Hauptdolomit (Norian) — stratigraphy, paleogeography and diagenesis

Sedimentary Geology, 32 (1982) 195--231

195

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

HAUPTDOLOMIT

(NORIAN)

-- STRATIGRAPHY,

PALEOGEOGRAPHY

AND DIAGENESIS

I. FRUTH and R. SCHERREIKS

Bayerische Staatssammlung fi~r Allgemeine und Angewandte Geologie, Munich (F.R.G.) (Received June 30, 1981, revised and accepted January 12, 1982)

ABSTRACT Fruth, I. and Scherreiks, R., 1982. Hauptdolomit (Norian) -- stratigraphy, paleogeography and diagenesis. Sediment Geol., 32: 195--231. Geochemical and carbonate petrographical methods were coordinated in facies investigations and environmental reevaluations, related to the Hauptdolomit (Hd.)= main dolmite formation (± Norian) of the Northern Calcareous Alps. It is practical to distinguish eight, environmentally controlled, facies units (1--8) and three geochemical groups (I--III). Superimposed upon the environment pattern (tidal complex, lagoonal complex, barrier bar and shoal complex) is a predictable (geochemical) dolomitization and non-carbonate distribution. The vertical and lateral facies associations, their waxing and waning in the geologic columns, allow paleogeographic reconstructions. Especially important are clayey, ±bituminous facies, commonly known as "Seefeld facies", which are interpreted to be of mainly intertidal to very shallow near-shore, rather than of deep-water, origin. Threefold stratigraphy can be substantiated and is found to be practicable for the Hd. formation in a large part of the Northern Calcareous Alps. In an attempt to explain some of the phenomena associated with dolomitization in the Hd. formation, a model of anaerobic dolomitization has been considered, outlining steps of early diagenetic dolomitization.

INTRODUCTION

Regional setting Hauptdolomit (Hd.), +Norian, of the central and southern belts of the N o r t h e r n C a l c a r e o u s A l p s is a t h i c k (+ 2 0 0 0 - - 2 5 0 0 m ) s e q u e n c e o f d o l o m i t e , some limestone ("Plattenkalk" and other calcareous intercalations) and occasional clayey-bituminous carbonates and marls. Hd. forms the greater part of t h e N o r t h e r n C a l c a r e o u s A l p s , t h i n n i n g t o w a r d s t h e n o r t h e r n m a r g i n . I t is c o m m o n l y e n v i s a g e d t o h a v e f o r m e d o n a m o r e o r less s t e a d i l y s i n k i n g p l a t form (or shelf) on the southern flank of the paleo-european landmass known as t h e V i n d e l i c i a n A r c h . F i g u r e 1 is a n i d e a l i z e d t r a v e r s e t h r o u g h t h e s e d i 0037-0738/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

196

Pclteogeologic Troverse Through the Norion of the Northern Limestone Alps .

.

.

.

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.

Hauptdolomit

end

SuprQtidO{

focies

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Houptdolomlt

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I

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o~" ~

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-

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80fr,er_ ,$(0nd$

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Fig. 1. Paleogeologic traverse through the Norian of the Northern Limestone (Calcareous) Alps, showing the facies pattern which developed in east--west and north--south directions.

mentary environments thought to have developed here (the distribution of the Dachsteinfacies is simplified after Zankl (1971); the distribution of the Hd. facies is according to interpretations made in this paper). Hd. deposition was preceded b y the more terrestrially influenced, +Karnian Raibl facies (clastic sediments, dolomites with or without evaporites and limestones) and succeeded b y the shallow less restricted marine, -+t{haetian KSssen facies (limestones and marls). Problems and aims

Until now, the paleogeography of Hd. has remained rather generalized. In addition to this, differing views have developed concerning Hd. facies distribution and interpreted environments, especially of some laminated dolomite facies (supratidal and intertidal, or supratidal through subtidal?) and clayeybituminous occurrences known as Seefeld facies (deep-water, or shallow, or tidal environment?). Advances have been made in stratigraphy, although macroscopic and microscopic obscurity are widespread with some exceptions of local clarity: Lower, Middle and Upper Hd.. In previous work (Fruth and Scherreiks, 1975), geochemical parameters were tested in sections of Upper Hd., and were found to be useful to some extent for local lithostratigraphy and facies investigations. These results gave rise to the new research presented in this paper which has the following aims: (1) To coordinate and interpret geochemical and carbonate petrographical data. (2) To investigate quantitative and qualitative aspects of Hd. facies and to establish useful facies units.

197

Main area x'y of Study ,V~

(

~MUNICH

Reconnaissance 0

lOOkm I

%

I

I

Fig. 2. Investigated region (geographical positions of study areas are refered to in detail in Figs. 4, 8 and 9).

(3) To investigate the order of succession of facies units and to establish their ideal transgressional--regressional sequence. (4) To reevaluate environmental interpretations. (5) To interpret diagenetic aspects linked with sedimentary environment, whereby a model of early diagenetic dolomitization fitted to the Hd. situation was sought and is roughly outlined. (6) To contribute towards the clarification of regional stratigraphy and paleogeography. Figure 2 shows the main study areas, which are depicted in more detail in later figures.

Laboratory methods Routine methods (in part described previously: Fruth and Scherreiks, 1975) were employed for the chemical analyses. The full analyses were performed with energy-dispersive X-ray fluorescense (EDRFA), type Tefa 6110 (EG & G Co., Ortec). As a control, some elements (e.g. Ca, Mg) were analyzed by atomic absorption methods (AAS}, type SP 1900 (Philips) and titration. X-ray diffraction was employed (PW 1730/1965, Philips)mainly for d-spacing calculations for dolomites and calcites. EVALUATION OF GEOCHEMICAL DATA

Frequency distributions and correlation coefficients for the geochemical data have helped to define quantitatively the rock modes occurring in the Hd. formation and they have disclosed some useful relationships between calcite, dolomite and carbonate versus non-carbonate content. In the following discussion, " c a r b o n a t e " and "non-carbonate" are loosely defined as: carbonate = CaO + MgO + ignition loss = calcite + dolomite (mineral); non-

198 TABLE I Geochemical EDRFA data for 270 Hd. samples. The averages and two standard deviations are listed. In cases of positive or negative skewness, log. or reciprocal log. values were used in normal distribution calculations. The samples are grouped according to the two CaO or respectively MgO modes that are developed in frequency distribution patterns Analytical parameters (wt.%)

A Dolomite to slightly calcitic dolomite C a / M g ~ 4 ; n = 230

B Calcitic dolomite to limestone Ca/Mg~4;n=40

Carbonate

~

+_ 2s

~

+ 2s

Ignition loss MgO CaO

46.32 19.24 31.75

43.3 15.8 26.4

42.44 2.95 48.78

39.3 0.7 42.5

Carbonate sum

97.31

--49.3 --23.3 --37.1

--45.3 --12.1 --55.0

94.17

Non-carbonate

SiO2 A1203 K20 Fe203 MnO Na20 P2Os TiO2

0.56 0.24 0.095 0.19 0.054 0.075 0.034 0.042

0.05 -0.03 -0.03 -0.02 -0.02 -0.00x-0.02 -0.02 --

Non-carbonate sum

1.29

0.2

Ca/Mg (from individual CaO and MgO data)

1.94

1.45 -- 2.6

carbonate = the T a b l e I. Carbonate

6.2 2.0 0.3 1.5 0.1 0.7 0.05 0.08

-- 8.4

2.59 0.85 0.097 0.34 0.086 0.047 0.078 0.067

0.4 --17.0 0.2 -- 4.0 0.01 -- 0.7 0.03 -- 3.6 0.01 -- 0.5 0.00x-- 0.9 0.06 -- 1.0 0.037-- 0.12

