; however, a complete skeleton of a Simosaurus has been discovered (von Huene, 1959), and plant remains (Voltzia fraasi, Pagiophyllum foetterlei ) are occasionally found (Hagdorn and Simon, 1985).
Description. This facies association occurs chiefly in the lower half of the studied sequence and consists of relatively massive, grey anhydrite a n d / o r gypsum units (massive gypsum of Bachmann, 1974). Their bedding is sometimes subparallel on a scale of cm to dm and is accentuated by mm-thin seams of clay or dolomite (Fig. 4). The bedding may be disrupted by dm-scale channel-like erosional features, filled with laminated gypsum and marl. These resemble "gutter casts" (Aigner and Futterer, 1978) in size and cross-section, but they cannot be followed in three dimensions. Generally, the individual sulfate beds are more or less structureless, except for gypsarenites with occasional remnants of ripple cross-lamination and possibly scoured bed bases. However, there are frequently intercalated layers, 1-7 cm thick, that consist of clearly recognizable selenite crystals, while others only display crystal relics or faint
Interpretation. The most salient feature in this facies is the occurrence of selenitic gypsum, which indicates the precipitation of primary gypsum crystals under very shallow subaqueous (subtidal) conditions (Schreiber, 1978; Warren, 1983; Kendall, 1984). Layering in the selenitic units is likely to represent (seasonal?) changes in the brine composition and dissolution of the selenite crystals, while preserved zig-zag lamination indicates continuous crystal growth over several years (Warren, 1982). Similar layering styles with alternating selenite crusts, gypsarenites and clay have been described by Orti Cabo, Mur, Geisler-Cussey and Dulau (1984) from coastal salinas of Spain. Whether the small channels represent a form of gutter cast, or are due to erosion by dense brines, or are caused by subaerial exposure can not be evaluated at present. Beds of pure, crystalline gypsum, remnants of ripples and scours suggest
10
Fig. 5. (a) Stratification and facies in the thin-bedded sulfates. Cm-thick alternations of claystone layers and gypsum layers. Obersontheim quarry. (b) Very similar facies from modern MacLeod Basin (photo courtesy M. Shephard).
some redeposition of gypsum as gypsarenite. A few of the contorted beds may be due to slumping. According to Richter (1985), the strontium contents in gypsum and anhydrite of the Gipskeuper are typical for marine sulfates.
Thin-bedded sulfate / claystone intervals Description. These more "platy" sulfate rocks (Bachmann, 1974) contain a variety of intergradational facies ranging between two extreme end members: (a) relatively massive but thin-bedded
alternations of cm-thick gypsum layers with mmthick claystone or dolomite seams, and (b) cmthick alternations of gypsum and claystone layers (Fig. 5a). Gypsum layers are both gypsites and gypsarenites (sensu Warren, 1982). Their colours are mostly light grey or white, but may also be greenish or reddish, particularly upward in the Gipskeuper sequence. Similarly, claystone layers may be grey, greenish or reddish in colour. This facies contains a wealth of sedimentary structures and textures. Many of the thin sulfate
11
Fig. 6. Sedimentary structures in the thin-bedded sulfates. (a) Clastic gypsum bed; note basal scours (b) Graded and partly wave-tippled gypsum layers intercalated into claystones. (c) Close-up of graded gypsum bed, probably representing a gypsum tempestite. Obersontheimquarry.
beds are structureless gypsites. However, gypsarenites often show sharp, erosional bases with scours, graded bedding and ripple marks (Fig. 6a-c). Some layers contain abundant gypsum porphyroblasts (Fig. 7a), 1-20 mm in diameter, within the finer-grained matrix. Halite "pseudomorphs", showing hopper features, have also been recorded (Fig. 8a). Mud cracks in the intercalated claystone layers are often well-recorded as gypsum-filled casts on lower bedding surfaces. Most spectacular, however, are frequent tepee
horizons within the thin-bedded gypsum facies (see below, Fig. 8b, c).
Interpretation. The basic motif of this very varied facies association is that it records various types of intermittently operating depositional processes: (1) Episodes of high-energy deposition. Gypsarenites, basal scoured surfaces, grading, ripples, etc., in the sulphates testify to subaqueous clastic deposition of sulphates by periodic high-energy events ("gypsum tempestites"). Kendall (1984)
12
Fig. 7. (a) Gypsum porphyroblasts on bedding plane. Lorenzenzimmernquarry. (b) Similar gypsum porphyroblasts ("hemipyramidal gypsum") from the sediment surface of the modern MacLeod salina (West Australia). Lense cap diameter 5 cm.
notes that clastic sedimentation is a dominant feature in the higher-energy environments of intertidal or supratidal-lagoonal evaporite settings. He interprets laminations as storm deposits when evaporite fiats were flooded by sediment-charged waters, by analogy with the formation of storm laminae in other tidal-flat sediments ("storm flats"). (2) Flood deposition. The thin multicoloured claystones intercalated into the sulphate facies could result from mechanical transport of the clay
during times of heavy rain, as described by Warren (1982). Thicker claystones may represent very distal end members of river floods, similar to flood deposits brought into the modern West Australian MacLeod salina by ephemeral streams (Logan and Brown, 1986; Logan, 1987). (3) Brine sheet deposition. M a n y of the laminated, but otherwise structureless gypsum layers with or without claystone alternations were most likely deposited from evaporiting "brine sheets". This appears the dominant mode of
13
3 :
~.
.
.
.
~
j.
.
.
.
.
Fig. 8. Indicators for subaerial exposure in the thin-bedded sulfates. (a) Halite "pseudomorphs", Triensbach quarry. (b) Tepee composed of fine-grained gypsum; note also porphyroblasts in layers below tepee. Obersontheim quarry. (c) Horizon with large tepees, note partly capped tops. Raibach quarry.
gypsum precipitation the m o d e m M a c L e o d evaporite basin (Logan and Brown, 1986; Logan, 1987; Fig. 5b). However, the structureless gypsite may also be compared to m o d e m gypsite forming in the zone of soil moisture (Warren, 1982) as subaerially exposed gypsum "caps". Mud cracks and tepee horizons document repeated and periodic sub-
aerial exposure of the depositional environment. Intervals with frequent porphyroblasts may also indicate exposure. Butler et al. (1982) report a similar association of discoidal gypsum crystals growing in the sediment below anhydrite polygons from Abu Dhabi. The gypsum prophyroblasts may indicate the paleo-brine level at the sediment surface, similar to the "hemipyramidal gypsite"
14 described by Logan and Brown (1986) and Logan (1987) from the MacLeod salina (Fig. 7b). In analogy to the modem Bristol Dry Lake (Handford, 1982), the halite pseudomorphs probably formed displacively in the capillary fringe just beneath an exposed saline mudflat. In summary, this facies seems to represent a suite of inter-to supratidal (possibly also some very shallow subtidal) flats with intermittent episodes of flooding and subaerial exposure. In this context, the terms supra-, inter- and subtidal are not meant to imply classical tidal flat environments but are used to indicate the position of facies with respect to main sea level.
Fig. 9. Nodular gypsum in red claystoneprobably representing a gypcrete soil ("Gwinner's Gipskruste"), Talheim quarry. Hammer for scale.
Disturbed sulfate intervals (tepees, nodules) Description. The thin-bedded gypsum/claystone
who could trace these layers over large parts of the southwest German Keuper basin.
facies as described above is frequently deformed into tepee horizons. These are normally only a few dm thick, but when stacked on top of each other, may form intervals up to 3 m thick. The tepees are characterized by beds of finely crystalline gypsum which bend upward and are often broken on top (Fig. 8b). The height of the individual tepee is normally a few cm to 1 dm, but horizons with diapir-like tepees as high as 0.5 m have also been observed (Fig. 8c). When flattening out the beds that form the tepees, it is obvious that they are longer than the available space, i.e. grew by expansion. Tepee tops are commonly eroded. In plan view, the tepees form polygons. Some tepee horizons appear extremely folded and disrupted, and some are brecciated and conglomeratic. Thus clear-cut tepee structures may grade into beds of nodular appearance. Another type of nodules occurs in a matrix of claystones. These nodules range in diameter from a few cm up to 2 dm, show cauliflower-like surfaces and consist of white, light grey to light reddish, crystalline gypsum (Fig. 9). The nodular beds are only a few dm thick and are generally eroded on top. Some of the disturbed sulphate intervals are laterally extremely widespread despite their limited thickness. They include the "Gekr~Ssegipse" of Bachmann (1974) and Brunner and Wurm (1983)
Interpretation. To our knowledge, tepees have only very rarely been described from evaporites. Exceptions are the recent anhydrite "polygons" from Abu Dhabi (Butler et al., 1982), "distorted beds" from Bristol Dry Lake, California (Handford, 1982), and tepee-like structures in carbonate residues from the North German Gipskeuper (Hauschke, 1987). However, gypsum tepees are very abundant in our study area. In a recent paper, Warren and Kendall (1987) reviewed the origin and setting of tepees from carbonate sediments. They emphasize that tepee formation generally involves three processes: (1) early cementation of the surface sediments, (2) fracturing of the resulting crust, and (3) filling of these fractures with sediment and cement. It appears that the present sulfate tepees formed by lateral expansion of indurated gypsum beds. Bed induration could be caused by precipitation of gypsum from evaporating capillary waters; Warren (1982) described modern gypsite crusts from South Australia. Subsequent cracking of such indurated beds was probably related to thermal expansion and contraction, or simply to desiccation. Successive episodes of crack-infill and drying-out could have caused the sulfate beds to expand and overthrust to form tepees, similar to the mechanism described by Warrren and Kendall
15 (1987) from modem carbonate tepees in coastal lakes of South Australia. Brecciated and conglomeratic layers appear to be due to extreme overthrusting, collapse and crumbling of tepees, in certain cases overprinted by mechanical reworking as well as compaction. Langbein (1987) envisages compaction as the main cause for deformed sulfates. However, many of the clasts and nodules in this succession clearly represent mechanically reworked remnants of tepee crusts. By analogy with modem examples, tepee formation, brecciation and nodular growth is likely to be due to prolonged phases of subaerial exposure and desiccation. This is also indicated by abundant red claystone seams, mudcracks and the often eroded tops of both tepee and nodular layers. The longer periods of subaerial exposure appear to be caused by (most likely only minor) lowerings of sealevel. Thin dolomite layers sometimes overlying these units, and infiltrating down into them, probably represent a renewed submergence by a subsequent transgression. The transgression probably also accounts for mechanical reworking seen in some "conglomeratic" sulfate layers. Some of the nodular gypsum horizons, however, occurring in the upper part of the studied sequence within a claystone matrix (Fig. 9) may be compared to modem soil crusts and "gypcrete" (Gwinner, 1970; Kulke, 1974) or have the characteristics of interstitial sulphate growth near the sediment surface, as described by numerous authors from recent sabkhas (e.g. Butler et al., 1982; Shinn, 1983).
Grey shales Description. Laminated shales only a few dm thick, occur as a single laterally persistent horizon. They are mudcracked at the top, and thin silt laminae but no gypsum was observed within them. Lingula shells and phosphatic grains have been found on bedding planes.
thickness of this unit as well as the mudcracks suggest that such conditions were of ephemeral nature. Clearcut environmental interpretations are difficult, but an ephemeral lacustrine to restricted shallow-marine setting may be envisaged. These shales are similar to the much thicker grey shales in the uppermost Gipskeuper (Graue Estherienschichten).
Green-grey and reddish claystones Description. Green-grey and reddish, more or less unbedded claystones become more abundant in the uppermost part of the studied sequence. They form up to meter-thick packages with few diagnostic sedimentary structures. They include horizons with mudcracks, a few, cm-thick sandstone and dolomite beds, and planar, frequently graded gypsarenites with or without tepee structures. The above mentioned gypsum nodules are common and vary from a few cm to about 30 cm in diameter. They are mostly light reddish in colour and occur preferentially in discrete horizons (Fig. 9).
