Aeolian sands in continental red beds of the Middle Buntsandstein (Lower Triassic) at the western margin of the German Basin

Sedimentary Geology, 31 (1982) 191--230

191

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AEOLIAN SANDS IN CONTINENTAL RED BEDS OF THE MIDDLE BUNTSANDSTEIN (LOWER TRIASSIC) AT THE WESTERN MARGIN OF THE GERMAN BASIN DETLEF MADER

BEB Gewerkschaften Brigitta und Elwerath Betriebsfiihrungsgesellschaft mbH, Hannover (F.R.G.) (Received November 18, 1980; revised and accepted September 21, 1981)

ABSTRACT Mader, D., 1982. Aeolian sands in continental red beds of the Middle Buntsandstein (Lower Triassic) at the western margin of the German Basin. Sediment. Geol., 31: 191-230. Aeolian sands occur widespread in continental red beds of the Middle Buntsandstein (Lower Triassic) at the western margin of the German Basin (Middle Europe), in the Eifel area. Cross-bedded sands were mainly deposited by grainfall on lee-slopes of barchanoidtype dunes in unimodal wind regime in both low-energy and high-energy environments. Occasionally, sets are internally truncated by reactivation surfaces. Crest height of solitary-set dunes varies in the range of some meters. Locally, sequences of cosets, both of tabular-planar and wedge-planar type, capped by fiat or gently windward dipping topset beds, represent larger dune complexes of about 10--20 m in height and at least 80--100 m in lateral extent. Horizontal-laminated deposits originated as sand sheets in interdune areas. The dune sands were deposited by trade winds of the northern hemisphere in low palaeolatitude, predominantly by southeasterly and southwesterly trade winds in summer when the intertropical convergence zone was shifted to the north. Aeolian sands built up an extensive dune belt in the Eifel, intersected by braided to anastomosing rivers. Fluviatile incursions or heavy ephemeral rainfall led to aquatic redeposition of aeolian sediments and origin of shallow lakes in interdune depressions. Distribution of aeolian and fluviatile sediments in Karlstal-Schichten is different in the Southern, Western and Northern Eifel. Middle Buntsandstein sequence in the Southern Eifel reflects an evolution of the fluviatile depositional environment also incorporating the occurrence and +distribution of aeolian sands. Evolution is characterized by increasing spacing and sinuosity of channels, weakening supply of coarse detritus from the source areas and decreasing braiding of the river systems. Aeolian sands occur in the final phase of this evolution which led from local alluvial fans via cobbly and pebbly braided rivers to sandy braided streams and finally to an intertonguing of aeolian dunes and braided to anastomosing rivers, matching with the climax of regional diversification of depositional milieu. The aeolian sand sea in the Eifel covered an area of nearly 2500 km 2 at the time of its largest extension. The dune belt ended in the Northern Eifel, the alternating sequence passed into an entirely alluvial succession which graded northwards by continuously diminishing grain size into the central playa/lacustrine to marine sediment series in Northwest Germany. Palaeogeography, sedimentology and genesis of the Middle Buntsandstein at the western margin of the Mid-European Basin and of Rotliegendas in Northwest Germany appear to be partially similar as is revealed by certain characteristics of the palaeogeographical reconstructions. Concurrence of Middle Buntsandstein palaeogeographical models for both Eifel and the 0037-0738/82/0000---0000/$02.75 © 1982 Elsevier Scientific Publishing Company

192 whole German Basin most clearly delineates the key position of the Eifel for interpretation of the Buntsandstein sedimentary environment and the palaeogeography in Middle Europe.