4.15

0.75 --21.3

19.3

4.17 --89.2

s u m o f t h e a n a l y t i c a l d a t a i n d i c a t e d as n o n - c a r b o n a t e in

content

B i m o d a l i t y is e s p e c i a l l y e v i d e n t in t h e f r e q u e n c y d i s t r i b u t i o n s o f C a O a n d MgO, r e f l e c t i n g t h e t w o m o d a l t y p e s " d o l o m i t e " a n d " l i m e s t o n e " . O f t h e 270 samples a n a l y z e d , 2 3 0 f o r m a d i s t i n c t m a x i m u m in the c a r b o n a t e c a t e g o r y - r a n g e " d o l o m i t e t o s l i g h t l y c a l c i t i c d o l o m i t e " ( C a / M g < 4) a n d 4 0 s a m p l e s f o r m a c o n c e n t r a t i o n in t h e " c a l c i t i c d o l o m i t e t o l i m e s t o n e " r a n g e ( C a / M g > 4). T a b l e I lists t h e a v e r a g e s a n d t h e t w o - s t a n d a r d - d e v i a t i o n r a n g e s o f t h e individual EDRFA-data for the samples corresponding to these modes. Data o n c a l c i t e a n d d o l o m i t e m i n e r a l c o n t e n t s a n d C a / M g r a t i o s are l i s t e d i n T a b l e II. T h e a v e r a g e s a n d r a n g e s o f t h e s e r e s u l t s are s h o w n , w h e r e b y t h e d o l o m i t i c m o d e o f T a b l e I, c o l u m n A, is s u b d i v i d e d i n t o a d o l o m i t e a n d s l i g h t l y calc i t i c d o l o m i t e g r o u p ( c o l u m n A~ a n d A2).

199 TABLE II Calculated calcite and dolomite (mineral) contents from AAS, titration and X-ray diffraction data Parameters

Calcite Wt.% complete range Wt.% average d-spacing range

A1 Dolomite

A2 Slightly calcitic dolomite

Ca/Mg < 2, n = 180

Ca/Mg < 4, n = 50

B Calcitic dolomite to limestone Ca/Mg > 4, n = 40

0.03-34.5 5.62 3.021--3.035

31.1-96.8 76.8 3.027--3.034

3.0308 98.3/1.69 5.54/0.08 96.2

3.0305 98.2/1.79 75.6/1.16 90.6

74.7--99.9 97.2 2.885--2.901 2.8895 51.3/48.7 54.0/43.2 1.74

59.2--98.8 92.0 2.885--2.905 2.8954 53.3/46.7 52.9/39.1 1.88

2.10--58.3 18.0 2.885--2.906 2.9020 55.5]44.5 10.7/7.3 2.05

97.2

97.6

94.8

1.74

2.07

14.2

(calcite below detection level)

d-spacing average CaCO3/MgCO3 molar ratio CaCO3/MgCO3 wt. ratio Ca/Mg ratio Dolomite Wt.% complete range Wt.% average d-spacing range d-spacing average CaCO3/MgCO3 molar ratio CaCO3/MgCO3 wt. ratio Ca/Mg ratio

Carbonate sum (Calcite + dolomite) Total Ca/Mg ratio

Dolomite D o l o m i t e ( d o l o s t o n e ) to slightly calcitic d o l o m i t e comprises an e s t i m a t e d 80 to 90 vol.% o f the studied Hd. c o l u m n s . However, t h e mineral d o l o m i t e was f o u n d as a c o n s t i t u e n t in all the a n a l y z e d samples, o c c u r r i n g in a m o u n t s ranging f r o m a b o u t 2 to over 99 wt.%. Thus, the studied l i m e s t o n e s all conrain at least s o m e d o l o m i t e , whereas the m a j o r i t y o f the d o l o m i t i c r o c k s conrain n o d e t e c t a b l e calcite (Table II). This a p p a r e n t o m n i p r e s e n c e o f d o l o m i t e in the Hd. f o r m a t i o n shows t h a t the d o l o m i t i z a t i o n process, in w h a t e v e r w a y it f u n c t i o n s , a f f e c t e d all Hd. facies, s o m e o n l y slightly o t h e r s t o t a l l y . T h e e x t e n t o f d o l o m i t i z a t i o n was also investigated f r o m the p o i n t o f view o f C a C O 3 : M g C O 3 m o l - r a t i o - c o m p o s i t i o n of the d o l o m i t e mineral f r o m X-ray d i f f r a c t i o n analyses. D o l o m i t e d-spacings were f o u n d to range f r o m a p p r o x i m a t e l y the ideal value (2.885) to a b o u t 2.906, w h i c h c o r r e s p o n d s to a CaCO3 : MgCO3 mol-ratio-range o f a b o u t 50 : 50 t o 58 : 42 (see e.g. Goldsmith and Graf, 1 9 5 8 ; F f i c h t b a u e r and G o l d s c h m i d t , 1 9 6 5 ; L i p p m a n n , 1 9 7 3 ,

200

for m e t h o d and error sources). Total dolomite (wt.%) was found to correlate significantly and negatively with d-spacing of dolomite (x = d-spacing, y = wt.% dolomite, r = --0.6946, n = 223, t = 14.35, confidence level over 99.9%, lineare regression: y = 9450.36 -- 3237.76x). The inverse relationship with calcite (wt.%) is similarly significant (x = d-spacing of dolomite, y = wt.% calcite, r = 0.4674, n = 80, t = 4.69, confidence level over 99.9%, linear regression: y = 9223.42 + 3193.68x). This statistically predictable relationship, observed previously (Ffichtbauer and Goldschmidt, 1965) in the Zechstein and Upper Jurassic, reveals that Caexcess in dolomite is directly proportional to wt.% of calcite and inversely proportional to wt.% of dolomite. Fiichtbauer and Goldschmidt (1965) interpret the concept of early diagenetic dolomitization of calcite to be substantiated by such observations. We are reminded in this connection of the step-by-step dolomitization of calcite described by Alderman (1965) and Von der Botch (1965) in Australian occurrences of Recent dolomite. Workers on the Zechstein and Keuper formations respectively, (F/ichtbauer and Goldschmidt, 1965; Marschner, 1968) interpret the relative shifts of CaCO3 : MgCO3 mol-ratios in dolomite as indicators of fluctuating hypersalinity, considered to be linked with early diagenetic dolomitization. In later chapters we show that the Hd. e n v i r o n m e n t was probably not especially hypersaline, implying that this condition, alone, is n o t the only factor that may have led to early diagenetic dolomitization.

Similarly, our observations show that Ca-excess in dolomite is also environmentally controlled: Ca-excess increases and total dolomite content decreases towards the subtidal zone. Such control substantiates previously made interpretations (Scherreiks, 1971; Fruth and Scherreiks, 1975) that marine transgressions and regressions coincide with calcite/dolomite distribution in Hd. and that this distribution is important in stratigraphy and paleogeography.

Calcite Calcite is a minor to major mineral constituent in only about 30% of the analysed samples (0.5--97 wt.%). The occurrences of calcareous rocks in the Hd. formation are highly variable from column to column. Only an estimated 10 to 20 vol.% of total Hd. has a Ca/Mg ratio greater than 4. The results of the X-ray diffraction analyses show calcite d-spacings ranging from above 3.021 to 3.035, corresponding to MgCO3-excess of from 0.3 to not quite 5 mol %. Inasmuch as the contents of Fe203 and MnO are considerably lower than MgO, and no significant relationship could be determined between these parameters and the d-spacing of calcite, it was assumed in calculations concerning wt.% calcite (Table II) that mainly Mg-excess causes the observed d-spacing shifts [Lippmann (1973) summarizes aspects of this t h e m e ].

201

Non-carbonate content

Non-carbonate content averages about 1% in the dolomitic mode and about 4% in the calcareous mode, although the observed range is similar in both modes (Table I). Interrelationships of some trace elements and their relations to insoluble residue contents have been considered previously (Fruth and Scherreiks, 1975). In accordance with E D R F A data, the non-carbonate minerals, recognized in Hd. with the help of X-ray diffraction methods and in thin section studies (see also Miiller-Jungbluth, 1971), are quartz, possibly muscovite, illite (occasionally pyrite and celestite). A significant, but non-linear, relationship exists between Ca/Mg ratio and non-carbonate content (Fig. 3) : samples having relatively high non-carbonate content are more frequently encountered amongst intermediary mixed carbonate groups than amongst either "pure" dolomite or "pure" limestone. This substantiates similar, but inconclusive, previous observations (Fruth and

Distribution

of Non-Carbonate

Relative

Total

IO0

i I

L

i

[1

Groups

n=331 I

--

f3.