Interpretation. This
facies is similar to inland sabkhas and saline mudflat deposits described by several authors (e.g. Eugster and Hardie 1975; Handford, 1982; Kendall, 1984). In such environments, evaporites precipitate mostly in ephemeral brine ponds or interstitially from hypersaline brines. Nodular gypsum layers probably represent paleo-groundwater levels. The ephemeral brine ponds and the highly saline groundwater were probably fed by temporary run-off of the hinterland o r / a n d by occasional marine storm floods. Greenish claystones suggest generally a reduc, ing environment, caused by more frequent flooding or by ephemeral lacustrine conditions. In contrast, red claystones probably represent longer exposed mudflats with less frequent flood events.
Thin sandstones
Interpretation. The dark grey colour and the pre-
Description. Greenish quartz sandstones occur as
served lamination of this facies indicate that deposition occurred subaqueously under somewhat "quiet water" conditions. However, the limited
cm-thick layers on erosional surfaces in between evaporites and claystones. The base of each is irregular and scoured, the top frequently covered
16
with oscillation ripples. Phosphatic grains as well as reworked claystone pebbles are also present. Occasionally, casts of Costadoria goldfussi, small shark teeth as well as undiagnostic trace fossils are found on the upper sandstone bedding planes. The sandstones are overlain by thin dolomitic beds or by claystones. As documented in stratigraphic cross-sections by von Freyberg (1965), they represent the very distal expressions of thicker sandbodies at the basin margin.
ing heavy rainstorms). However, subsequent marine ingressions reworked the sheetflood sands, as indicated by the faunas present in some of them. Facies cycles Detailed bed-by-bed logging of the Gipskeuper sequence reveals a marked cyclicity (Fig. 10). This has not been recognized so far, although the cyclicity is punctuated by the traditionally used "marker beds". In general, the small-scale cycles recognized here vary between 1 and 4 m in thickness. These cyles are in several ways reminiscent of the widespread shallowing-upwards cycles in shallow-water carbonate environments (e.g. Wilson, 1975; Goodwin and Anderson, 1985).
Interpretation. The thin sandstone layers appear to record rapid and laterally very extensive inundation of an extensive mudflat. Since the thin sandstones grade towards the basin margin into thick sandbodies, the sand material must have been derived from there (possibly via sheetfloods dur-
INGRESSION/sheet flood
"Y
--
SUPRATIDAL
mudcrec~'~s
gypcrele
~
15
molt ,d
oOC storm floods (hypersallne marine}
,~ tepees
oct storm floods brine ponds
,re q
C/st, Gyp
SUPRA -/INTERTIDAL comm subaeriat
]1
I (/sl
gygn
distal sheet f~ood$ inundation
k k (;y#
INTER ISUBTIOAL hypersali~e marine freqsubeerlo[
t~>
•
Sh
~l,oguto
~st
~ C <~ ~ s ~ ~ ~>nod~
Gekrbse gips 3 ,[i~yp
(2Dnod cont bed,~ ~gyp porphyro btosts
SUBTiDAL
dgy
gngy
" ~ "'-freq
grad
(~
S~STIDAL:INGRESSION
Oc ~ --s ~y bia¢k pebbles
~
restricted merino
~~
:
,~>/)
hypers0hne ~arine
treq sub~er,a~
freq 9rQd
Gekr6se gips 1
SUBTIOAL
'.'
red, wh 9ngy'
i l oct subaerial
OL
Do
a at mot
,~yc,
channel
~__.
s"~"t-Ool OC ~ ~ rnit
~
~(Z) D dUotkpebb~ ocrete s g'
reslricted maline
C/st
X r~ts
SUPRA /INTERTIDAL comm subQerieL o~ storm floods &
freq Subaet ~1 occ $1orm floods & distol sheet floods
I i I iI
II ~-~ i,I ~I ! ,'
SUPRA -/INTERTIDAL comm subo erio[ occ slorm floods ~
/ II)
i I ~, ' i
INTER-/SUBTIOAL hyper~tin e m~rme heq 5ubaedQI
,~i
Ii , h~cYcPerss2bTgr,~2rne I, occ suboer~a
[ I
L
I
Ii SUBTIDAL INGRESSION
subaerlal
,x
(~)C gl P ~yp
/ ~,
dlstoJ sheet floods
SUBTIDAL INGRESSION
.... gY
I(
INGRESSION/sheet flood SUPRAIIDAL
subaerlal
gngY, red, ~h
i --
subaeri=l SUBAQUEOUS testr marine/brackish
INTER-/SUSTIDAL
hyperse[ine mar,no
~ottz,o
hypersGUnemarine
SUPRATtDAL
nod
GekrGsegips 2
INTERISUBTIDAL
Gyp
sUB SUPRATIOAL
distal sheet flaods
hypersoline morine
Symm r,pple marks s - - s t a r v e d Ool
I
l
I i
GYP
:B
1
suboeria~
00t
Bank 6
. . ~
INGRESSION/sheet tlodd SUPRA-tlNTER1tDAL comm ~ubfleria J oct slotm floods
I
5UaTIOAL
hypersallne marrne
Do/
I/~
0o/
i
~, I
- -s
rs:stb~ieCrt,:~ . . . . . . . Bank
!'
g~
SUBTIOAL 7 hypersa[I ne ma.~e jI~ j I iNGRE Sl N ,e r marine .. J
Fig. 10. Detailed sedimentological log of the Gipskeuper in Obersontheim (cycles 1-12) and in Talheim quarry (cycles 13-18). The arrows indicate small-scale shoaling-upward cycles (cycle numbers are referred to in the text). Symbols and abbreviations adapted from Shell Standard Legend.
17
DOl sd II~gl moU r~
::-_:::-~----::
SUPRATIDAL
GIst
redbn/
gygn
~:--:_-:::_-:_=~
distat s h ~ t floods
mott r~
~_:_-:;
Gyp
GWINNERs Gipskruste
I:2Dnod gyporete
nz-
C/St(Gyp)
gygn
SUBrlOAL:INGRESSION restricted mQrine
SUPRATIDAL ~c
dista~ sheet floods
C/st
SUPRATIDAL
Clst
gygn
dlstal sheet ftoods inundation
redbn
vi redbn
SUPRA-/INTERTIOAL
C/st, Gyp
c o m m subQerial oc¢ storm floods & distat s h ~ t floods brine pOndS
SUPRATIOAL
C/st
gy redl>n
Gyp [3 Dunkelviolettes Grenzlager
oct distal sheet floods
vi t e d ~
Gyp
SUPRATIOAL
C/5t
C/st, Gyp ~
~vk
occ. dkstal sheet f i n d s
gygn
SUPRA */INTERTIDAL c ~ m ~b~erio( o¢c. storm f i n d s t distal s h ~ t f i n d s
freq grod gygn/ tedTon
Gyp
(~nod
gypcrete
$UPRATIOAL
C/st
viredb~
occ distal sheet floods
Clst
vign
distQ~ sheet floods inundation
brine ponds
SUB-/INTERTIDAL o c t storm floods
mo~ red
Gyp
gygn
~nod
mort gn
gypcrete
SUPRATIOAL SUPRATIOAL
C/st
redbn
suboerial ~.distalsheetfioods
C/st
~/pcrete nod~red
Q~ BOChi
rB.
redbn
~c
distal sheet floods
/Gyp) Dol, cly
gngy
gygn
INUNOATION DO/ s d y , . . . ~ _ . . ~ g ~ . ~ . ~ . C/st gngy
5st
sheet flood
Fig. 10. (continued).
"'Base carbonate cycles" (Fig. 11a) Description. This type of cycles is characteristic for
the lower part of the studied sequence (e.g. cycles 1-4 in Fig. 10). Each cycle starts with a basal carbonate layer (see section "Carbonate intervals"), in a few cases also a thin sandstone, which may be followed by massive sulphates with selenite intercalations (see section "Massive sulfates with selenite"). This may be overlain by thinner bedded gypsarenite and gypsite with claystone laminae and tepee structures. The top of a cycle, just below the basal carbonate layer of the following cycle, often shows abundant gypsum porphyroblasts. Upwards in the cycles the thin-bedded gypsum becomes more abundant. There is also an upwards increase in the redness of the claystone intervals, and in the abundance of tepees.
Interpretation . The "base carbonate" cycles start with a marine ingression, are followed by shallow subtidal evaporite precipitation (selenite) and lead under progressive upwards shallowing to eventual subaerial exposure (gypsum tepees). Sequences commencing with a thin carbonate layer, caused by an ingression, followed by selenitic gypsum, are very similar to the initial stages of the modem MacLeod salina (Figl 12; Logan and Brown, 1986; Logan, 1987). The sulfate parts of the present cycles are very similar to shallowing-up evaporite cycles described by Warren (1982, 1983) from modem coastal salinas of South Australia, which also pass from selenites upwards into laminated gypsarenites and gypsites. The cycles bear some resemblance to cycles described by Loucks and Longman (1982) from the Lower Cretaceous of Texas. These also start
18
Fig. 11. Example of "base Carbonate cycle" with basal carbonate layer and tepees at top (Lorenzenzimmern quarry east of Schw~ibisch Hall). (b) Example of "top disturbed cycle" (Raibach quarry west of Schwiibisch Hall). Note basal bedded sulfate overlying disturbed horizon (top of previous cycle) passing up into sequence with increasing amount of tepees.
with a thin marine carbonate unit, followed by subtidal and finally supratidal sulfates. However, these are deposited in a lagoonal setting.
"'Top disturbed" cycles (Fig. 11b) Description. This type of cycles is more typical for the middle part of the studied sequence (e.g. cycles 5 - 7 in Fig. 10). Basal carbonate layers may be present, but they are far less prominent or are not
developed at all. This type of cycles show fewer, if any, selenite intervals. Instead, they are dominated by the thin bedded g y p s u m / c l a y s t o n e alternations with claystones and tepees increasing in abundance upwards in each cycle (Fig. l l b ) . Claystones tend to change in colour from greygreenish to more reddish upwards. The top of the cycles is a strongly disrupted interval with extreme tepee development, i.e. the traditional "Gekr/)segipse".
19
Fig. 12. (a) Wave-rippled calcarenite layer overgrown by selenitic gypsum at the base of a small-scale transgressive/regressive cycle from the Gipskeuper. Bank delta, Raibach quarry. (b) Almost identical succession from the modern MacLeod salina: wave-rippled calcarenite, representing a Holocene ingression (Logan and Brown, 1986), overgrown by subaqueously grown selenite crystals. Lense cap diameter 5 cm.
Interpretation. The record of marine ingressions at the base of the " t o p disturbed" cycles is very subtle. This suggests either a weaker ingressive pulse, or a more landward setting, or both. A more landward setting is indicated by more abundant red claystone layers. However, clear upward shallowing trends are also present, where the "disturbed tops" testify to longer periods of subaerial exposure. The thin sandstone layers overlying the "disturbed tops" of certain cycles (e.g. cycle 7 in Fig. 10) are most likely sheetflood-derived. However, these sands became reworked by the ingression of the subsequent cycle as indicated by the occasional occurrence of shelly fauna.
10) and is less regularly developed than the evaporitic cycles described above. Cycles have a sharp base, and m a y show a cm-thick sandstone. This is followed upwards by an up to 1 m thick interval of thin-bedded reddish gypsum often with tepees and graded gypsarenite beds. Upwards the bedded gypsum passes into greyish or reddish claystones, the red colours becoming more abundant towards the top. The upper part of each cycle is characterized by discrete horizons of gypsum nodules, a few cm to 3 dm in diameter, within a claystone matrix. When dissolved near the present-day surface their relicts are still recognizable by red mottling. The cycles are generally topped by a mudcracked horizon.
Argillaceous cyles
Interpretation. The thin sandstone layers at the
Description. This type of cycle occurs in the upper
cycle base (or at the top of the preceding cycle) were derived via sheetfloods from the hinterland. However, these sands became reworked by subse-
part of the studied sequence (cycles 8 - 1 2 in Fig.