INTRODUCTION

Aeolian dune sands are widespread in continental sequences throughout geological history. Today, dune belts not only cover large areas of the Earth's recent crust, but sand-sea formation is an important process also extra-terrestrically, as is proved by the occurrence of dune fields on Mars (Cutts and Smith, 1973). Important periods of continental red-bed formation in the geological past in Europe have been the Siluro--Devonian (Old Red) and Permian--Triassic (New Red). During the Permian and Triassic, Middle Europe was situated at a low palaeolatitude on the northern hemisphere. Palaeogeographical position and a semi-arid to arid climate twice led to the origin of continental red-bed sequences, first in the Upper Rotliegendes (Lower Permian) and second in the Buntsandstein (Lower Triassic). Terrestrial deposits of both sequences largely consist of fluviatile and lacustrine sediments sometimes also containing salt layers, and partly also of aeolian dune sands. In spite of numerous studies of widespread Rotliegendes dune sands (Glennie, 1972; Lutz et al., 1975; Marie, 1975; Van Veen, 1975; Dachroth, 1976; Nairn and Smithwick, 1976; Ellenberg et al., 1976; Plein, 1978; Glennie et al., 1978; Brookfield, 1978, 1980; Nagtegaal, 1979; Falk et al., 1979; Wijhe et al., 1980), Triassic aeolian sediments which occur in the Middle Buntsandstein have up to now only been recognized in few parts of the Mid-European Basin, and this paper is the first to describe Middle Buntsandstein dune sands in detail. The red bed sequence of the Buntsandstein (Lower Triassic) in the German Basin (Northwest and Middle Europe) consists of sediments of various origins. In the last decades, numerous authors established the predominantly aquatic nature of the Buntsandstein deposits (for literature compilation see Mader, 1981a). The water-laid rocks are mainly interpreted as fluviatile and brackish-marine sediments. From the source areas (Gallian Swell in the west and Vindelician Swell in the east; Brinkmann, 1933;Wurster, 1964a, 1964b, 1968; Grumbt, 1974; Hoppe, 1976), the detritus was transported by large rivers to the northern central basin where brackish-marine conditions existed (Sindowski, 1957; Leggewie et al., 1977). Aeolian sediments from the Buntsandstein in the Mid-European Triassic Basin have been previously recognized -- apart from the Eifel -- in the Pfalz and Saar areas (Prof. Dr. W. Dachroth, pers. commun., 1978; Dachroth, 1980), at the eastern margin of the Rhenish Massif (Dr. K.W. Tietze, pets. commun., 1980), in the Hessian Depression (Wycisk, 1977, 1978, 1981), in Helgoland (Clemmensen, 1979; Wunderlich, 1979), in Jutland/Denmark (Pedersen and Andersen, 1980) and in the Holy Cross Mountains/Poland (Gagol and Karpiniec, 1974; Gradzinski et al., 1979). From all these occur-

193 rences, however, only dune sands have been reported which either are merely locally present in greater thickness or which extend over smaller areas as intercalations of varying thicknesses in the aquatic sequence. This paper now first describes newly discovered aeolian sediments of greater thickness and distribution from the German Buntsandstein. GEOLOGICAL SETTING BUNTSANDSTEIN

AND

STRATIGRAPHY

OF

THE

EIFEL

MIDDLE

The investigations were mainly carried out at the western margin of the German Basin, in the Eifel North--South-zone. This region, a depression zone of Variscan fold axes (Schenk, 1938) and a transgression path both for the Middle Devonian (Struve, 1961) and the Lower Muschelkalk seas (Fuchs and Mader, 1980), is divided into northern, western and southern parts. Triassic deposits occur in the following areas (see Fig. 1): Northern Eifel: Mechernich area; Western Eifel: Stadtkyll area, Oberbettingen area, Northern Trier area; -- Southern Eifeh Southern Trier area. Stratigraphically (see Fig. 2), the classical subdivision of the Middle Buntsandstein at the western margin of the German Basin into Trifels-Schichten, Rehberg-Schichten and Karlstal-Schichten (set up by Thiirach, 1894) also is possible in the Southern Eifel (Heitele, 1979). In the Western Eifel, KarlstalSchichten immediately overlie the eroded Variscan basement. In the Northern Eifel, subdivision of Middle Buntsandstein is still uncertain due to the unusual coarse facies of the deposits in this region. Apart from investigations in the Eifel, further evidence for distribution of aeolian sands was obtained by comparative studies in various other parts of the Mid-European Triassic Basin. -

-

-

-

SEDIMENTOLOGY

O F T H E EIFEL B U N T S A N D S T E I N

The Buntsandstein of the Eifel consists of sediments of various origins: alluvial-fan, mud-flow, aeolian, fluviatile, lacustrine, deltaic sediments and (only in Upper Buntsandstein) palaeosols (Mader, 1979, 1981b). Fluviatile sediments often display cyclic composition. Sedimentation units ideally consist of channel lag, channel bar, topstratum deposits (according to Allen, 1965) and palaeosol (Mader, 1981a). Alluvial-fan and mud-flow deposits occur only locally at the base of the Buntsandstein sequence; deltaic sediments are restricted to the top of the red-bed succession, leading to the shallow marine Muschelkalk transgression; lacustrine deposits are present as intercalations in aeolian sediments. These deposits are of minor importance. The Buntsandstein sequence consists mainly of aeolian and fluviatile sediments. Fluviatile deposits occur both in the Middle and Upper Buntsandstein (the Lower Buntsandstein is not present at the western margin of the German

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Fig. 2. Stratigraphy of the Middle Buntsandstein (Lower Triassic) in the Eifel (Germany). Southern Eifel: after Krieger (1973, 1978), Negendank ( 1 9 7 4 ) and Heitele (1979). PreBuntsandstein: Rotliegendes or Devonian. Western Eifel: after Mader (1979). Pre-Buntsandstein: Devonian. Northern Eifel: after Miiller et al. (1960) and Hendriks and Schrader (1980). Pre-Buntsandstein: Permian or Devonian. Legend: 1 = Pre-Buntsandstein (Permian or Devonian); 2 = boulders, cobbles and pebbles; 3 = cobbles and pebbles; 4 = pebbles; 5 = sand.