Contents

to C a / M g - R a t i o

I -

I

non- carb

Ill

--

o L

groups

l

( 2 wt%

t~

)2-4

g o

u i c

50-

)4-8

7 " ~ ) B-10

E

g

~)10 0

~o Z

-I

i~ 3~ 6~ ,28

o o IO

~_r_~.~_l.~_~

I~

,_~t ,~ ~O

o

0 I~,~Ol ~ 0

i-~ E

E

I

E,'F EF'= °~

Fig. 3. N o n - c a r b o n a t e distribution relative to Ca/Mg ratio groups. Partially d o l o m i t i z e d rocks contain above average non-carbonate more o f t e n than rocks having a Ca/Mg ratio b e l o w a b o u t 4 or above about 64. The g e o c h e m i c a l groups (I, II, III) c o i n c i d e with interpreted e n v i r o n m e n t s in a predictable w a y , s u m m a r i z e d in Fig. 5.

202 Scherreiks, 1975). Correlation coefficients of the linear relationship of wt.% dolomite or calcite and non-carbonate give rise to insecure and conflicting r-values. The distribution of non-carbonate (Fig. 3) is interpreted to reveal three paleogeographic--geochemical groups: (I) An environment conducive for dolomitization but not for non-carbonate accumulation. (II) An environment in which the extent of dolomitization is very variable, associated with highest non-carbonate accumulations. (III) An environment neither favourable for dolomitization nor for non-carbonate deposition. The stratigraphic distributions of calcite, dolomite and non-carbonate are shown in Fig. 4, and will be referred to in later sections. Paleogeographicgeochemical groups I, II and III are depicted in Fig. 5 where they are coordinated with carbonate petrographic data. LITHOFACIES AND ENVIRONMENT The main lithologies of Hd. had already been differentiated in 1861 in a comprehensive work on the Bavarian Alps by Giimbel (1861), and they have maintained their significance, especially in mapping, for later workers (Hradil and Falser, 1930; Trusheim, 1930; Ampferer, 1932; Sander, 1936) and even up to the present: (a) Dolomite ( " H a u p t d o l o m i t " ) comprises the main mass of the Hd. formation. (b) "Plattenkalk" (platy limestone), an often fossiliferous sequence of very variable thickness (0--200 m), occurring toward the top of the Hd. formation, and transitional to the Rhaetian KSssen formation. ("Plattenkalk" is considered in this paper to be a part of the Hd. formation and not a seperate stratigraphical entity). (c) Various calcareous intercalations in the dolomite sequences (a). (d) Dolomitic through calcareous clayey intercalations in the dolomitic facies, which are often strikingly bituminous ((~lschiefer, Asphaltschiefer, + Seefeld facies). (e) Sedimentary and tectonic breccia. Contrary to older literature in which Hd. is described as totally subtidal in origin (e.g. Trusheim, 1930; Sander, 1936), modern carbonate petrographical work of the last 15 years depicts the depositional environment of Hd. lithofacies as lagoonal, intertidal and supratidal. Decisive for this conceptual change of environment were reports on analogous Recent carbonate depositional environments and the work by Fischer (1965) in the contemporary Dachstein formation and Bosellini (1967) in the Southern Alps. In carbonate petrographical investigations, Miiller-Jungbluth (1968, 1971) describes intertidal and supratidal environments for much of the Hd. in the Northern Calcareous Alps; these findings were supported by Zankl (1967, 1971) and later by Scherreiks (1971). With consideration of geochemical parameters, Fruth

203 and Scherreiks investigated lateral and vertical transitions of subtidal to intertidal to supratidal facies. Much still remains unclear, however, and a number of decisive, conflicting views concerning facies distributions and environments have developed. For example, Zankl (1971) interpretes some of the completely dolomitized facies as subtidal or lagoonal, whereas Scherreiks (1971), and Fruth and Scherreiks interpret a stricter association of early diagenetic dolomitization with the supratidal and intertidal zone. The clayey-bituminous, usually highly dolomitic, occurrences known as Seefeld facies have been interpreted as deep-water environments, of about 100 m depth (Trusheim, 1930; Sander, 1936; Mfller-Jungbluth, 1971), whereas other workers consider them to be of shallow, near-shore to intertidal origin (Hradil and Falser, 1 9 3 0 - near shore -- Zankl, 1971 -- euxinic pools). Eight facies units have been distinguished according to carbonate petrographical features. They were numbered in an order which corresponds to their vertical and lateral association with one another (Walther's Principle: Walther, 1893). Unit 1 is interpreted as the highest tidal, farthest onshore Hd. facies and unit 8 as the farthest offshore Hd. facies. Major geochemical data are coordinated with the facies units in the proposed sedimentary model in Fig. 5.

Facies unit 1 :non-laminate, sparry (intraclast) dolomite mudstone Description Often massive or very thick irregularly bedded (>1 m), light brownishgrey to dark grey dololutite without apparent laminations. Dolospar fabrics range from small discrete patches, like Fischer's (1965) "shrinkage pores", but not distinctly laminate, to more or less complete, sucrose grain-growth mosaics (dolosparites). Some dolospar fabrics are not unlike "knife-stab" structures (Milller-Jungbluth, 1971), interpreted as relics of gypsum or celesrite which are n o w replaced by dolomite. However, neither gypsum nor celestite could be directly verified by us in any of the dolomitic facies. Characteristic for this unit are scattered intraclasts and intraclast layers, often developing from still more or less attached sheet-crack fabrics to discontinuous flat pebble laminae. Environmental interpretation These deposits are interpreted to be supratidal, partly reworked and severely diagenetically altered algal mats (unit 2 below) and intertidal muds (unit 3 below). As such they grade into algal-mat and tidal mud-flat facies {interpreted below). The non-laminate character of unit 1 may be due in part to the growth of gypsum (Reineck and Singh, 1975). However, the lack of preserved evaporites, with the exception of some possible pseudomorphs, contradicts a supratidal interpretation, a problem considered previously {Fruth and Scherreiks, 1975): gypsum and other more soluble salts may have

204 been dissolved at approximately the same rate of formation by incoming tides and occasional rainfall. In addition to dissolution, sulfates may have been decomposed by the activity of sulfate-reducing bacteria, which are capable of being active (halophites) in nutrient solutions containing from 6 to 30% salt (ZoBell, 1958). Thus, below the more or less desiccated supratidal surface, a wet intergranular microenvironment can be envisaged where anaerobic decomposition of fine organic detritus and reduction of (possible) sulfate minerals may have taken place (see also section on dolomitization). Nevertheless, the fact remains that Hd. does not contain CaSO4 at present, whereas many dolomitic formations are known to be associated with CaSO4, which is even the case for the Raibl formation, stratigraphically below the Hd. formation. The evaporite facies of the Raibl formation may indicate a more extreme, landward, supratidal environment, which in the case of the Hd. formation is n o t exposed and may be tectonically covered. Facies unit 2 : stromatolitic dolomite Description Hd. stromatolites are composed of dark grey to almost black, fetid, undulatory to fine-crenulated, even parallel to non-parallel dololutite laminae, which alternate with middle grey dololutite on a millimeter to centimeter scale. The latter dololutite may contain a few ostracods and intraclasts. Bedding is medium to thick (> 50 cm). Using a modified classification scheme, proposed by Logan et al. (1964), Hd. stromatolites may be described with the formula:

PLF ~ - - - ~ ~ LLH-s LLH-s< . . . . + LLH-c e L F (after Miiller~Jungbluth, 1971) is used to signify the -+parallel-laminated, finely crenulated, stromatolitic type. It dominates over spaced, laterally linked, hemispheroidal structure (LLH-s). Microstructurally, spaced laterally linked hemispheres (LLH-s) can grade into crenulated close laterally linked hemispheres (LLH-c). Shrinkage pores, sheet-cracks and beddingparallel burrows, containing internal sediment and diagenetic dolomite cement, are locally well developed features. Environmental interpretation An algal mat environment is indicated by stromatolitic fabrics. Blue-green algae flourish in the near-high-tide zone (Gebelein, 1977), trapping and binding incoming carbonate mud and organic detritus, forming extensive, thin slimy, rubbery mats. Aiding their development and later preservation are the longer periods of drying at this tidal level which are unfavourable for significant populations of bioturbating sediment feeders (contrary to facies unit 3). The LLH structures are characteristic for protected intertidal mud flats (Logan et al., 1964).

vert,cal

1

Kratzer-

50

Laminated,

B

clayey

supratldal

El

10

5

bloclast

(Unltl)

Dolomite-

dolomite

w

0:5%

m

-----

---_ ,...