20
quent marine ingressions (shelly fauna). The ingression led to ephemeral but very extensive "brine ponds" with deposition of gypsum as seen in the
basinal
lower part of the cycles. Frequent tepees indicate alternating periods of subaqueous and subaerial conditions. The transition to the claystone-
15 km
marginal
m
Gekrbsegips 2 °
"
': i~',,,,
,'""
I
.~e~¢~
w-
I
~
4
//
/~/
L
//
Fig. 13. Variation in facies succession of three shoaling-upward cycles. Note thinning of carbonate layers, increase m the amount of tepees toward basin margin (right) and occurrence of thin sandstones. Note also less-well developed subcycles within cycles 4 and 5. Legend and number of cycles see Fig. 10. Locations see Fig. 2b.
21 dominated lithologies upwards in the cycles probably represents a change to a dryer mudflat-like setting. Thin layers of gypsum were formed on the mudflats by evaporiting brine sheets and by clastic redeposition as gypsarenites. Mudcracks and sulphate nodules at the top of cycles indicate exposed inland sabkha conditions (cf. Gwinner, 1970). Principally similar cycles have been modelled by Kendall (1984) and have recently also been described by Hauschke (1987) from the North German Gipskeuper. A partial m o d e m analogue are cycles described by Handford (1982) from Bristol Dry Lake: in these a lower "salt pan" unit passes upwards into "saline mudflat" deposits. Basin-fill patterns
Distribution of small-scale cycles Laterally, each cycle may vary in facies succession (Fig. 13). The basal carbonates tend to become A -~ sequences
less fossiliferous and less prominent towards the basin margin, while clay interbeds, thin sandstones, as well as the abundance of tepees increases to the basin margin. However, the cycles can be correlated over considerable distances, as mentioned above. This is well illustrated by the detailed microstratigraphic work of Bachmann (1974) and Brunner and Wurm (1983), who could trace both the carbonate intervals (" Bank alpha to delta") as well as the disturbed sulphate horizons ("Gekrtsegipse 1-3) as marker beds throughout the entire SW-German Basin (Fig. 14).
Vertically, the Gipskeuper appears as a sequence of stacked shallowing-upward cycles, each beginning with a marine ingression (some probably in combination with sheet floods). Such a vertical stacking of cycles is a very common motif in both many carbonate and in evaporite sequences (e.g. Wilson, 1975; Goodwin and Anderson 1986). Such cycles are valuable tools for chronostratigraphic A'
basinal
marginal m
6
l/ Fig. 14. Basin-fill pattern of the Gipskeuper in transect from margin towards basin center (profile A-A" in Fig. 2b). Note how small-scale shoaling-up cycles(arrows) are superimposedonto a larger-scaleregressivecycle(large arrow). Lithostratigraphymodified after Brunner and Wurm (1983). Number of cyclesrefer to numbers in Fig. 10.
22 correlation based on event-stratigraphy. This applies especially for largely unfossiliferous evaporite basins.
Cycle hierarchy Viewing the studied Gipskeuper sequence as a whole, an overall trend in basin-fill is apparent. Most massive and pure sulphate rocks are restricted to the base. Clay interbeds become more abundant upwards, until they dominate the sequence completely. Successive minor cycles show systematic upwards changes: the record of marine ingressions (carbonate layers with marine fauna) becomes more and more subtle, while simultaneously all indicators for subaerial exposure and for continentality increase upwards (Fig. 15). It thus appears that the stack of small-scale trans/ regressive cycles is superimposed on an overall, larger-scale regressive cycle. Given the uncertainty in biostratigraphically correlating the Germanic and Alpine Triassic, the overall regressive cycle
~.
.~=~
Gyp (2Dnod C/st,reddish ~ C/st, gygn ---- -- --_----~.
C/st,
Gyp,
thin-bedded
freq grad
~yp, thin-bedded,tepees~ 3 (~) nod (~
C/Stint ....
Gyp,thin-bedded C/Stinterv., gn/red / . . freq grad ~
~s
Gyp,~sthick-bedded'massive ~
j
Gyp,thick-bedded, selenite
massive
~
~O/ (2) block pebbles
(~C ~ ~
,SSt (~,,....~..~%
Ba88
1 2 3 ~ 5 6 7 8 9 101112131z, 15161718
-o SUPRATIDAL ~: occ. distal sheet floods o
.... s ~ Gyp~ thin-bedded~ w a v "-Lry.~
SUPRATIDAL distal sheet flood
gypcrete
~
gygn/red
SEQUENCES
INTERPRETATION
LITHOLOGY / TEXTURES ~ . ~ . . . . •..... .j
probably corresponds to the lowering in sealevel recognized by Brandner (1984) for the entire Mediterranean region in the lower part of the Carnian (note contrast with Haq et al., 1987). The small-scale cycles are more difficult to interpret. However the extremely widespread lateral continuity of small-scale cycles bears on their origin. Clearly, local autocyclic mechanisms are unlikely. Instead small-scale transgressive/ regressive pulses by (probably minor) fluctuations in eustatic sealevel can be envisaged. Arthurton (1980) considered eustatic fluctuations for rhythmic sequences in the British Upper Triassic Keuper Marl, and van Houten (1987) described Triassic lacustrine deposits which appear to record astronomically controlled "Milankovitch cycles" from New Yersey. Also, the Carnian of the Alpine Triassic is characterized by an extremely cyclic depositional pattern, for which a eustatic Milankovitch-type control had also been proposed (Hardie et al., 1986). In sequence stratigraphic terms (van Wagoner et al., 1987) the overall re-
more freq. distal sheet floods/ inundation
l
SUPRA-/INTERTIDAL subaerial, occ. storm floods & "~ sheet floods, brine ponds ~ SUPRA-/INTERTIDAL _E comtn, suboerial, oct. storm ~[ floods & distal sheet floods i J
--i"]-~ /
INTER-/SUBTIDAL hypersaline marine, freq. subaerial, occ. storm floods & distal sheet floods SUBTIDAL
occ.hypersalinesubaerialmarine
J
J J
SUBTIDAL hypersaline marine occ. suboerial SUBTIDAL:INGRESSION
restricted marine
Sst: reworked sheet flood
Fig. 15. "Ideal" shallowing-upward cycle of the Gipskeuper. Black bars show sequences of actually observed cycles from bottom to top of the studied sequence (numbers of cycles refer to numbers in Fig. 10). Note the systematic upward changes in cycle type, recording the superimposed overall regressive trend.
23
tecture. This has been done by a hierarchical analysis of three levels of sedimentary sequences: (1) Stratification types record a variety of depositional processes. These range from marine ingressions, shallow subaqueous gypsum precipitation, clastic redeposition of gypsarenites, subaerial exposure, distal flood events, lacustrine and mudflat sedimentation. (2) Facies sequences show generally shallowing-upward trends and reflect small-scale trans/ regressive cycles. Their apparently basin-wide distribution indicates allocyclicity which may be caused by small-scale variations in eustatic sealevel. (3) The basin-fill consists of a vertical stacking of the small-scale trans/regressive facies cycles. These are superimposed onto a larger-scale regressive cycle, recording the transition of marginal marine to continental redbed deposition. This hierarchy of cycles in the basin architecture of the Gipskeuper is thus principally similar to the infill style of the underlying Haupt-
gressive cycle probably represents part of a "third-order sequence", while the small-scale cycles can be regarded as "paracycles". Conclusions (Fig. 16)
Many single depositional features in the Gipskeuper may be explained by comparison with both coastal salinas and inland sabkhas and playas in the modern. The general setting of the Gipskeuper, however, differs from these "actualistic" examples, particularly in its huge extent and in its very low relief. Gipskeuper evaporites show a number of marine characteristics, but clearly were deposited in an intracontinental context. The lack of true actualistic analogues for epeiric evaporite sedimentation thus imposes certain limits in applying "comparative sedimentology" alone to understand the depositional processes. To this end, the well-established lithostratigraphy of the southwest German Gipskeuper has been used as a base to analyse the dynamics of the stratigraphic archi-
Dynamic stratigraphy FACIES
• tepees/
CYCLES
CYCLIC
emergence
~
B A S I N - FILL
r~-~-~-
~ _~ :,.:.?:!::~
A III l = "-11
\
1/
II[~'!'~ /l/~'~'.~ snoa,,ng
fiats
basins
_ ..
~
i
~
~
I:~/:t
Ill
/~---~-~
J ~ . -
i~;~ Ill/~-_~-~J~ F~".-~ I I I F ~ - / : j ~ : , _ ~
l
"~ "
. . . .
Ill ~'~/~::~ :~:.:-:~:"~
0 Fig. 16. Summary of the "dynamic stratigraphy" of the Gipskeuper. Stratification types indicate the depositional dynamics of the various subenvironments ranging from shallow-marine to alluvial sheetfloods. Facies sequences record minor ingression/regression cycles. Vertically stacked minor cycles constitute a major regressive sequence of the studied Gipskeuper basin fill, shown here as a schematic margin (fight) to basin (left) cross-section where the marine influence (transgressive carbonate marker beds) decreases upwards in the sequence, while the regressive terrigenous influx gradually increases.
24 m u s c h e l k a l k ( A i g n e r , 1985) a n d s e e m s t y p i c a l for the s t r a t i g r a p h y o f e p e i r i c basins. T h e G i p s k e u p e r thus exemplifies dynamics of evaporite accumulat i o n a n d allows us to u n d e r s t a n d
some of the
p r o c e s s e s t h a t l e a d to a l a y e r - c a k e s t r a t i g r a p h y of a n e p i c o n t i n e n t a l basin.
Acknowledgements We gratefully acknowledge the owner's permiss i o n to w o r k in several G i p s k e u p e r
q u a r r i e s in
W i i r t t e m b e r g . T . A . is g r a t e f u l to B. L o g a n ,
R.
B r o w n a n d M. S h e p h a r d for g u i d i n g a field trip to t h e m o d e r n M a c L e o d salina, w h i c h p r o v i d e d s o m e i d e a s for a n " a c t u a l i s t i c " i n t e r p r e t a t i o n o f s o m e features
in
the
Gipskeuper.
M.
Shephard
is
t h a n k e d for the f o t o o f Fig. 5b. T h i s p r o j e c t has been
c a r r i e d o u t in o u r free t i m e o u t s i d e
our
professional duties and the views expressed herein are strictly o u r o w n .
References Aigner, T., 1985. Storm Depositional Systems: Dynamic Stratigraphy in Modern and Ancient ShaUow-Marine Sequences. Springer-Verlag, Berlin, 177 pp. Aigner, T. and Futterer, E., 1978. Kolk-Tt~pfe und -Rinnen (pot and gutter casts) lm Muschelkalk-Anzeiger Fiir Wattenmeer? Neues Jahrb. Geol. Paliiontol. Abh., 156: 285-304. Arthurton, R.S., 1980. Rhythmic sedimentary sequences in the Triassic Keuper Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geol. J., 15: 43-58. Bachmarm, G.H., 1974. Grundglpsschichten und Bochinger Horizont (Mittlerer Keuper) in Nordost-Wiirttemberg. Jahresh. Geol. Landesamtes Baden-Wtirttemb., 16: 79-96. Bachmann, G.H. and Gwinner, M.P., 1971. Algen-Stromatolithen vonder Grenze Unterer/Mittlerer Keuper (Obere Trias) bei Schw~ibisch Hall (Nordwiirttemberg, Deutschland). Neues Jahrb. Geol. Pal~iontol., Monatsh., 1971: 549-604. Beutler, G. and Schiiler, F., 1979. Ober Vorkommen salinarer Bildungen in der Trias im Norden der DDR. Z. Geol. Wiss., 7: 903-912. Brandner, R., 1984. Meeresspiegelsehwankungen und Tektonik in der Trias der NW-Tethys. Jahrb. Geol. Bundesanst., 126: 435-475. Brunner, H. and Wurm, F., 1983. Stratigraphie und M~ichtigkeiten der unteren Gipskeuper-Schichten (km 1, GrabfeldFolge) in Baden-Wiirttemberg. Jahresber. Mitt. Oberrheinischen Geol. Ver., N.F., 65: 307-344. Butler, G.P., Harris, P.M. & Kendall, C.G.St.C., 1982. Recent evaporites from the Abu Dhabi coastal flats. Soc. Econ. Paleontol. Mineral., Core Workshop, 3: 33-64.