Basin, compare Weiler, 1972), whereas aeolian sands are restricted to the Middle Buntsandstein. AEOLIAN SANDS IN THE EIFEL MIDDLE BUNTSANDSTEIN

Description Distinction of aeolian and fluviatile deposits, respectively aeolian and generally water-laid sediments, has recently become more and more ambiguous due to the increasing convergence of several criteria formerly taken as diagnostic for aeolian environment with characteristics of aquatic sediments (for example see Kaldi and Allen, 1980). The confusion is emphasized by various reinterpretations of deposits formerly regarded as undoubtedly aeolian sediFig. 1. Geological sketch of the Eifel (redrawn after Dahlgriin, 1939; Schriel, 1939; and Geib et al., 1972), geographic subdivision of the area and location o f the region in Northwest Europe (see insert in upper right). Legend: I = Devonian basement; 2 = Buntsand-

stein (Lower Triassic); 3 = Muschelkalk, Keuper and Liassic (Middle to Upper Triassic and Lower Jurassic); 4 = Tertiary and Quaternary (only considered in the Niederrheinische Bucht north o f the Mechernich area in the Northern Eifel, otherwise removed). Tectonics not respected. Numbers below and left of the map: coordinatea of the German GaussKriiger grid. Small numbers: coefficients; large numbers: kilometers.

196

ments (Pryor, 1971a, 1971b; Freeman and Visher, 1975; Strack, 1978; Strack and Stapf, 1980). Definite differentiation of aeolian and fluviatile deposits in the Eifel Middle Buntsandstein has become possible by a lot of criteria being taken together. Identification is based on various both sedimentological and petrographical characteristics as follows: {1 ) Typical micro-lamination (reported from aeolian sands by Wright, 1956; Opdyke, 1961; Stokes, 1964; McKee, 1966; Oppermann, 1971; Glennie, 1972; Dachroth, 1976; Nemec and Porebski, 1977; Plein, 1978; Brookfield, 1978, 1980; and Clemmensen, 1979), consisting of alternating laminae of coarse, well-rounded and fine, subrounded to angular grains (for detailed description see Mader, 1980b). The micro-lamination occurs in both horizontal- and cross-stratified aeolian sands. (2) Occurrence of thin layers or narrow lenses of millet-seed sand (Kaviarsand), consisting of very well rounded, very well sorted, very coarse grains with frosted surfaces. Millet-seed sand is present as layers of up to some centimeters thickness at the base of micro-laminated series or is lenticularly concentrated in syngenetic small-scale bedding deformations {Fig. 3-6). Milletseed sand is interpreted as residual layers of unusually coarse grains {creep population) at the base of the dunes. (3) Cross-bedding types (after Allen, 1963): alpha-, omikron- and pi-crossstratification. The predominant cross-stratification is of tabular-planar type (after McKee and Bigarella, 1979b), but wedge-planar sets occur too. Trough cross-stratification as usual in aeolian sands is very rare {Wilson, 1958; Hatchell, 1967). Aeolian ripples, as described by McKee et al. {1971), Walker and Harms (1972), Steidtmann {1974), Sanderson (1974), Brookfield (1977) and Gradzinski et al. (1979) have not been observed. (4) Size of cross-stratification sets. In most cases, the size of the aeolian foresets ranges between 30 cm and 1.5 m, but sets of up to 4 m thickness also occur (plate I-l). Occasionally, sets of up to 6 m thickness are observed. The lateral extent of the largest sets exceeds 50 m. (5) Occurrence of foreset and backset beds. Foreset beds (avalanche deposits, Bagnold, 1941; slipface deposits, Ahlbrandt and Fryberger, 1980) show steep dip angles (usually more than 25 ° , occasionally up to 40 ° , often increasing towards the top of the cross-stratification set), whereas backset beds (accretion deposits, Baguold, 1941; topset deposits, Shotton, 1937; Ahlbrandt and Fryberger, 1980) are gently (10--15 °) dipping in windward direction (Fig. 3-1). Backset beds in aeolian sediments are described by Solger (1920), Smith (1940), McKee (1940, 1945, 1957), Opdyke (1961) and Sharp (1966). Individual foreset sequences sometimes show upward progression from subhorizontal to steeply inclined laminae (compare McKee and Bigarella, 1979b). Occasionally, topset deposits conform to the topography of the dune and change their inclination from backset direction to horizontal stratification and finally to foreset direction from stoss to lee side across the dune corn-