(Unit

L) @

Stromatolite

for

Clayey

(Unit

-

-

on

f bltumlnous

D

0.5

/I

3-L

3

(Unit

w

dolomite

5)

Scale

-3.1

Hochalpl

Karwendel

Bedded

log.

-

..-‘-_...::::.‘lr6

...

Figures

----_--6_

2)

-content

. . ..._._,,.,

---_

Legend

....

-----

-i

100 50

Groual

Noncarbonate

. ... . .

(Lorea

Alps

(Algal)

.

Schonblchl K

Lechtal

j %

.E

Calcareous

mudstone

3)

mudstone

(Unit

’

to

packstone

Z

2

0

a

0

E 0 -

(Unit

Fecal

(mostly

6 -7)

(mostly

‘.

:

comes lamelllbranchs)

Natlca

_’

:’

Amman)

:

(log. scale) and corresponding forming the boundary MHd./

(millolids)

lamelllbranchs

pellets

Foramlnlfera

Ostracoda

Blomorph

Bioclasts

Gastropoda

.cr-44rI

(Rohntal/HlnterrlO)

Fig. 4. Geologic section through Hauptdolomit in the Lechtal and Karwendel Alps. The sections show the distribution of calcite, dolomite and non-carbonate facies units (described in text). Natica facies in the eastern Lechtal Alps indicate an extensive lagoonal encroachment into intertidal and supratidal environments, UHd. They are laterally associated with clayey, bituminous facies.

Higher

m

Calclte-

1

tal

Scale

Eastern

(Heiterwand-Salvesental)

Lsoom

-0

Salvesen

I

205-201

Environments

Carbonate

model for Hauptdolomit. Hd. investigations.

Petrographic/GeochemicaI

Fig. 5. Schematic carbonate petrographic/geochemical as a guide and key in stratigraphic and paleogeographic

Jnterpreted

Schematic

208-210

sequence

is shown

of Hauptdolomit

An ideal transgressional

Model

with corresponding

Li thofacies

environments

Units

and geochemical

Geochemical

groups.

This scheme

Groups

may be used

/-

211 Contrasting to this is Dachstein stromatotite (observations in Lofer area): the domal structures (LLH-s) are larger and better developed, grading laterally and vertically into stacked hemispheroids of both c- and v-modes (LLH-~ SH), which are totally lacking in Hd. (see also Miiller-Jungbluth, 1971). Sander (1936) shows excellent photos of SH types in Dachstein facies. SH structures are characteristic for exposed intertidal headlands (Logan et al., 1964). This contrast is important for paleogeographic interpretations and reconstructions.

Diurnal and longer periods of drying, especially for algal mats well above the high-tide level, are responsible for various shrinkage structures and desiccation. These semidurable surface crusts can be torn up, producing scattered clasts or intraclast-laden laminae (hydraulically similar occurrences in the Alpine Ladinian are described by German, 1969). The subsurface intergranular environment is considered to have turned anaerobic, due to high organic content (algal mats), also indicated by the dark colors and fetid odors. Facies unit 3 : homogenic to intermittently laminated dolomite mudstone (dololutite) Description Thick to medium bedded (>1 m - - > 1 0 cm) brownish-grey to dark grey, usually fetid, mostly homogenic mudstone to wackestone. Seldomly occurring are wavy silt-in-mud or mud-in-mud laminations. Color bands, diffuse pyrite and cloudy or mottled patches (bioturbation) are main features in the otherwise prevalent homogenic dololutite groundmass. Isolated arenitic bioclasts, pellets, intraclasts and silt-grains are u n c o m m o n (wackestone}. Dolosparfilled vugs and grain-growth fabrics are not as c o m m o n as in facies units 1 and 2. This facies unit forms the thickest and laterally the most extensive sequences. Environmental interpretation This most important, Hd. facies is interpreted to have been a very broad intertidal mud flat (Mfiller-Jungbluth, 1971, on the contrary, interprets the predominant Hd. facies to be supratidal). The fine-grained textures indicate a mudflat type, which must have been protected from strong currents and wave action, where sedimentation rate was slow, allowing bioturbating invertebrates, working in the upper few millimeters to centimeters, enough time to destroy most laminations (Reineck and Singh, 1975). A high percentage of fine organic detritus is supposed to have nourished this population of bioturbating sediment feeders, similarly to Recent tidal environments. Below this uppermost zone, reducing conditions may be expected to have evolved as a result of oxygen depletion by aerobic organic decomposition. The anaerobic zone is considered, by the present authors, to have been especially important for the diagenetic development of Hd. (see section on dolomitization).

212 Facies unit 4 :laminated clayey dolomite mudstone/wachestone/grainstone Description Strikingly fetid {bituminous), dark brown to black, medium to thin beds {<30 cm), composed of very thin ( 3 - ~ 1 mm), even-parallel to non-parallel laminae (thinly interlayered sand and mud, called "tidal bedding" after Reineck and Singh, 1975). Individual laminae contain arenitic accumulations of pellets, ostracods, few oncoids and silty to fine arenitic bioclast detritus. Dololutite matrix dominates over dolospar-bound allochems. Thin clay and coal partings are characteristic of the transition to facies unit 5. Environmental interpretation A characteristic of Recent intertidal areas is the transition from muddy, bioturbated, non-laminate intertidal flats (unit 3) into sandy, laminated intertidal flats, towards low-tide-level (Reineck and Singh, 1975). Bioturbation is negligible due to relatively high depositional rates. This zone, known as the mixed intertidal flat and sand flat is interpreted to be represented in Hd. by laminated fine arenitic muds of facies unit 4. The dominant fine arenitic allochems are ostracods and clayey (!) pellets (see unit 5), and indefinable (partly silt size) bioclasts. These facies grade into sandy and m u d d y channel and m u d d y pond sediments. Facies unit 5 : dolomitic to calcareous, bituminous marls Description This fetid and highly bituminous {15--20%) Hd. facies represents the onceproductive part of the Seefeld facies (Hradil and Falser, 1930). It is a black laminated marl (often referred to as shale). The non-carbonate content ranges from a b o u t 20 to 80%. The carbonate portion consists of a mixture of predominantly dolomite and variable amounts of calcite. Pellets, often squashed b e y o n d recognition, and ostracods are prevalent allochems. This facies is also well known for its fossil fish and plant remains (Trusheim, 1930). Environmental interpretation The clayey, bituminous Hd. facies are interpreted as intertidal and nearshore subtidal accumulations of pools, channels, tidal deltas, with occasional extensions into shallow lagoonal areas, and perhaps also into mangrove-swamp-like areas or marsh, suggested b y plant remains and coal partings. Inasmuch as suspended clay cannot appreciably settle out of even slightly agitated waters, the high clay contents of facies unit 5 appear to contradict this interpretation. We base our interpretation mainly on observations made by Pryor (1975) on clay accumulations in the tidal areas and shallow near-shore zone of southern East Coast and Gulf Coast of the U.S.A. He discusses mechanisms, which may also have been effective in Hd. environments,

213

such as evaporation, downward drainage from ponds and enhancement of clay flocculation due to the mixing of higher and lower saline waters. The latter mechanism can be envisaged to have had some effect on clay flocculation, in case of Hd., where suspended-clay-bearing near-shore waters encountered higher saline supra- and intertidal run-off. Most significant, however, are Pryor's (1975) observations on filter-feeding organisms and their capacity of producing astonishing accumulations of argillaceous fecal pellets. The pellets contain up to 90% clay minerals and 10% organic matter. The average rates of a c c u m u l a t i o n are given as 12 metric tons (dry wt.) of pelleted m u d per km 2 per year, a p p r o x i m a t e l y equal to accretions of 4.5 m m / y r . The pellets are fairly resistant, behave hydraulically like sand-grains and form layers ranging from 5 to 30 cm thickness and, locally, intertidal delta masses up to 60 cm thick.