Duchrow, H., 1984. Keuper. In: H. Klassen (Editor), Geologie des Osnabrticker Berglandes. Naturwiss. Museum, Osnabrtick, pp. 221-333. Eugster, H.P. and Hardie, L.A., 1975. Sedimentation in an ancient playa-lake complex: the Wilkins Peak Member of the Green River Formation of Wyoming. Geol. Soc. Am. Bull., 86: 319-334. Frank, M., 1930. Stratigraphie und Bildungsgeschichte des siiddeutschen Gipskeupers. Jahresber. Mitt. Oberrheinischen Geol. Ver., N.F., 19: 24-77. Geyer, O.F. and Gwinner, M.P., 1986. Geologie von BadenWtirttemberg. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart. Goodwin, P.W. and Anderson, E.J., 1985. Punctuated aggradational cycles: a general hypothesis of episodic stratigraphic accumulation. J. Geol., 93: 515-533. Gwinner, M.P., 1970. Gipskrusten im Gipskeuper bei Obersontheim (Baden-Wiirttemberg)? Neues Jahrb. Geol. Palaontol., Monatsh., 1970: 88-90. Hagdorn, H. and Simon, T., 1985. Geologie und Landschaft des Hohenloher Landes. Thorbecke Verlag, Sigmaringen, 186 pp. Handford, C.R., 1982. Sedimentology and evaporite genesis in a Holocene continental sabkha-playa basin--Bristol Dry Lake, California. Sedimentology, 29: 239-253. Hardie, L. et al., 1986. Repeated subaerial exposure of subtidal carbonate platforms, Triassic, northern Italy: evidence for high-frequency sea level oscillations on a 104 year scale. Paleoceanography, 1: 447-457. Haq, B.U., Hardenbol., J. and Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic. Science, 235: 1156-1167. Hauschke, N., 1987. Knollige und tepeeartige Strukturen-Indikatoren fiir die friihdiagenetische Bildung von Ca-Sulfaten unter Playa-Bedingungen im Unteren Gipskeuper (km 1) des Lippischen Berglandes. Neues Jahrb. Geol. Pal~iontol., Abh., 175: 147-179. Huth, R., 1956. Zur Geologie der Steinmergelb~inke, insbesondere der Bleiglanzbank, im Gipskeuper. Z. Angew. Geol., 1: 15-17. Kendall, A.C., 1984. Evaporites. Geosci. Can., Reprint Ser., 1: 259-296. Kendall, C.G.St.C., 1987. A review of the origin and setting of tepees and their associated fabrics. Sedimentology, 34: 1007-1027. Kozur, H., 1973. Biostratigraphie der Germanischen Mitteltrias. Freiberger Forschungsh. C, 280: I-III, 127 pp. Kulke, H., 1974. Zur Geologie und Mineralogie der Kalk- and Gipskrusten Algeriens. Geol. Rundsch., 63: 970-998. Langbein, R., 1987. The Zechstein sulphates: the state of the art. In: T.M. Peryt (Editor), The Zechstein Facies in Europe. Lecture Notes in Earth Sciences, Vol. 10. Springer-Verlag, Berlin, pp. 143-188. Logan, B.W., 1987. The MacLeod evaporite basin, Western Australia. Am. Assoc. Pet. Geol., Mem., 44. Logan, B. and Brown, R., 1986. Sediments of Shark Bay and MacLeod Basin, Western Australia. Field Seminar Handbook, Sedimentol. Res. Group, Dep. Geol., Univ. W. Aust., 229 pp.
25 Loucks, R.G. and Longman, M.W., 1982. Lower Cretaceous Ferry Lake Anhydrite, Fairway Field, East Texas: product of shallow-subtidal deposition. Soc. Econ. Paleontol. Mineral., Core Workshop, 3: 130-173. Orti Cabo, F., Mur, J.J.P., Geisler-Cussey, D. and Dulau, N., 1984. Evaporite sedimentation in the coastal salinas of Santa Pola (Alicante, Spain). Rev. Inv. Geol. (Barcelona), 38/39: 169-220, Richter, D.K., 1985. Die Dolomite der Evaporit- und der Dolcrete-Playasequenz im mittleren Keuper bei Coburg (NE Bayern). Neues Jahrb. Geol. Pal~iontol., Abh., 170: 87-128. Schreiber, B.C., 1978. Environments of subaqueous gypsum deposition. Soc. Econ. Paleontol. Mineral., Short Course, 4: 43-73. Schreiber, B.C., 1986. Arid shorelines and evaporites. In: G.H. Reading (Editor), Sedimentary Environments and Facies. Biackwell, London, 2rid ed., pp. 189-228. Shinn, E.A., 1983. Tidal flat. Am. Assoc. Pet. Geol., Mem., 33; 171-210. van Houten, F.B., 1987. Late Triassic cyclic sedimentation: Upper Lockatong and lower Passaic Formations (Newark Supergroup), Delaware Velley, west and central New Yersey. Geol. Soc. Am. Centennial Field Guide, Northeastern Section, pp. 81-86.
van Wagoner, J.C., Mitchum, R.M., Posamentier, H.W. and Vail, P.R., 1987. Key definitions of sequence stratigraphy. In: A.W. Bally (Editor), Atlas of Seismic Stratigraphy, Vol. 1. Am. Assoc. Pet. Geol., Stud. Geol., 27: 11-14. von Freyberg, B., 1965. Cyclen und stratigraphische Einheiten im Mittleren Keuper Nordbayerns. Geol. Bavarica, 55: 130-145. von Huene F., 1959, Simosaurus guilielmi aus dem Oberen Mittelkeuper yon Obersontheim. Palaeontographica, 113. Warren, J.K., 1982. The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia. Sedimentology, 29: 609-637. Warren, J.K., 1983. On the significance of evaporite lamination. 6th Int. Symp. on Salt, 1983, Vol. 1: Salt Institute, pp. 161-170. Warren, J.K. and Kendall, C.G.St.C., 1985. Comparison of sequences formed in marine sabkha (subaerial) and salina (subaqueous) settings--modern and ancient. Am. Assoc. Pet. Geol., Bull., 69: 1013-1023. Wilson, J.L., 1975. Carbonate Facies in Geologic History. Springer, Berlin, 471 pp. Wurster, P., i964. Delta sedimentation in the German Keuper Basin. Dev. Sedimentol., 1: 436-446. Ziegler, P.A., 1982, Geological Atlas of Western and Central Europe. Elsevier, Amsterdam, 130 pp.
5
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin T H O M A S A I G N E R 1 and G E R H A R D H. B A C H M A N N
2
1 KSEPL (Shell Research), Volrnerlaan 6, 2288 GD Rijswijk (The Netherlands) 2 PREUSSAG AG, ErdiJl und Erdgas, Karl-Wiechert-Allee 4, 3000 Hannover 61 (F.R.G.)
Received June 2, 1988; revised version accepted November 14, 1988
Abstract Aigner, T. and Bachmann, G.H., 1989. Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin. Sediment. Geol., 62: 5-25. The dynamics of evaporite accumulation are studied in the Gipskeuper (Triassic) of the southwest German epicontinental basin based on a hierarchical, three-level stratigraphic analysis: (1) Among the various stratification types, the following are most important: (a) thin carbonate intervals with marine faunas; (b) massive sulfates with selenite, indicating very shallow subaqueous gypsum precipitation; (c) thin-bedded gypsarenites with a variety of sedimentary structures suggesting clastic evaporite accumulation during high-energy events ("gypsum tempestites"); (d) laminated gypsum with spectacular tepee structures, testifying inter- to supratidal settings; (e) green-grey and reddish claystones with nodular gypsum indicating evaporitic playa mudflats. (2) Facies sequences occur in several varieties and commonly form 1-4 m thick transgressive/regressive cycles with shallowing-upwards trends. The long recognized carbonate "marker beds" represent short-term trans- or ingressions into the almost flat Keuper Basin. Highly disturbed tepee horizons form widely correlatable regressive peaks. These sequences form evaporite equivalents to the ubiquitous shallowing-upward cycles known from carbonate systems. (3) The basin-fill consists of a small-scale cycles being vertically stacked upon each other to form the basic building block of the layer-cake architecture of the Gipskeuper succession. The stacking of small-scale cycles is superimposed onto a larger-scale, regressive cycle, which records a general shift from restricted marginal marine to hypersaline marine to the more continental red bed deposition of the overlying Keuper. This hierarchy of cycles probably reflects various orders of eustatic sealevel fluctuations. The Gipskeuper thus exemplifies dynamics of evaporite accumulation and allows us to understand some of the processes that produce a "layer-cake" stratigraphy. Its cycle hierarchy is a common motif in the entire southwest German Basin fill, and appears typical for epicontinental basins.
Introduction T h e depositional processes that lead to the acc u m u l a t i o n of evaporite sequences are still less u n d e r s t o o d t h a n those i n c a r b o n a t e a n d siliciclastic rocks, although significant insights have b e e n gained d u r i n g the past years (e.g. Kendall, 1984; W a r r e n a n d Kendall, 1985; Schreiber, 1978, 1986; Logan, 1987). This study aims to c o n t r i b u t e to the u n d e r s t a n d i n g of the d y n a m i c s of evaporite accumulations. A variety of s e d i m e n t a r y fabrics a n d 0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
sequences, some of which have n o t b e e n described from evaporite successions, are d o c u m e n t e d from a relatively thin U p p e r Triassic evaporite-to-red b e d sequence of the southwest G e r m a n G i p s k e u p e r F o r m a t i o n . Based o n a three-level analysis of (1) the stratification types, (2) the facies cycles, a n d (3) the stacking a n d hierarchy of cycles, a n att e m p t is m a d e to r e c o n s t r u c t the " d y n a m i c stratigr a p h y " (cf. Aigner, 1985) of this sequence (Fig. 1). The G i p s k e u p e r offers favourable c o n d i t i o n s for such a study, because it is well exposed in
DYNAMIC STRATIGRAPHY
J
J
0,na ,. I ,ac,e
II
]
as,n 0,nam,. ]
Fig. 1. General approach of this study: process-orientedanalysis of three hierarchical levels of sedimentary sequences. First step analyses individual stratification types, second step looks at arrangement of strata into facies sequences, and third step analyses packaging of cycleswithin the entire basin infill.
quarries, and a well-documented lithostratigraphy as a base for process-oriented studies has been established (Frank, 1930; Bachmann, 1974; Brunner and Wurm, 1983). Our studies focussed on the quarries east of Heilbronn in Northern Wiirttemberg (Fig. 2). This initial work concentrates on major sedimentologic/stratigraphic features in the lower part of the Gipskeuper Formation (Grundgips, Bochinger, Dunkelrote Mergel Members). Further investigations are planned for the future.
General setting The Triassic in the epicontinental German Basin is characterized by a tripartite subdivision into (1) the continental Buntsandstein, (2) the shallowmarine Musclielkalk, and (3) the marginal-marine to continental Keuper. The whole succession shows a well-developed "layer-cake" stratigraphy. Most units, be it only very thin, can be readily correlated over several hundred kilometers, and many have traditionally been used as lithostratigraphic
marker beds. The "layer-cake" stratigraphy suggests an extremely gentle depositional relief in this intracontinental basin, with slight changes in baselevel affecting huge areas instantaneously. The basin-fill therefore represents a particularly sensitive record of environmental changes. Within the Middle Keuper, evaporites (mostly anhydrite and gypsum, but also halite in some areas) are widespread throughout the Central European Basin (Fig 2a). In the southwest German subbasin (Fig. 2b), the Gipskeuper Formation records the transition from marginal marine to continental red bed conditions. While the basal Gipskeuper includes some 15-20 m of massive evaporite rocks, its upper part is dominated by red, greenish, greyish and varicoloured claystones, marlstones, and some sandstone and thin dolomitic intervals. Dependent on the type of exposure, the sulfates occur either as gypsum or anhydrite. The Gipskeuper is overlain by the Schilfsandstein, interpreted as a delta compley by Wurster (1964). For a schematic stratigraphic overview, see Fig. 2c.