Many filter feeders are tidal-zone inhabitants. For example, the decapod Callianassa is highly populous in protected tidal pools and shore zones. Callianassa populations can filter and pelletize over 600 metric tons of suspended solids per km 2 each year. Some of the fecal pellets observed in Hd. are strikingly similar to those of Callianassa (see Plate I). To be seen are haemorrhodilates in longitudinal and transverse view, almost identical to those depicted in Pryor {1975, fig. 5). PLATE I

_ ~e.4 ~4

C

Fecal pellets (aff Callianassa): bar equals 1 ram. Fecal pellets of filter-feeding invertibrates contain significant c o n t e n t s of clay minerals and organic m a t t e r and thus such pellet accumulations can produce clayey-bituminous facies in relatively high energy environments where suspended clay otherwise c a n n o t settle.

214 Facies unit 5, but also 4 and 6, often contain pelletal laminae or lenses. Pellet accumulations are often squeezed together very tightly, a typical feature of compaction observed also by Pryor (1975). The environment of facies unit 5 is not considered by the present authors to have been euxinic above the sediment/water interface, although below this surface anaerobic conditions undoubtedly prevailed. The sedimentation rate and tidal currents are considered to have been relatively high, hindering surface euxinic conditions, but affecting the accumulations of pelletal sand and ostracod lags together with the occasional burial of fish and plants. Interpreted in this way, facies unit 5 represents facies lenses in an enviromental belt that mainly was near-shore and intertidal, and thus is a guide and key for reconstructing Hd. paleogeography and stratigraphic development. Facies unit 6 .' slightly clayey, calcareous to slightly dolomitic m u d s t o n e / wackestone and packstone Description Dark grey to black, fetid, medium to thin ( < 3 0 cm) beds. Medium to thick laminae (3--30 mm). Pellets, ostracods and arenitic bioclasts (packstone) are interlaminated with wackestone and mudstone laminae (tidal bedding). This unit is only partially dolomitized (calcareous dolomite to dolomitic linestone). Especially striking intercalations contain mass-accumulations of fine rudaceous gastropods (mostly Natica comes Ammon) in calcilutite to calcisiltite matrix (Natica facies: Scherreiks, 1967, 1971). Selective celestite replacement of originally aragonite shell substance occurred in some lenses (Scherreiks, 1970} and represents a special diagenetic environment (see section diagenesis). Environmental interpretation Subtidal, current-bedded, channel lags and near-shore low relief mounds are interpreted to be represented by facies unit 6. It is transitional mostly between intertidal and near-shore facies (unit 4, 5) and off-shore m u d d y lagoonal facies (unit 7). Facies unit 7:calcareous mudstone and wackestone Description Dark grey to black, fetid, thick to thin-bedded (50--10 cm) mudstone and wackestone. Sporadic, discontinuous, even parallel laminae ( < 1 0 mm) seldom occur in this mostly homogenic calcilutite. Bioturbation is common. Scattered pyrite occurs. Dolomite content is usually small (< 5%). Environmental interpretation Facies unit 7 is comparable to Recent m u d d y lagoon sediments, which are typically and extensively bioturba'ted (Reineck and Singh, 1975; Gebelein,

215 1977). The intergranular microenvironment of the Hd. lagoonal muds probably was anaerobic, considering the dark colors, fetid odor and occasional pyrite grains, but this anaerobic zone must have set in below the zone of bioturbation.

Facies unit 8 : calcareous packstone and grainstone with rudaceous lenses Description Dark to light grey, sometimes slightly fetid, medium to thin bedding ( < 3 0 cm). Medium to thin (<10 mm), even and nonparallel laminae. Bioclast packstone and grainstone (fine rudaceous calcibiosparite lenses), in which pelecypods dominate over other bioclasts, alternate with silty wackestone. Oolitic grainstones and packstones are diagnostically important intercalations. Oncoid lenses occur also. Environmental interpretation The facies types indicate higher energy environments, supposed to represent barrier beaches, mounds and shoals. These arenitic and rudaceous accumulations sheltered the lower energy environments of the Hd. lagoon and tidal zone from most wave and strong current action. It was, for the most part, a well-oxygenated sediment. The frequent occurrence of sparites (grainstones) is considered to indicate that cementation was favoured by occasional (perhaps diurnal) subaerial exposure as low barrier beaches, influenced now and then by meteoric water. Other facies types Slump breccia (indicated in Fig. 5) Massive to poorly bedded dolomite breccia with more or less chaotically distributed to subangular components (0 1 mm--10 cm) in dololutite matrix and/or sparry dolomite cement. The matrix is characteristically darker than components, which are mostly of unit 1, 2 and/or 3 and intercalated in facies unit 3 or 4. Breccias also occur in facies unit 5, in which case they are composed of fragments of units 4 to 5 in a dark, -+clayey dololutite matrix. Simple slump structures have been observed in units 2 and 3. The environment is interpreted as tidal channel to tidal deltaic. Conglomerate The few observations of dolomitic/calcareous conglomerates contain rounded to subangular dololutite and calcilutite fragments (¢ 1 mm--1 cm) in dolomitic-calcareous groundmass. The environment is supposed to be near-shore, indicating longer periods of working and possibly transgressive phases.

216

Fault breccia and mylonite Tectonic breccia, permeated by small faults and associated with larger, often bedding-parallel faults, is very c o m m o n in the dolomite sequences. Although rotated or displaced fragments occur, the tectonic breccias mainly give a shattered appearance (crush-breccia). The rock crumbles readily in small polygonal fragments as most fractures are hardly healed. Sparry dolomite sometimes occurs as a joint and void filler, which, in this case, must be considered to be late diagenetic in origin. Mylonite is an arenitic and usually friable breccia variety associated with directly visable larger faults. It is light grey to almost white, often with pink tones (MnCO3?). DIAGENESIS

Cementation Grainstones (or sparites) sometimes form thin laminations in units 4 and 6 (pelletat ostracodsparite), b u t they are mainly associated with and typical for unit 8 (bioclast sparite and oosparite). The cement-filled pores consist of an outer lining of short fibrous cement A and an internal mosaic of only a few larger calcite crystals, representing cement B. In some oolite grainstone, cement A is either extremely thin or only intermittently developed, in which case there is instead a mosaic of numerous small calcite crystals. This may indicate periodic subaerial exposure (Ffichtbauer and MSller, 1970). On the other hand, grain growth appears to have destroyed especially cement-A fabrics in some cases. A significant part of stromatolite dolomites, dolointrasparites and very fine pore spaces of dololutite facies consist of sparry dolomite, considered to have been initially largely CaCO3-cement.

Grain-growth fabrics Grain-growth features appear to be associated especially with facies having low clay contents. Units 1, 2 and 3 and the grainstones (sparites) of unit 8 are mainly affected. Similar observations, where clay contents above about 2% hinders grain growth, have been described previously (various observers discussed in Ffichtbauer and Mfiller, 1970).

Dolomitization The observations on the composition and distribution of dolomite (rock and mineral) in Hd. facies have led the present authors to consider possible physicochemical conditions responsible not only for complete dolomitization of supra- and intertidal, but also for partial dolomitization of subtidal facies. Hypersalinity, which probably was not e x t r e m e - because of almost complete lack of evaporites - - , appfars to be only one of a number of pos-

217

sible factors which may have influenced early diagenetic dolomitization. A dolomitization model fitted to the Hd. situation is envisaged, taking into consideration an "organic dolomitization" concept proposed by Lippmann (1973). In the following preliminary conception carbon-isotope analyses have been omitted although in future work they may bring evidence for or against an organic dolomitization model. Lippmann (1973, pp. 185--186) suggests that proof or disproof of organic vs. anorganic origin of carbonates is not necessarily disclosed by 12C/13 C ratios, because original composition may be changed in the course of diagenesis (the work of Gould and Smith, 1979, is also especially interesting in this respect).

Anaerobic dolomitization

Dolomitization is proposed to be caused by physicochemical changes produced by anaerobia which are known to promote significant fluctuations in Mg/Ca ratio, pH, carbonate alkalinity (ZoBell, 1958; Abd-E1-Malek and Rizk, 1963; Lippmann, 1973). The dolomitization process supposedly functions in two main intervals: (1) Anaerobia become active in interstitial solutions at a depth perhaps of only a few millimeters, where aerobic decomposition of organic matter has depleted oxygen. Anaerobic decomposition which is largely, but not solely, linked with sulfate reduction, increases carbonate alkalinity. This results in aragonite precipitation, relative Ca 2÷ depletion and increased Mg/Ca ratio. Mg-calcite formation supposedly is enhanced by high carbonate alkalinity, one of the important factors in breaking hydration envelopes, normally hindering Mg2÷ incorporation in precipitating calcite (Lippmann, 1973). Mg-calcite in turn supposedly is transformed, in many dissolution-precipitation steps, into unordered dolomite phases (Glover and Sippel, 1967; Alderman, 1965; Von der Borch, 1965). A supporting mechanism may be the formation of Mg-complexes (Garrels and Thompson, 1962) and hydrous magnesium gels or compounds such as hydromagnesite (Von der Borch, 1965), playing the role of dolomite precursors or temporary magnesium reservoirs (Lippmann, 1973). (2) A number of conditions probably should develop intermittently which temporarily are not conducive for anaerobic metabolism (ZoBell, 1958), such as build-up of toxic metabolic products, SO~÷ depletion and lowered Mg2÷-ions, causing a second interval of relative anaerobic dormancy. Endoand exodiffusion, working towards the equalization of ionic concentration gradients (Engelhardt, 1960), should then tend to "normalize" interstitial conditions by supplying depleted ions and exporting toxic metabolites. Permeability in the mostly very fine-grained Hd. sediments was most likely relatively low and flow-rates must have decreased with burial, cementation and compaction. Nevertheless, mechanisms such as "capillary concentration" (Friedman and Sander, 1967), "seepage refluxion" (King, 1947; Adams and Rhodes, 1960) or "evaporative pumping: (Hsii and Siegenthaler, 1969) may also have played some role, in addition to diffusion, in balancing pore-space concentrations.

218

With gradual alkalinity and pH normalization, and Mg 2÷, Ca 2+, SO~÷ etc. replenishment, interval 1 should then repeat again followed by interval 2 and so forth as long as utilizable carbon for anaerobic activity can be supplied. Interval I is mainly a precipitation phase, interval 2 one of dissolution, b u t generally high alkalinity supposedly promotes the incorporation and holding of magnesium in hydrous and anhydrous carbonate compounds, or dolomite precursors, eventually leading to complete dolomitization. Very little dolomitization t o o k place in the Hd. lagoonal environment, whereas dolomitization increases to 100% in facies representing the intertidal and supratidal zone. Two reasons which are linked with one another are suggested to have caused this dolomite distribution: high organic content and increased salinity, when they occur together, are the optimal conditions for dolomitization. Occurring separately, or in unbalanced proportions, dolomitization is only partial. This could explain the only very slight dolomitization of lagoonal mud facies which probably did contain some organic matter b u t most likely lacked high enough pore space salinities, whereas the tidal sediments contained higher salinities as well as high organic contents. Bitumen, being an important aspect of Hd. and being especially enriched in Seefeld facies, may indicate that there was a general organic-matter surplus b e y o n d the amount necessary for anaerobic dolimitization. Anaerobic dolomite may be a quantitatively calculable by-product of bitumen formation or vice versa.

Organic carbon requirements of anaerobic dolomitization Lippmann (1973) calculates that 13 g organic carbon, from anaerobic decomposition of organic matter, are theoretically necessary for 100 g of dolomite, being the amount of carbon in CaMg(CO3)2. This calculation applies to the reaction: Ca 2÷ + Mg 2÷ + 2CO~- {organic) -* CaMg(CO3)2. Actually, however, only 6.5 g of anaerobically derived carbon are theoretically necessary to produce 100 g of dolomite from a CaCO3-sediment, because 6.5 g carbon are initially present in the original carbonate (allochems and mud): Ca 2÷ + COD- (original + Mg 2÷ + COD- (organic) -~ CaMg(CO3)2. Figure 6 depicts this schematically in terms of vol.%, for a hypothetical sediment containing 30.2 vol.% calcite, 50% pore space and 19.7 vol.% organic matter. In this e x a m p l e , organic m a t t e r is a s s u m e d t o c o n t a i n o n t h e average 50% c a r b o n a n d to have a specific gravity of 1, a n d for simplicity, t h e original c a r b o n a t e s e d i m e n t is supp o s e d to be calcite.

The final dolomite is composed of 26.6 vol.% dolomitized allochems and 26.6 vol.% dolomitized CaCO3 cement. The dolomitized CaCO3 cement contains the Ca surplus of the dolomitized allochems. In this example about 20 g Mg must be supplied (by way of porous flow and/or diffusion). This (minimum) amount is contained in about 15 1 of normal seawater, so that no

219

Schematic Materiat Batance Exampl.e of Anaerobic Doiomitization 100 T,,,,,, ~ I ~ I ~ 3 or, real ,-

/

I

Volume loss (arbitrarily chosen)

,,,.,,

a,,o¢hems

X" ~

, 50-

--

(50)

o

"

E ~'- >

~

~allochems

___ (26,61

Pore space ~ n,, ~,,_;~?-:~.., ~(arbitrarily chosenl~'t'~cu~

0

.... 0

matter (50% C~SG=I)

~ ~ ~

(Time and Depth)

Fig. 6. In the schematic example, dolomitization of about 30 vol.% calcite (allochems and/or mud) results in a total of 53 vol.% dolomite. Necessary carbonate is supposedly produced by anaerobic decomposition of about 20 vol.% of organic matter.

p r o b l e m o f Mg-supply is apparent. A t o t a l o f a b o u t 152 g o f d o l o m i t e is p r o d u c e d b y the a n a e r o b i c t u r n o v e r o f a b o u t 10 g organic c a r b o n (or 6.5 g organic c a r b o n are p r o p o r t i o n a l to 100 g d o l o m i t e ) . In a material balance model for the mechanism of dolomitization, whereby increased Mg/Ca ratio and brine reflux were viewed to be the main prerequisites for dolomitization, MSller and Rajagopalan (1976) calculate the volume of brine needed for the formation of 1 g dolomite to be in the range of 0.1 to 3 l, depending on the molar ratio of Mg/Ca of the brine, the figure of 0.1 1 is also the minimum amount that we calculate (152 g dolomite: 15 l = 1 g dolomite: 0.098 1.