0
l l i l l l
50 km
• gypsumquarry Berlin
)
y//
300 krn
':Stuttgart Fm. andstein M
100 -Anatina-B. ~
Engelhofer~ :
"(o
u_
Platte Mittlerer ~---~ horizont
~^Gips
50-
Strassburg/
EstherienSchichten
2
}l
__~_^ ~ ^
Bleigtanz Bo,nk
~ Ounkelrot e = ~ c ~ ^ Mergel Bochinger - - Bank -=-~:~-"~ Bochinger H.
~
^
/x~'~l~teO /
~
f
r : O,(Jbingen
Neckar
O
'" /
/
~"~
lll'lt
Grundgips-
B.o.-6 ~ S c h i c h t e n
dolomit lettenkeuperFm C
~1 b
/-
Fig. 2. (a) Simplified paleogeographic sketch of Central European Keuper Basin (after Ziegler, 1982). (b) Isopach map of the Gipskeuper Formation in the southwest German subbasin, after Geyer and Gwinner (1986). A - A ' = profile of Fig. 14. Quarries: 1 = Obersontheim, 2 = Talheim, 3 = Lorenzenzimmern, 4 = Raibach. (c) Generalised stratigraphic column of the Gipskeuper in southwest Germany. The studied interval comprises "Grundgipsschichten" to "Dunkelrote Mergel" members.
Stratification and facies types
Carbonate interoals Description. Carbonates occur within either the claystones or the evaporites as units up to several d m in thickness. Most of the carbonates are regionally widespread and are used as marker beds in the existing lithostratigraphic framework. For instance, the basal " G r e n z d o l o m i t " (cf. Fig. 2c) is correlatable as far as northern G e r m a n y over a distance of more than 500 km (Huth, 1956; Beutler and Schtiler, 1979; Duchrow, 1984). Although carbonate beds vary significantly in thickness, composition and internal structures, they show some general, recurring patterns. Each
has a sharp base, often erosional, partly scoured, and in some cases it m a y even be burrowed (Fig. 3a). The beds are dolomitic and vary texturally between mudstones (dolomicrites), and shelly, pelletoidal or oolitic wacke-, pack-, or grainstones. Some beds contain some quartz and phosphatic grains. Carbonate intraclasts m a y be very abundant as well as black pebbles and reworked selenitic clasts (Fig. 3b). Intervals with clasts can display imbrication, while shelly layers m a y be graded (Fig. 3c) and carbonate sands tend to be cross-bedded. Some clasts were found encrusted with stromatolites (Bachmann and Gwinner, 1971; Fig. 3d). Algae m a y also have contributed to the fine, crinkly lamination seen in some dolomicrites.
Fig. 3. Stratification and facies in the carbonate intervals. (a) Note sharp erosional base (arrow) with possible burrows and several wave ripple horizons at the top, each overgrown by selenite crystals. Lense cap diameter 5 cm. (b) Imbrication of selenite clasts within carbonate layer. Bank gamma, Raibach quarry. (c) Composite carbonate layer with black pebbles in lower and graded shell bed in upper part. Grenzdolomit, Obersontheim quarry. (d) Stromatolites on carbonate clast. Grenzdolomit, Hessental.
The carbonate units can display internal erosion surfaces, which m a y be lined with selenite crystals that appear to have grown in situ on the surface. Bed tops are often wave-rippled (Fig. 3a). The shelly fauna in the carbonate units is dominated by the mass occurrence of the bivalve Costadoria goldfussi, and some minute gastropods. Locally, the bivalves Myophoria transversa, and Unionites occur in addition ( H a g d o r n and Simon, 1985).
Interpretation. Our interpretation is based on the following main arguments: (1) The sharp lithological and textural contrast between evaporites, or claystones, and the
carbonate layers, together with evidence of considerable reworking (intraclasts) indicate a major and sudden event that temporarily changed environmental conditions. Sedimentary fabrics suggest shallow subaqueous (subtidal) deposition (crossbedding, grading, ripples, imbrication). Occasionally subaerial exposure m a y have occurred (black pebbles). (2) The sudden and episodic occurrence of a restricted marine fauna in the carbonate layers calls for episodic improvement of the ecological conditions, p r o b a b l y as during phases of marine influx (Bachmann, 1974). However, the dominance of one bivalve species, and its mass occur-
rence does not indicate a fully marine but rather a somewhat stressed, marginal marine (and at times perhaps also brackish) setting. Although the shell material is reworked, it is unlikely that the fauna was transported from marine areas into the Gipskeuper setting. (3) The extremely widespread occurrence of the carbonate intervals excludes local causes but calls for at least regional, probably basin-wide events. Possible causes include: (a) climatic changes with more freshwater runoff and mixing, allowing a restricted fauna to develop, or (b) marine ingressions. The latter appears more likely (see also von Freyberg, 1965; Richter, 1985). Kozur (1973) found faunal affinities to the western Mediterranean, indicating short-term ingressions from the south into the German Basin. The carbonate intervals can thus be interpreted as marine ingressions interrupting the more restricted evaporitic conditions with the supply of marine waters during relative rises in sealevel. In contrast, some dolomites in the upper part of the Gipskeuper (Estherienschichten) represent playadolomites (Richter, 1985). These are entirely lacking in fauna.
Massive sulfates with selenite
Fig. 4. Stratification in the massive sulfates. Selenite with marked vertical textures; note preserved gypsum crystal tops. Obersontheim quarry.
vertical textures (Fig. 4). Both preserved and truncated selenite prisms are observed. Fossils are rare; however, a complete skeleton of a Simosaurus has been discovered (von Huene, 1959), and plant remains (Voltzia fraasi, Pagiophyllum foetterlei ) are occasionally found (Hagdorn and Simon, 1985).
Description. This facies association occurs chiefly in the lower half of the studied sequence and consists of relatively massive, grey anhydrite a n d / o r gypsum units (massive gypsum of Bachmann, 1974). Their bedding is sometimes subparallel on a scale of cm to dm and is accentuated by mm-thin seams of clay or dolomite (Fig. 4). The bedding may be disrupted by dm-scale channel-like erosional features, filled with laminated gypsum and marl. These resemble "gutter casts" (Aigner and Futterer, 1978) in size and cross-section, but they cannot be followed in three dimensions. Generally, the individual sulfate beds are more or less structureless, except for gypsarenites with occasional remnants of ripple cross-lamination and possibly scoured bed bases. However, there are frequently intercalated layers, 1-7 cm thick, that consist of clearly recognizable selenite crystals, while others only display crystal relics or faint
Interpretation. The most salient feature in this facies is the occurrence of selenitic gypsum, which indicates the precipitation of primary gypsum crystals under very shallow subaqueous (subtidal) conditions (Schreiber, 1978; Warren, 1983; Kendall, 1984). Layering in the selenitic units is likely to represent (seasonal?) changes in the brine composition and dissolution of the selenite crystals, while preserved zig-zag lamination indicates continuous crystal growth over several years (Warren, 1982). Similar layering styles with alternating selenite crusts, gypsarenites and clay have been described by Orti Cabo, Mur, Geisler-Cussey and Dulau (1984) from coastal salinas of Spain. Whether the small channels represent a form of gutter cast, or are due to erosion by dense brines, or are caused by subaerial exposure can not be evaluated at present. Beds of pure, crystalline gypsum, remnants of ripples and scours suggest
10
Fig. 5. (a) Stratification and facies in the thin-bedded sulfates. Cm-thick alternations of claystone layers and gypsum layers. Obersontheim quarry. (b) Very similar facies from modern MacLeod Basin (photo courtesy M. Shephard).
some redeposition of gypsum as gypsarenite. A few of the contorted beds may be due to slumping. According to Richter (1985), the strontium contents in gypsum and anhydrite of the Gipskeuper are typical for marine sulfates.
Thin-bedded sulfate / claystone intervals Description. These more "platy" sulfate rocks (Bachmann, 1974) contain a variety of intergradational facies ranging between two extreme end members: (a) relatively massive but thin-bedded
alternations of cm-thick gypsum layers with mmthick claystone or dolomite seams, and (b) cmthick alternations of gypsum and claystone layers (Fig. 5a). Gypsum layers are both gypsites and gypsarenites (sensu Warren, 1982). Their colours are mostly light grey or white, but may also be greenish or reddish, particularly upward in the Gipskeuper sequence. Similarly, claystone layers may be grey, greenish or reddish in colour. This facies contains a wealth of sedimentary structures and textures. Many of the thin sulfate
11
Fig. 6. Sedimentary structures in the thin-bedded sulfates. (a) Clastic gypsum bed; note basal scours (b) Graded and partly wave-tippled gypsum layers intercalated into claystones. (c) Close-up of graded gypsum bed, probably representing a gypsum tempestite. Obersontheimquarry.
beds are structureless gypsites. However, gypsarenites often show sharp, erosional bases with scours, graded bedding and ripple marks (Fig. 6a-c). Some layers contain abundant gypsum porphyroblasts (Fig. 7a), 1-20 mm in diameter, within the finer-grained matrix. Halite "pseudomorphs", showing hopper features, have also been recorded (Fig. 8a). Mud cracks in the intercalated claystone layers are often well-recorded as gypsum-filled casts on lower bedding surfaces. Most spectacular, however, are frequent tepee
horizons within the thin-bedded gypsum facies (see below, Fig. 8b, c).
Interpretation. The basic motif of this very varied facies association is that it records various types of intermittently operating depositional processes: (1) Episodes of high-energy deposition. Gypsarenites, basal scoured surfaces, grading, ripples, etc., in the sulphates testify to subaqueous clastic deposition of sulphates by periodic high-energy events ("gypsum tempestites"). Kendall (1984)
12
Fig. 7. (a) Gypsum porphyroblasts on bedding plane. Lorenzenzimmernquarry. (b) Similar gypsum porphyroblasts ("hemipyramidal gypsum") from the sediment surface of the modern MacLeod salina (West Australia). Lense cap diameter 5 cm.
notes that clastic sedimentation is a dominant feature in the higher-energy environments of intertidal or supratidal-lagoonal evaporite settings. He interprets laminations as storm deposits when evaporite fiats were flooded by sediment-charged waters, by analogy with the formation of storm laminae in other tidal-flat sediments ("storm flats"). (2) Flood deposition. The thin multicoloured claystones intercalated into the sulphate facies could result from mechanical transport of the clay
during times of heavy rain, as described by Warren (1982). Thicker claystones may represent very distal end members of river floods, similar to flood deposits brought into the modern West Australian MacLeod salina by ephemeral streams (Logan and Brown, 1986; Logan, 1987). (3) Brine sheet deposition. M a n y of the laminated, but otherwise structureless gypsum layers with or without claystone alternations were most likely deposited from evaporiting "brine sheets". This appears the dominant mode of
13
3 :
~.
.
.
.
~
j.
.
.
.
.
Fig. 8. Indicators for subaerial exposure in the thin-bedded sulfates. (a) Halite "pseudomorphs", Triensbach quarry. (b) Tepee composed of fine-grained gypsum; note also porphyroblasts in layers below tepee. Obersontheim quarry. (c) Horizon with large tepees, note partly capped tops. Raibach quarry.
gypsum precipitation the m o d e m M a c L e o d evaporite basin (Logan and Brown, 1986; Logan, 1987; Fig. 5b). However, the structureless gypsite may also be compared to m o d e m gypsite forming in the zone of soil moisture (Warren, 1982) as subaerially exposed gypsum "caps". Mud cracks and tepee horizons document repeated and periodic sub-
aerial exposure of the depositional environment. Intervals with frequent porphyroblasts may also indicate exposure. Butler et al. (1982) report a similar association of discoidal gypsum crystals growing in the sediment below anhydrite polygons from Abu Dhabi. The gypsum prophyroblasts may indicate the paleo-brine level at the sediment surface, similar to the "hemipyramidal gypsite"
14 described by Logan and Brown (1986) and Logan (1987) from the MacLeod salina (Fig. 7b). In analogy to the modem Bristol Dry Lake (Handford, 1982), the halite pseudomorphs probably formed displacively in the capillary fringe just beneath an exposed saline mudflat. In summary, this facies seems to represent a suite of inter-to supratidal (possibly also some very shallow subtidal) flats with intermittent episodes of flooding and subaerial exposure. In this context, the terms supra-, inter- and subtidal are not meant to imply classical tidal flat environments but are used to indicate the position of facies with respect to main sea level.