Time estimates T h e time r e q u i r e m e n t s f o r the f o r m a t i o n o f small a m o u n t s o f d o l o m i t e are k n o w n to be on t h e order o f 103 yrs f r o m r a d i o c a r b o n - d a t e s r e p o r t e d f r o m R e c e n t e n v i r o n m e n t s (e.g. Shinn, 1968). T h e r e f o r e , it is a p p a r e n t t h a t a great a m o u n t of time w o u l d be necessary if anaerobic d o l o m i t i z a t i o n is to be held responsible for c o m p l e t e l y d o l o m i t i z e d sediments. Using data r e p o r t e d b y ZoBell ( 1 9 4 2 ) , c o n c e r n i n g the rates o f a n a e r o b i c c o n s u m p t i o n

220 of organic matter in sediments, it is possible to arrive at time-estimates required for bacteria to use up the amounts of organic matter called for in an anaerobic dolomitization model: In the anaerobic zone of a sediment, ZoBell (1942) observed that organic matter decreases with depth at a rate of 1 mg organic matter per gram w e t s e d i m e n t per foot. Assuming that this rate remains approximately the same for long time periods and greater depth, we have recalculated the data in our model in wt.%, which amounts to 0.15 g of organic matter per 1 g sediment (plus pore-space water). At the reported rate of decrease, 0.15 g of organic matter near the surface of the anaerobic zone would be used up at a depth of 150 feet or about 50 m. Calculating that about 0.2 mm of Hd. is equivalent to 1 yr (the entire thickness equals 2500 m, the entire time-span for the Norian is thus estimated to be about 12.5 m.y.), the decrease of 0.15 g of organic matter to zero would take 250,000 yr. In another approach to the problem, one can use ZoBell's (1942) estimate, from laboratory observations, that one bacteria cell is capable of consuming 1 0 -11 mg of organic matter per hour. Considering that thousands of bacteria per gram sediment are, on the average, active in anaerobic microenvironments, we come to a time/subsidence figure which is remarkably similar to the one arrived at above. Calculating for an average activity of 10,000 cells per gram sediment, 0.15 g of organic matter per gram sediment would be consumed in about 175,000 yrs, during which time the sediment would have been buried and have subsided to a depth of 34 m. The main significance of these figures is that a sizable portion of time during which dolomitization may conceivably go to completion is calculated. "Celestitization "

The original aragonite shell substance of gastropods (Natica) has been selectively replaced by celestite in some bioclast lenses of facies unit 6. This "celestitization" of aragonite was interpreted (Scherreiks, 1970) as an early diagenetic process having taken place in an aerobic environment into which H~S, migrating from anaerobic facies, was being oxidized to sulfuric acid by sulfur bacteria (Fig. 7). The source of Sr 2+ could have been twofold: first, aragonite releases a greater part of its ubiquitous Sr-content upon inversion to calcite (Noll, 1934; Turekian and Kulp, 1956). Such aragonite--calcite inversion could have been taking place below or adjacent to the Natica beds in connection with anaerobic dolomitization (see above); secondly there could have been evaporitic celestite, known to occur in Recent tidal facies (Evans and Shearman, 1964), which would have been dissolved in an anaerobic, sulfate reducing environment. Consequently, anaerobic conditions could not have developed in the Natica beds either during or after celestite formation, otherwise none would be found.

221

C e l e s t i t i z a t i o n of Natica

beds seo lever

02

\

/ \ \

/ k

~_.....~"~

~02

/ /

H 2 S 04 • Col CO 3 • Sr

I i I I

I I I 1

I

L

,

I

,s,2. Anaerobic

',.2s 1

~

dolomitization

of buried intertidal and

supratidal

i sediments

Fig. 7. Celestitization of Natica beds. Aragonite parts of gastropod tests are found to be selectively replaced by celestite in some Natica-bearing lenses (Lorea Group).

Lithostratigraphy of Hauptdolomit Traditionally, Hd. has been divided into two lithofacies parts: a lower dolomite sequence ( " H a u p t d o l o m i t " ) and an upper limestone sequence ("Plattenkalk"). The limestone sequence varies greatly in thickness and dolomite content, so that it was soon recognized by early workers as a diachronous facies. Bituminous marls (Seefeld facies) are well known and have occasionally been mapped. Since Gtimbel (1861) they have been observed to occur sporadically in about the middle and upper part of the Hd. formation. However, stratigraphical inferences were not made until 1968 (Miiller-Jungbluth, 1968). Contrary to this, Trusheim {1930), finding them at various levels, negates their use for stratigraphical purposes. A stratigraphical division into Lower and Upper Hd. (including "Plattenkalk" facies) was proposed by Scherreiks (1967, 1970) based on the occurrence of a striking gastropod-bearing (Natica), -+clayey, limestone horizon, which continuously and predictably appears about 1000 to 1200 m below the KSssen beds in the eastern Lechtal Alps, in an area covering up to 100 km 2. Miiller-Jungbluth (1968), working in the southern part of the Lechtal Alps, proposed a more detailed stratigraphy, based on changing carbonate petrographical features which he describes as being environmentally con-

222

trolled as a result of more or less steady subsidence. In 1971 he proposed its over-regional application: Upper Raibl beds (Karnian) -- Subtidal. Lower Hd. Supratidal. Middle Hd. Begins with subtidal facies but is mostly intertidal to supratidal. Bituminous Hd. Subtidal. Upper Hd. Return to intertidal and supratidal and toward the top again subtidal ("Plattenkalk"). Scherreiks (1971), basically substantiates these views and subdivisions: Lower Hd. -- Begins locally with a thin basal calcareous facies (subtidal) containing conglomerates, followed b y a dolomitic sequence (return to tidal environment).~ Middle Hd. -- Begins with thin clayey, -+bituminous and slightly calcareous facies (intertidal to subtidal) followed by a return to dolomitic intertidal and supratidal facies. Upper Hd. -- Begins with a thin subtidal calcareous facies (Natica facies) which laterally passes into clayey bituminous facies, equivalent to "Bituminous Hd." (Mtiller-Jungbluth, 1968, 1971). The remainder of Upper Hd. was shown to consist of many regressional and transgressional sequences, which culminate in "Plattenkalk", ending with the transition to (Rhaetian) KSssen marls. From the present work we conclude that this threefold stratigraphy is practicable for Hd. over a wide region if not for most of the Northern Calcareous Alps (Figs. 4 and 8). F r o m o u r p o i n t o f view, t h e s u b d i v i s i o n " b i t u m i n o u s H d . " , p r o p o s e d by Miiller-Jungb l u t h ( 1 9 6 8 , 1971), s h o u l d be i n c l u d e d as p a r t o f t h e transgressional s e q u e n c e f o r m i n g t h e Middle H d . / U p p e r Hd. t r a n s i t i o n . F u r t h e r m o r e , " b i t u m i n o u s " is descriptive o f a single c h a r a c t e r i s t i c w h i c h m a y be o b s e r v e d at m a n y levels in t h e Hd. f o r m a t i o n . T h e fact t h a t b i t u m e n is k n o w n to be diagenetically m o b i l e has also r e s t r a i n e d us f r o m a t t e m p t i n g t o use it as a q u a n t i t a t i v e p a r a m e t e r . Nevertheless, we d o agree t h a t b i t u m e n c o n t e n t is o n e o f a n u m b e r of valuable facies f e a t u r e s w h i c h are especially well d e v e l o p e d and, a l t h o u g h i n t e r m i t t e n t , are widespread in t h e t r a n s i t i o n z o n e f r o m u p p e r Middle to l o w e r U p p e r Hd.

Inasmuch as the four major transgressions (Raibl/LHd., LHd./MHd., MHd./UHd., UHd./KS) responsible for this threefold stratigraphy are diachronous in nature and are often divisible into a number of lesser transgressions and regressions, the attempt has been made to place division boundaries at the base of the supposedly strongest or culminating transgression, rather than choosing one single facies type to represent a boundary in all areas. This rule is superfluous where the recognized transgression and regression is thin and does not pertain to the uppermost boundary UHd./KS. In

227 this latter case, the occurrence of typical marls (often clayey coquina) is the widely accepted lithofacies boundary we use. PALEOGEOGRAPHIC--STRATIGRAPHICDEVELOPMENT The lateral relationships between Hd. and Dachstein facies are important for a first schematic understanding of the paleogeography (Fig. 1). According to Zankl (1967, 1971), the transition of Dachstein facies into contemporary Hd. takes place gradually, whereby the far back-reef tidal islands of the Dachstein facies are envisaged to develop transitionally into larger and larger banks which represent Hd. In our further development of Zankl's (1967, 1971) concept, we view the tidal islands of Dachstein facies as a barrier complex which sheltered the Hd. lagoonal and tidal zones from most strong wave and current action. The vertical, stratigraphical development is a complicated lateral, diachronous, back-and-forth shifting of the Hd. facies as a result of, often abrupt, subsidence, corresponding to transgressions (Figs. 8 and 9); periods of relative tectonic quiescence correspond to regressional sequences, which are viewed to result from a process of sediment accretion and seaward-shifting facies, analogous to the accretionary process at work in the Persian Gulf t o d a y (Shinn, 1973). Fischer (1965) postulates cyclical eustatic sea-level fluctuations in order to explain repetitious sequences of subtidal through supratidal for Norian Dachstein facies, which we can neither refute nor substantiate, but appear to us unnecessary to explain observed phenomena. L o w er Hd.

According to Jerz (1966), Hd. develops out of various carbonate facies of the Raibl formation (limestone, dolomite and evaporitic dolomites}; the dolomitic and evaporitic facies retreat northwards, thought to be a result of faulting on the southern margin of the Vindelician landmass, causing the Raibl/Lower Hd. transgression. Because of the diachronous character of the transgression, the lithofacies along the border Raibl/Lower Hd. do not necessarily coincide with the transition Karnian/Norian. In the sections investigated for this paper (Figs. 4, 8 and 9), it was convenient to place the Raibl/ LHd. boundary at the base of a calcareous facies, which is not unlike many Hd. intercalations. In the Lechtal Alps the transgression is marked by a finegrained conglomerate. Farther northwards, LHd. is preceded by dolomitic evaporitic Raibl facies (Jerz, 1966). We interpret this to show that the influence of the transgression was weaker towards the north, but it was nevertheless effective enough in bringing an intertidal LHd. development to an area where previously high-supratidal, evaporitic, deposits (upper Raibl northern facies) were accumulating. The further development of LHd. is dominantly a time of tectonic quiescence and regressive with occasional

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228 short and weak episodes of subsidence, so that only few clayey, laminated dolomites (unit 4) interrupt series of tidal flat mudstones (unit 3) with rare signs of stromatolites (unit 2). Middle Hd. Lower intertidal to lagoonal facies occur about 200--300 m above the Raibl/LHd. boundary, marking the begin of Middle Hd. These sometimes calcareous and slightly clayey facies (units, 4, 6) may be observed in the Lechtal and Karwendel Alps (Figs. 4, 8, 9). Their extension into other areas is not yet substantiated. The further development of MHd. is similar but more extreme and much longer (800--1000 m) than the development of LHd.. The regressive character of MHd. is exemplified by thick, tidal flat, mudstone sequences (unit 3) and locally by stromatolites (unit 2) and supratidal-mudstones and sparites (unit 1). Gradual to abrupt and intermittent reappearances of clayey, laminated dolomites (unit 4) in intertidal mudstones (unit 3) brings MHd. to a close. Upper Hd. Upper Hd. begins with near-shore and lagoonal sedimentation (units, 4, 5, 6, 7), the culmination of a development that begins in upper MHd. Figure 9 shows the envisaged paleogeography during the culmination of the MHd./ UHd. transgression. The lagoonal extent of this transgression is characterized by a near-shore bioclast wacke- to packstone, Natica facies (Figs. 4 and 8). The Natica facies are transitional with tidal zone, clayey bituminous facies (units 4, 5) and calcareous lagoonal mudstones (unit 7). Ideally seen, the Natica facies are sandwiched in between clayey and sometimes highly bituminous facies. The advance and retreat of the Natica facies are probably not preserved in the southern Karwendel Alps (due to erosion); Seefeld facies in this area are thus interpreted to be mainly upper MHd. but not higher. In the Ammergau Alps and Northern Karwendel Alps and east of Kufstein, only laminated, clayey and bituminous dolomite marks the advance and retreat of this transgression (1--10 m). In the eastern Lechtal Alps, up to 15 m are observed (Fig. 4). The Dachstein facies, between St. Ulrich and Lofer, is most likely contemporary with UHd., represented by the uppermost ( - 1000 m) o f calcareous/dolomitic Dachstein facies complex (see also Fruth and Scherreiks, 1975). The lower third to two-thirds of UHd. is a regressive sequence of intertidal mudstones, dominating over occasional supratidal stromatolitic intercalations. Thin horizons (less than one or a few meters) of clayey, laminated dolomites to clayey calcareous facies (units 4, 5, 6) indicate abrupt, shortlived episodes of subsidence. Beginning very early in the Lechtal Alps, a series of about five to seven transgressions of variable intensity gradually bring dominatingly lagoonal

229

conditions, which appear increasingly delayed (much thinner) towards the north and west (Fig. 8). For the first time in Hd. oolitic mounds and coarse bioclast sparites (grainstones) make their appearance, interpreted to represent the barrier bar complex b e y o n d which, in a seaward direction (S and SE), an outer lagoonal environment was situated, where Dachstein facies and later Rhaetian facies developed. The oolitic and bioclast mounds appear quite early in the eastern Lechtal Alps, receding and reappearing a few times. A final more intensive transgressional surge spread the Hd. lagoonal facies over most of the Northern Calcareous Alps ("Plattenkalk") followed by the, often transitional, deeper water, black marls, clayey limestones and patch reefs of the KSssen and Rhaetian types (Fabricius, 1966). ACKNOWLEDGEMENTS

Field and laboratory work was supported by the Bayerische Staatssammlung fiir Allgemeine und Angewandte Geologic and the Geological Institute of the University of Munich. The authors are indebted to Prof. Dr. K. WeberDiefenbach for his help concerning a E D R F A carbonate (dolomite) program, and especially thankful for the chemical-technical work carried out by Miss I. HSrmann, and partly by Miss A. Strunz, and the drawing work done by Mrs. I. Rappel. Valuable discussions, concerning X-ray diffraction analyses, were held with Dr. H. Marschner and Dr. R. Snethlage, to whom we are very grateful. For helpful constructive criticism of the original manuscript, we wish to thank Prof. Dr. H. Fiichtbauer. REFERENCES Abd-E1-Malek, Y. and Rizk, S.G., 1963. Bacterial sulfate reduction and the development of alkalinity. J. Appl. Bacteriol., 26: 7--26. Adams, J.E. and Rhodes, M.L., 1960. Dolomitization by seepage refluxion. Bull. Am. Assoc. Pet. Geol., 44: 1912--1920. Alderman, A.R., 1965. Dolomitic sediments and their environment in the south-east of South Australia. Geochim. Cosmochim. Acta, 29: 1355--1365. Ampferer, O., 1932. Erl~iuterungen zu den Geologischen Karten der Lechtaler Alpen 1 : 25.000. Geol. BA, Wien, 125 pp. Bosellini, A., 1967. La tematica, deposizionale della dolomia principale (Dolomiti E Prealpi Venete). Boll. Soc. Geol. Ital., 86: 133--169. Evans, G. and Shearman, D.J., 1964. Recent celestine from the sediments of the Trucial Coast of Persian Gulf. Nature, 202: 385--386. Fabricius, F.H., 1966. Beckensedimentation und Riffbildung an der Wende Trias/Jura in den Bayerisch--Tiroler Kalkalpen. Internat. Sediment. Petrogr. Ser., 9: 1--143. Fischer, A.G., 1965. The Lofer cyclothems of the Alpine Triassic. Symp. Cyclic Sedimentation, Kans. Geol. Surv. Bull., 169: 107--149. Friedman, G.M. and Sander, J.E., 1967. Origin and occurrence of dolostones. In: Chilingar et al. (Editors), Carbonate Rocks, Part A. Elsevier, New York, N.Y., pp. 267--348. Fruth, I. and Scherreiks, R., 1975. Facies and geochemical correlation in the Upper Hauptdolomit (Norian) of the eastern Lechtaler Alps. Sediment. Geol., 13: 27--45. Fiichtbauer, H. and Goldschmidt, H., 1965. Beziehungen zwischen Calciumgehalt und Bildungsbedingungen der Dolomite. Geol. Rundsch., 55: 29--40.

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