Fig. 9. Nodular gypsum in red claystoneprobably representing a gypcrete soil ("Gwinner's Gipskruste"), Talheim quarry. Hammer for scale.
Disturbed sulfate intervals (tepees, nodules) Description. The thin-bedded gypsum/claystone
who could trace these layers over large parts of the southwest German Keuper basin.
facies as described above is frequently deformed into tepee horizons. These are normally only a few dm thick, but when stacked on top of each other, may form intervals up to 3 m thick. The tepees are characterized by beds of finely crystalline gypsum which bend upward and are often broken on top (Fig. 8b). The height of the individual tepee is normally a few cm to 1 dm, but horizons with diapir-like tepees as high as 0.5 m have also been observed (Fig. 8c). When flattening out the beds that form the tepees, it is obvious that they are longer than the available space, i.e. grew by expansion. Tepee tops are commonly eroded. In plan view, the tepees form polygons. Some tepee horizons appear extremely folded and disrupted, and some are brecciated and conglomeratic. Thus clear-cut tepee structures may grade into beds of nodular appearance. Another type of nodules occurs in a matrix of claystones. These nodules range in diameter from a few cm up to 2 dm, show cauliflower-like surfaces and consist of white, light grey to light reddish, crystalline gypsum (Fig. 9). The nodular beds are only a few dm thick and are generally eroded on top. Some of the disturbed sulphate intervals are laterally extremely widespread despite their limited thickness. They include the "Gekr~Ssegipse" of Bachmann (1974) and Brunner and Wurm (1983)
Interpretation. To our knowledge, tepees have only very rarely been described from evaporites. Exceptions are the recent anhydrite "polygons" from Abu Dhabi (Butler et al., 1982), "distorted beds" from Bristol Dry Lake, California (Handford, 1982), and tepee-like structures in carbonate residues from the North German Gipskeuper (Hauschke, 1987). However, gypsum tepees are very abundant in our study area. In a recent paper, Warren and Kendall (1987) reviewed the origin and setting of tepees from carbonate sediments. They emphasize that tepee formation generally involves three processes: (1) early cementation of the surface sediments, (2) fracturing of the resulting crust, and (3) filling of these fractures with sediment and cement. It appears that the present sulfate tepees formed by lateral expansion of indurated gypsum beds. Bed induration could be caused by precipitation of gypsum from evaporating capillary waters; Warren (1982) described modern gypsite crusts from South Australia. Subsequent cracking of such indurated beds was probably related to thermal expansion and contraction, or simply to desiccation. Successive episodes of crack-infill and drying-out could have caused the sulfate beds to expand and overthrust to form tepees, similar to the mechanism described by Warrren and Kendall
15 (1987) from modem carbonate tepees in coastal lakes of South Australia. Brecciated and conglomeratic layers appear to be due to extreme overthrusting, collapse and crumbling of tepees, in certain cases overprinted by mechanical reworking as well as compaction. Langbein (1987) envisages compaction as the main cause for deformed sulfates. However, many of the clasts and nodules in this succession clearly represent mechanically reworked remnants of tepee crusts. By analogy with modem examples, tepee formation, brecciation and nodular growth is likely to be due to prolonged phases of subaerial exposure and desiccation. This is also indicated by abundant red claystone seams, mudcracks and the often eroded tops of both tepee and nodular layers. The longer periods of subaerial exposure appear to be caused by (most likely only minor) lowerings of sealevel. Thin dolomite layers sometimes overlying these units, and infiltrating down into them, probably represent a renewed submergence by a subsequent transgression. The transgression probably also accounts for mechanical reworking seen in some "conglomeratic" sulfate layers. Some of the nodular gypsum horizons, however, occurring in the upper part of the studied sequence within a claystone matrix (Fig. 9) may be compared to modem soil crusts and "gypcrete" (Gwinner, 1970; Kulke, 1974) or have the characteristics of interstitial sulphate growth near the sediment surface, as described by numerous authors from recent sabkhas (e.g. Butler et al., 1982; Shinn, 1983).
Grey shales Description. Laminated shales only a few dm thick, occur as a single laterally persistent horizon. They are mudcracked at the top, and thin silt laminae but no gypsum was observed within them. Lingula shells and phosphatic grains have been found on bedding planes.
thickness of this unit as well as the mudcracks suggest that such conditions were of ephemeral nature. Clearcut environmental interpretations are difficult, but an ephemeral lacustrine to restricted shallow-marine setting may be envisaged. These shales are similar to the much thicker grey shales in the uppermost Gipskeuper (Graue Estherienschichten).
Green-grey and reddish claystones Description. Green-grey and reddish, more or less unbedded claystones become more abundant in the uppermost part of the studied sequence. They form up to meter-thick packages with few diagnostic sedimentary structures. They include horizons with mudcracks, a few, cm-thick sandstone and dolomite beds, and planar, frequently graded gypsarenites with or without tepee structures. The above mentioned gypsum nodules are common and vary from a few cm to about 30 cm in diameter. They are mostly light reddish in colour and occur preferentially in discrete horizons (Fig. 9).
Interpretation. This
facies is similar to inland sabkhas and saline mudflat deposits described by several authors (e.g. Eugster and Hardie 1975; Handford, 1982; Kendall, 1984). In such environments, evaporites precipitate mostly in ephemeral brine ponds or interstitially from hypersaline brines. Nodular gypsum layers probably represent paleo-groundwater levels. The ephemeral brine ponds and the highly saline groundwater were probably fed by temporary run-off of the hinterland o r / a n d by occasional marine storm floods. Greenish claystones suggest generally a reduc, ing environment, caused by more frequent flooding or by ephemeral lacustrine conditions. In contrast, red claystones probably represent longer exposed mudflats with less frequent flood events.
Thin sandstones
Interpretation. The dark grey colour and the pre-
Description. Greenish quartz sandstones occur as
served lamination of this facies indicate that deposition occurred subaqueously under somewhat "quiet water" conditions. However, the limited
cm-thick layers on erosional surfaces in between evaporites and claystones. The base of each is irregular and scoured, the top frequently covered
16
with oscillation ripples. Phosphatic grains as well as reworked claystone pebbles are also present. Occasionally, casts of Costadoria goldfussi, small shark teeth as well as undiagnostic trace fossils are found on the upper sandstone bedding planes. The sandstones are overlain by thin dolomitic beds or by claystones. As documented in stratigraphic cross-sections by von Freyberg (1965), they represent the very distal expressions of thicker sandbodies at the basin margin.
ing heavy rainstorms). However, subsequent marine ingressions reworked the sheetflood sands, as indicated by the faunas present in some of them. Facies cycles Detailed bed-by-bed logging of the Gipskeuper sequence reveals a marked cyclicity (Fig. 10). This has not been recognized so far, although the cyclicity is punctuated by the traditionally used "marker beds". In general, the small-scale cycles recognized here vary between 1 and 4 m in thickness. These cyles are in several ways reminiscent of the widespread shallowing-upwards cycles in shallow-water carbonate environments (e.g. Wilson, 1975; Goodwin and Anderson, 1985).
Interpretation. The thin sandstone layers appear to record rapid and laterally very extensive inundation of an extensive mudflat. Since the thin sandstones grade towards the basin margin into thick sandbodies, the sand material must have been derived from there (possibly via sheetfloods dur-
INGRESSION/sheet flood
"Y
--
SUPRATIDAL
mudcrec~'~s
gypcrele
~
15
molt ,d
oOC storm floods (hypersallne marine}
,~ tepees
oct storm floods brine ponds
,re q
C/st, Gyp
SUPRA -/INTERTIDAL comm subaeriat
]1
I (/sl
gygn
distal sheet f~ood$ inundation
k k (;y#
INTER ISUBTIOAL hypersali~e marine freqsubeerlo[
t~>
•
Sh
~l,oguto
~st
~ C <~ ~ s ~ ~ ~>nod~
Gekrbse gips 3 ,[i~yp
(2Dnod cont bed,~ ~gyp porphyro btosts
SUBTiDAL
dgy
gngy
" ~ "'-freq
grad
(~
S~STIDAL:INGRESSION
Oc ~ --s ~y bia¢k pebbles
~
restricted merino
~~
:
,~>/)
hypers0hne ~arine
treq sub~er,a~
freq 9rQd
Gekr6se gips 1
SUBTIOAL
'.'
red, wh 9ngy'
i l oct subaerial
OL
Do
a at mot
,~yc,
channel
~__.
s"~"t-Ool OC ~ ~ rnit
~
~(Z) D dUotkpebb~ ocrete s g'
reslricted maline
C/st
X r~ts
SUPRA /INTERTIDAL comm subQerieL o~ storm floods &
freq Subaet ~1 occ $1orm floods & distol sheet floods
I i I iI
II ~-~ i,I ~I ! ,'
SUPRA -/INTERTIDAL comm subo erio[ occ slorm floods ~
/ II)
i I ~, ' i
INTER-/SUBTIOAL hyper~tin e m~rme heq 5ubaedQI
,~i
Ii , h~cYcPerss2bTgr,~2rne I, occ suboer~a
[ I
L
I
Ii SUBTIDAL INGRESSION
subaerlal
,x
(~)C gl P ~yp
/ ~,
dlstoJ sheet floods
SUBTIDAL INGRESSION
.... gY
I(
INGRESSION/sheet flood SUPRAIIDAL
subaerlal
gngY, red, ~h
i --
subaeri=l SUBAQUEOUS testr marine/brackish
INTER-/SUSTIDAL
hyperse[ine mar,no
~ottz,o
hypersGUnemarine
SUPRATtDAL
nod
GekrGsegips 2
INTERISUBTIDAL
Gyp
sUB SUPRATIOAL
distal sheet flaods
hypersoline morine
Symm r,pple marks s - - s t a r v e d Ool
I
l
I i
GYP
:B
1
suboeria~
00t
Bank 6
. . ~
INGRESSION/sheet tlodd SUPRA-tlNTER1tDAL comm ~ubfleria J oct slotm floods
I
5UaTIOAL
hypersallne marrne
Do/
I/~
0o/
i
~, I
- -s
rs:stb~ieCrt,:~ . . . . . . . Bank
!'
g~
SUBTIOAL 7 hypersa[I ne ma.~e jI~ j I iNGRE Sl N ,e r marine .. J
Fig. 10. Detailed sedimentological log of the Gipskeuper in Obersontheim (cycles 1-12) and in Talheim quarry (cycles 13-18). The arrows indicate small-scale shoaling-upward cycles (cycle numbers are referred to in the text). Symbols and abbreviations adapted from Shell Standard Legend.
17
DOl sd II~gl moU r~
::-_:::-~----::
SUPRATIDAL
GIst
redbn/
gygn
~:--:_-:::_-:_=~
distat s h ~ t floods
mott r~
~_:_-:;
Gyp
GWINNERs Gipskruste
I:2Dnod gyporete
nz-
C/St(Gyp)
gygn
SUBrlOAL:INGRESSION restricted mQrine
SUPRATIDAL ~c
dista~ sheet floods
C/st
SUPRATIDAL
Clst
gygn
dlstal sheet ftoods inundation
redbn
vi redbn
SUPRA-/INTERTIOAL
C/st, Gyp
c o m m subQerial oc¢ storm floods & distat s h ~ t floods brine pOndS
SUPRATIOAL
C/st
gy redl>n
Gyp [3 Dunkelviolettes Grenzlager
oct distal sheet floods
vi t e d ~
Gyp
SUPRATIOAL
C/5t
C/st, Gyp ~
~vk
occ. dkstal sheet f i n d s
gygn
SUPRA */INTERTIDAL c ~ m ~b~erio( o¢c. storm f i n d s t distal s h ~ t f i n d s
freq grod gygn/ tedTon
Gyp
(~nod
gypcrete
$UPRATIOAL
C/st
viredb~
occ distal sheet floods
Clst
vign
distQ~ sheet floods inundation
brine ponds
SUB-/INTERTIDAL o c t storm floods
mo~ red
Gyp
gygn
~nod
mort gn
gypcrete
SUPRATIOAL SUPRATIOAL
C/st
redbn
suboerial ~.distalsheetfioods
C/st
~/pcrete nod~red
Q~ BOChi
rB.
redbn
~c
distal sheet floods
/Gyp) Dol, cly
gngy
gygn
INUNOATION DO/ s d y , . . . ~ _ . . ~ g ~ . ~ . ~ . C/st gngy
5st
sheet flood
Fig. 10. (continued).
"'Base carbonate cycles" (Fig. 11a) Description. This type of cycles is characteristic for
the lower part of the studied sequence (e.g. cycles 1-4 in Fig. 10). Each cycle starts with a basal carbonate layer (see section "Carbonate intervals"), in a few cases also a thin sandstone, which may be followed by massive sulphates with selenite intercalations (see section "Massive sulfates with selenite"). This may be overlain by thinner bedded gypsarenite and gypsite with claystone laminae and tepee structures. The top of a cycle, just below the basal carbonate layer of the following cycle, often shows abundant gypsum porphyroblasts. Upwards in the cycles the thin-bedded gypsum becomes more abundant. There is also an upwards increase in the redness of the claystone intervals, and in the abundance of tepees.
Interpretation . The "base carbonate" cycles start with a marine ingression, are followed by shallow subtidal evaporite precipitation (selenite) and lead under progressive upwards shallowing to eventual subaerial exposure (gypsum tepees). Sequences commencing with a thin carbonate layer, caused by an ingression, followed by selenitic gypsum, are very similar to the initial stages of the modem MacLeod salina (Figl 12; Logan and Brown, 1986; Logan, 1987). The sulfate parts of the present cycles are very similar to shallowing-up evaporite cycles described by Warren (1982, 1983) from modem coastal salinas of South Australia, which also pass from selenites upwards into laminated gypsarenites and gypsites. The cycles bear some resemblance to cycles described by Loucks and Longman (1982) from the Lower Cretaceous of Texas. These also start
18
Fig. 11. Example of "base Carbonate cycle" with basal carbonate layer and tepees at top (Lorenzenzimmern quarry east of Schw~ibisch Hall). (b) Example of "top disturbed cycle" (Raibach quarry west of Schwiibisch Hall). Note basal bedded sulfate overlying disturbed horizon (top of previous cycle) passing up into sequence with increasing amount of tepees.
with a thin marine carbonate unit, followed by subtidal and finally supratidal sulfates. However, these are deposited in a lagoonal setting.
"'Top disturbed" cycles (Fig. 11b) Description. This type of cycles is more typical for the middle part of the studied sequence (e.g. cycles 5 - 7 in Fig. 10). Basal carbonate layers may be present, but they are far less prominent or are not
developed at all. This type of cycles show fewer, if any, selenite intervals. Instead, they are dominated by the thin bedded g y p s u m / c l a y s t o n e alternations with claystones and tepees increasing in abundance upwards in each cycle (Fig. l l b ) . Claystones tend to change in colour from greygreenish to more reddish upwards. The top of the cycles is a strongly disrupted interval with extreme tepee development, i.e. the traditional "Gekr/)segipse".
19
Fig. 12. (a) Wave-rippled calcarenite layer overgrown by selenitic gypsum at the base of a small-scale transgressive/regressive cycle from the Gipskeuper. Bank delta, Raibach quarry. (b) Almost identical succession from the modern MacLeod salina: wave-rippled calcarenite, representing a Holocene ingression (Logan and Brown, 1986), overgrown by subaqueously grown selenite crystals. Lense cap diameter 5 cm.
Interpretation. The record of marine ingressions at the base of the " t o p disturbed" cycles is very subtle. This suggests either a weaker ingressive pulse, or a more landward setting, or both. A more landward setting is indicated by more abundant red claystone layers. However, clear upward shallowing trends are also present, where the "disturbed tops" testify to longer periods of subaerial exposure. The thin sandstone layers overlying the "disturbed tops" of certain cycles (e.g. cycle 7 in Fig. 10) are most likely sheetflood-derived. However, these sands became reworked by the ingression of the subsequent cycle as indicated by the occasional occurrence of shelly fauna.
10) and is less regularly developed than the evaporitic cycles described above. Cycles have a sharp base, and m a y show a cm-thick sandstone. This is followed upwards by an up to 1 m thick interval of thin-bedded reddish gypsum often with tepees and graded gypsarenite beds. Upwards the bedded gypsum passes into greyish or reddish claystones, the red colours becoming more abundant towards the top. The upper part of each cycle is characterized by discrete horizons of gypsum nodules, a few cm to 3 dm in diameter, within a claystone matrix. When dissolved near the present-day surface their relicts are still recognizable by red mottling. The cycles are generally topped by a mudcracked horizon.
Argillaceous cyles
Interpretation. The thin sandstone layers at the
Description. This type of cycle occurs in the upper
cycle base (or at the top of the preceding cycle) were derived via sheetfloods from the hinterland. However, these sands became reworked by subse-
part of the studied sequence (cycles 8 - 1 2 in Fig.
20
quent marine ingressions (shelly fauna). The ingression led to ephemeral but very extensive "brine ponds" with deposition of gypsum as seen in the
basinal
lower part of the cycles. Frequent tepees indicate alternating periods of subaqueous and subaerial conditions. The transition to the claystone-
15 km
marginal
m
Gekrbsegips 2 °
"
': i~',,,,
,'""
I
.~e~¢~
w-
I
~
4
//
/~/
L
//
Fig. 13. Variation in facies succession of three shoaling-upward cycles. Note thinning of carbonate layers, increase m the amount of tepees toward basin margin (right) and occurrence of thin sandstones. Note also less-well developed subcycles within cycles 4 and 5. Legend and number of cycles see Fig. 10. Locations see Fig. 2b.
21 dominated lithologies upwards in the cycles probably represents a change to a dryer mudflat-like setting. Thin layers of gypsum were formed on the mudflats by evaporiting brine sheets and by clastic redeposition as gypsarenites. Mudcracks and sulphate nodules at the top of cycles indicate exposed inland sabkha conditions (cf. Gwinner, 1970). Principally similar cycles have been modelled by Kendall (1984) and have recently also been described by Hauschke (1987) from the North German Gipskeuper. A partial m o d e m analogue are cycles described by Handford (1982) from Bristol Dry Lake: in these a lower "salt pan" unit passes upwards into "saline mudflat" deposits. Basin-fill patterns
Distribution of small-scale cycles Laterally, each cycle may vary in facies succession (Fig. 13). The basal carbonates tend to become A -~ sequences
less fossiliferous and less prominent towards the basin margin, while clay interbeds, thin sandstones, as well as the abundance of tepees increases to the basin margin. However, the cycles can be correlated over considerable distances, as mentioned above. This is well illustrated by the detailed microstratigraphic work of Bachmann (1974) and Brunner and Wurm (1983), who could trace both the carbonate intervals (" Bank alpha to delta") as well as the disturbed sulphate horizons ("Gekrtsegipse 1-3) as marker beds throughout the entire SW-German Basin (Fig. 14).
Vertically, the Gipskeuper appears as a sequence of stacked shallowing-upward cycles, each beginning with a marine ingression (some probably in combination with sheet floods). Such a vertical stacking of cycles is a very common motif in both many carbonate and in evaporite sequences (e.g. Wilson, 1975; Goodwin and Anderson 1986). Such cycles are valuable tools for chronostratigraphic A'
basinal
marginal m
6
l/ Fig. 14. Basin-fill pattern of the Gipskeuper in transect from margin towards basin center (profile A-A" in Fig. 2b). Note how small-scale shoaling-up cycles(arrows) are superimposedonto a larger-scaleregressivecycle(large arrow). Lithostratigraphymodified after Brunner and Wurm (1983). Number of cyclesrefer to numbers in Fig. 10.
22 correlation based on event-stratigraphy. This applies especially for largely unfossiliferous evaporite basins.
Cycle hierarchy Viewing the studied Gipskeuper sequence as a whole, an overall trend in basin-fill is apparent. Most massive and pure sulphate rocks are restricted to the base. Clay interbeds become more abundant upwards, until they dominate the sequence completely. Successive minor cycles show systematic upwards changes: the record of marine ingressions (carbonate layers with marine fauna) becomes more and more subtle, while simultaneously all indicators for subaerial exposure and for continentality increase upwards (Fig. 15). It thus appears that the stack of small-scale trans/ regressive cycles is superimposed on an overall, larger-scale regressive cycle. Given the uncertainty in biostratigraphically correlating the Germanic and Alpine Triassic, the overall regressive cycle
~.
.~=~
Gyp (2Dnod C/st,reddish ~ C/st, gygn ---- -- --_----~.
C/st,
Gyp,
thin-bedded
freq grad
~yp, thin-bedded,tepees~ 3 (~) nod (~
C/Stint ....
Gyp,thin-bedded C/Stinterv., gn/red / . . freq grad ~
~s
Gyp,~sthick-bedded'massive ~
j
Gyp,thick-bedded, selenite
massive
~
~O/ (2) block pebbles
(~C ~ ~
,SSt (~,,....~..~%
Ba88
1 2 3 ~ 5 6 7 8 9 101112131z, 15161718
-o SUPRATIDAL ~: occ. distal sheet floods o
.... s ~ Gyp~ thin-bedded~ w a v "-Lry.~
SUPRATIDAL distal sheet flood
gypcrete
~
gygn/red
SEQUENCES
INTERPRETATION
LITHOLOGY / TEXTURES ~ . ~ . . . . •..... .j
probably corresponds to the lowering in sealevel recognized by Brandner (1984) for the entire Mediterranean region in the lower part of the Carnian (note contrast with Haq et al., 1987). The small-scale cycles are more difficult to interpret. However the extremely widespread lateral continuity of small-scale cycles bears on their origin. Clearly, local autocyclic mechanisms are unlikely. Instead small-scale transgressive/ regressive pulses by (probably minor) fluctuations in eustatic sealevel can be envisaged. Arthurton (1980) considered eustatic fluctuations for rhythmic sequences in the British Upper Triassic Keuper Marl, and van Houten (1987) described Triassic lacustrine deposits which appear to record astronomically controlled "Milankovitch cycles" from New Yersey. Also, the Carnian of the Alpine Triassic is characterized by an extremely cyclic depositional pattern, for which a eustatic Milankovitch-type control had also been proposed (Hardie et al., 1986). In sequence stratigraphic terms (van Wagoner et al., 1987) the overall re-
more freq. distal sheet floods/ inundation
l
SUPRA-/INTERTIDAL subaerial, occ. storm floods & "~ sheet floods, brine ponds ~ SUPRA-/INTERTIDAL _E comtn, suboerial, oct. storm ~[ floods & distal sheet floods i J
--i"]-~ /
INTER-/SUBTIDAL hypersaline marine, freq. subaerial, occ. storm floods & distal sheet floods SUBTIDAL
occ.hypersalinesubaerialmarine
J
J J
SUBTIDAL hypersaline marine occ. suboerial SUBTIDAL:INGRESSION
restricted marine
Sst: reworked sheet flood
Fig. 15. "Ideal" shallowing-upward cycle of the Gipskeuper. Black bars show sequences of actually observed cycles from bottom to top of the studied sequence (numbers of cycles refer to numbers in Fig. 10). Note the systematic upward changes in cycle type, recording the superimposed overall regressive trend.
23
tecture. This has been done by a hierarchical analysis of three levels of sedimentary sequences: (1) Stratification types record a variety of depositional processes. These range from marine ingressions, shallow subaqueous gypsum precipitation, clastic redeposition of gypsarenites, subaerial exposure, distal flood events, lacustrine and mudflat sedimentation. (2) Facies sequences show generally shallowing-upward trends and reflect small-scale trans/ regressive cycles. Their apparently basin-wide distribution indicates allocyclicity which may be caused by small-scale variations in eustatic sealevel. (3) The basin-fill consists of a vertical stacking of the small-scale trans/regressive facies cycles. These are superimposed onto a larger-scale regressive cycle, recording the transition of marginal marine to continental redbed deposition. This hierarchy of cycles in the basin architecture of the Gipskeuper is thus principally similar to the infill style of the underlying Haupt-
gressive cycle probably represents part of a "third-order sequence", while the small-scale cycles can be regarded as "paracycles". Conclusions (Fig. 16)
Many single depositional features in the Gipskeuper may be explained by comparison with both coastal salinas and inland sabkhas and playas in the modern. The general setting of the Gipskeuper, however, differs from these "actualistic" examples, particularly in its huge extent and in its very low relief. Gipskeuper evaporites show a number of marine characteristics, but clearly were deposited in an intracontinental context. The lack of true actualistic analogues for epeiric evaporite sedimentation thus imposes certain limits in applying "comparative sedimentology" alone to understand the depositional processes. To this end, the well-established lithostratigraphy of the southwest German Gipskeuper has been used as a base to analyse the dynamics of the stratigraphic archi-
Dynamic stratigraphy FACIES
• tepees/
CYCLES
CYCLIC
emergence
~
B A S I N - FILL
r~-~-~-
~ _~ :,.:.?:!::~
A III l = "-11
\
1/
II[~'!'~ /l/~'~'.~ snoa,,ng
fiats
basins
_ ..
~
i
~
~
I:~/:t
Ill
/~---~-~
J ~ . -
i~;~ Ill/~-_~-~J~ F~".-~ I I I F ~ - / : j ~ : , _ ~
l
"~ "
. . . .
Ill ~'~/~::~ :~:.:-:~:"~
0 Fig. 16. Summary of the "dynamic stratigraphy" of the Gipskeuper. Stratification types indicate the depositional dynamics of the various subenvironments ranging from shallow-marine to alluvial sheetfloods. Facies sequences record minor ingression/regression cycles. Vertically stacked minor cycles constitute a major regressive sequence of the studied Gipskeuper basin fill, shown here as a schematic margin (fight) to basin (left) cross-section where the marine influence (transgressive carbonate marker beds) decreases upwards in the sequence, while the regressive terrigenous influx gradually increases.
24 m u s c h e l k a l k ( A i g n e r , 1985) a n d s e e m s t y p i c a l for the s t r a t i g r a p h y o f e p e i r i c basins. T h e G i p s k e u p e r thus exemplifies dynamics of evaporite accumulat i o n a n d allows us to u n d e r s t a n d
some of the
p r o c e s s e s t h a t l e a d to a l a y e r - c a k e s t r a t i g r a p h y of a n e p i c o n t i n e n t a l basin.
Acknowledgements We gratefully acknowledge the owner's permiss i o n to w o r k in several G i p s k e u p e r
q u a r r i e s in
W i i r t t e m b e r g . T . A . is g r a t e f u l to B. L o g a n ,
R.
B r o w n a n d M. S h e p h a r d for g u i d i n g a field trip to t h e m o d e r n M a c L e o d salina, w h i c h p r o v i d e d s o m e i d e a s for a n " a c t u a l i s t i c " i n t e r p r e t a t i o n o f s o m e features
in
the
Gipskeuper.
M.
Shephard
is
t h a n k e d for the f o t o o f Fig. 5b. T h i s p r o j e c t has been
c a r r i e d o u t in o u r free t i m e o u t s i d e
our
professional duties and the views expressed herein are strictly o u r o w n .
References Aigner, T., 1985. Storm Depositional Systems: Dynamic Stratigraphy in Modern and Ancient ShaUow-Marine Sequences. Springer-Verlag, Berlin, 177 pp. Aigner, T. and Futterer, E., 1978. Kolk-Tt~pfe und -Rinnen (pot and gutter casts) lm Muschelkalk-Anzeiger Fiir Wattenmeer? Neues Jahrb. Geol. Paliiontol. Abh., 156: 285-304. Arthurton, R.S., 1980. Rhythmic sedimentary sequences in the Triassic Keuper Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geol. J., 15: 43-58. Bachmarm, G.H., 1974. Grundglpsschichten und Bochinger Horizont (Mittlerer Keuper) in Nordost-Wiirttemberg. Jahresh. Geol. Landesamtes Baden-Wtirttemb., 16: 79-96. Bachmann, G.H. and Gwinner, M.P., 1971. Algen-Stromatolithen vonder Grenze Unterer/Mittlerer Keuper (Obere Trias) bei Schw~ibisch Hall (Nordwiirttemberg, Deutschland). Neues Jahrb. Geol. Pal~iontol., Monatsh., 1971: 549-604. Beutler, G. and Schiiler, F., 1979. Ober Vorkommen salinarer Bildungen in der Trias im Norden der DDR. Z. Geol. Wiss., 7: 903-912. Brandner, R., 1984. Meeresspiegelsehwankungen und Tektonik in der Trias der NW-Tethys. Jahrb. Geol. Bundesanst., 126: 435-475. Brunner, H. and Wurm, F., 1983. Stratigraphie und M~ichtigkeiten der unteren Gipskeuper-Schichten (km 1, GrabfeldFolge) in Baden-Wiirttemberg. Jahresber. Mitt. Oberrheinischen Geol. Ver., N.F., 65: 307-344. Butler, G.P., Harris, P.M. & Kendall, C.G.St.C., 1982. Recent evaporites from the Abu Dhabi coastal flats. Soc. Econ. Paleontol. Mineral., Core Workshop, 3: 33-64.
Duchrow, H., 1984. Keuper. In: H. Klassen (Editor), Geologie des Osnabrticker Berglandes. Naturwiss. Museum, Osnabrtick, pp. 221-333. Eugster, H.P. and Hardie, L.A., 1975. Sedimentation in an ancient playa-lake complex: the Wilkins Peak Member of the Green River Formation of Wyoming. Geol. Soc. Am. Bull., 86: 319-334. Frank, M., 1930. Stratigraphie und Bildungsgeschichte des siiddeutschen Gipskeupers. Jahresber. Mitt. Oberrheinischen Geol. Ver., N.F., 19: 24-77. Geyer, O.F. and Gwinner, M.P., 1986. Geologie von BadenWtirttemberg. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart. Goodwin, P.W. and Anderson, E.J., 1985. Punctuated aggradational cycles: a general hypothesis of episodic stratigraphic accumulation. J. Geol., 93: 515-533. Gwinner, M.P., 1970. Gipskrusten im Gipskeuper bei Obersontheim (Baden-Wiirttemberg)? Neues Jahrb. Geol. Palaontol., Monatsh., 1970: 88-90. Hagdorn, H. and Simon, T., 1985. Geologie und Landschaft des Hohenloher Landes. Thorbecke Verlag, Sigmaringen, 186 pp. Handford, C.R., 1982. Sedimentology and evaporite genesis in a Holocene continental sabkha-playa basin--Bristol Dry Lake, California. Sedimentology, 29: 239-253. Hardie, L. et al., 1986. Repeated subaerial exposure of subtidal carbonate platforms, Triassic, northern Italy: evidence for high-frequency sea level oscillations on a 104 year scale. Paleoceanography, 1: 447-457. Haq, B.U., Hardenbol., J. and Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic. Science, 235: 1156-1167. Hauschke, N., 1987. Knollige und tepeeartige Strukturen-Indikatoren fiir die friihdiagenetische Bildung von Ca-Sulfaten unter Playa-Bedingungen im Unteren Gipskeuper (km 1) des Lippischen Berglandes. Neues Jahrb. Geol. Pal~iontol., Abh., 175: 147-179. Huth, R., 1956. Zur Geologie der Steinmergelb~inke, insbesondere der Bleiglanzbank, im Gipskeuper. Z. Angew. Geol., 1: 15-17. Kendall, A.C., 1984. Evaporites. Geosci. Can., Reprint Ser., 1: 259-296. Kendall, C.G.St.C., 1987. A review of the origin and setting of tepees and their associated fabrics. Sedimentology, 34: 1007-1027. Kozur, H., 1973. Biostratigraphie der Germanischen Mitteltrias. Freiberger Forschungsh. C, 280: I-III, 127 pp. Kulke, H., 1974. Zur Geologie und Mineralogie der Kalk- and Gipskrusten Algeriens. Geol. Rundsch., 63: 970-998. Langbein, R., 1987. The Zechstein sulphates: the state of the art. In: T.M. Peryt (Editor), The Zechstein Facies in Europe. Lecture Notes in Earth Sciences, Vol. 10. Springer-Verlag, Berlin, pp. 143-188. Logan, B.W., 1987. The MacLeod evaporite basin, Western Australia. Am. Assoc. Pet. Geol., Mem., 44. Logan, B. and Brown, R., 1986. Sediments of Shark Bay and MacLeod Basin, Western Australia. Field Seminar Handbook, Sedimentol. Res. Group, Dep. Geol., Univ. W. Aust., 229 pp.
25 Loucks, R.G. and Longman, M.W., 1982. Lower Cretaceous Ferry Lake Anhydrite, Fairway Field, East Texas: product of shallow-subtidal deposition. Soc. Econ. Paleontol. Mineral., Core Workshop, 3: 130-173. Orti Cabo, F., Mur, J.J.P., Geisler-Cussey, D. and Dulau, N., 1984. Evaporite sedimentation in the coastal salinas of Santa Pola (Alicante, Spain). Rev. Inv. Geol. (Barcelona), 38/39: 169-220, Richter, D.K., 1985. Die Dolomite der Evaporit- und der Dolcrete-Playasequenz im mittleren Keuper bei Coburg (NE Bayern). Neues Jahrb. Geol. Pal~iontol., Abh., 170: 87-128. Schreiber, B.C., 1978. Environments of subaqueous gypsum deposition. Soc. Econ. Paleontol. Mineral., Short Course, 4: 43-73. Schreiber, B.C., 1986. Arid shorelines and evaporites. In: G.H. Reading (Editor), Sedimentary Environments and Facies. Biackwell, London, 2rid ed., pp. 189-228. Shinn, E.A., 1983. Tidal flat. Am. Assoc. Pet. Geol., Mem., 33; 171-210. van Houten, F.B., 1987. Late Triassic cyclic sedimentation: Upper Lockatong and lower Passaic Formations (Newark Supergroup), Delaware Velley, west and central New Yersey. Geol. Soc. Am. Centennial Field Guide, Northeastern Section, pp. 81-86.
van Wagoner, J.C., Mitchum, R.M., Posamentier, H.W. and Vail, P.R., 1987. Key definitions of sequence stratigraphy. In: A.W. Bally (Editor), Atlas of Seismic Stratigraphy, Vol. 1. Am. Assoc. Pet. Geol., Stud. Geol., 27: 11-14. von Freyberg, B., 1965. Cyclen und stratigraphische Einheiten im Mittleren Keuper Nordbayerns. Geol. Bavarica, 55: 130-145. von Huene F., 1959, Simosaurus guilielmi aus dem Oberen Mittelkeuper yon Obersontheim. Palaeontographica, 113. Warren, J.K., 1982. The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia. Sedimentology, 29: 609-637. Warren, J.K., 1983. On the significance of evaporite lamination. 6th Int. Symp. on Salt, 1983, Vol. 1: Salt Institute, pp. 161-170. Warren, J.K. and Kendall, C.G.St.C., 1985. Comparison of sequences formed in marine sabkha (subaerial) and salina (subaqueous) settings--modern and ancient. Am. Assoc. Pet. Geol., Bull., 69: 1013-1023. Wilson, J.L., 1975. Carbonate Facies in Geologic History. Springer, Berlin, 471 pp. Wurster, P., i964. Delta sedimentation in the German Keuper Basin. Dev. Sedimentol., 1: 436-446. Ziegler, P.A., 1982, Geological Atlas of Western and Central Europe. Elsevier, Amsterdam, 130 pp.