197

1

O,S m

1m

3

z,

5

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Fig. 3 . 1 = Gently windward dipping backset strata at the top of steeply inclined foreset sequence. Aeolian sands. Western Eifel, Stadtkyll area. Quarry at the western side of the Heidenkopf III east of Dahlem (Sheet Stadtkyll, r 39 890, h 82 980). 2 = Lateral pinch out of cross-stratified aeolian dune. Thickness of individual laminae decreases rapidly towards the margin. Fossil relief is preserved by the succeeding unit. Aeolian sands. Western Eifel, northern Trier area. Cut in a hollow way north of the Hochbtisch near Niederweiler (Sheet Waxweiler, r 31 000, h 44 350). 3 = Pebble-bearing fluviatile bar deposits, overlying with erosional lower boundary horizontal-laminated aeolian sands, are deflated at the top. The sand-sized population has been blown out, the pebbles are concentrated as aeolian lag deposits at the base of cross-stratified dune sands. Southern Eifel. Cut at a road at the southern side of the Gl~/sges-Berg northwest of Trier-Pallien (Sheet Trier, r 4 4 6 5 0 , h 14 980). 4 = Aquatically relaid dune sands, without distinct bedding. The younger series is irregularly truncating into the older unit and is overlain by cross-stratified dune sands. Western Eifel, Oberbettingen area. Cut at the sports ground of Birgel (Sheet Stadtkyll, r 45 190, h 76 090). 5 = Syngenetic slump fault of some meters throw. Post-fracture cross-bedded aeolian sands (right) are adapted to the fault plane and dip extraordinarily steep in the lower laminae. Towards the younger strata, the angles fiat to the usual values. Horizontal-laminated aeolian sands overlie with erosional lower boundary the faulted sequence. Southern Eifel. Rocks at the Genoveva cave southeast of Butzweiler (Sheet Welschbillig, r 46 320, h 18 930 to r 46 740, h 19 320). 6 = Small-scale syngenetic deformation structure in aeolian sands, probably originated by footprints of vertebrates or impact of coarse hailstones. The hollow is filled with a lenticular concentration of very coarse-grained sand (creep population) which equalizes the relief. Southern Eifel. Cut at a way at the northwestern side of the Vogelskopf southeast of Kordel (Sheet Welsehbillig, r 46 040, h 22 840).

198

199 plex ( d o w n w i n d a n d u p w i n d topsets, A h l b r a n d t a n d F r y b e r g e r , 1980). T h e s e t o p s e t beds overlie t h i c k c o m p l e x e s o f tabular-planar a n d w e d g e - p l a n a r foresets o f u p t o 15 m thickness a n d in suitable o u t c r o p s clearly delineate the palaeomorphology of the dune. I n t h i c k d u n e c o m p l e x e s c o m p o s e d o f series o f cosets, t h e individual foresets are o f t e n c u t b y internal e r o s i o n surfaces ( c o m p o u n d sets are built u p o f several intrasets; G r a d z i n s k i et al., 1 9 7 9 ) . (6) A b s e n c e o f cyclic c o m p o s i t i o n . U n i f o r m aeolian s e q u e n c e s (up t o s o m e tens o f m e t e r s t h i c k ) display n o cyclic c o m p o s i t i o n . T h e succession is m a i n l y built u p o f m u l t i s t o r e y s t e e p l y d i p p i n g f o r e s e t beds. I n fluviatile sequences

and successions with alternating water- and wind-laid deposits, thin aeolian sands occasionally occur at the top of the alluvial cyclothems. In contrast, fluviatile sequences consist either of stacked bar deposits or a series of cyclothems with occasional intercalations of multistorey channel sediments. (7) Occurrence of preserved lateral pinch outs of cross-bedded dunes. Occasionally, the rapid diminution of the individual foreset laminae's thickness at the sides of barchans is exposed (Fig. 3-2). In these rare cases, fossil dune reliefis preserved by the succeeding unit. (8) Occurrence of deflation layers (aeolian lag deposits). At the eroding base of some foreset sequences, small pebbles (up to 2 cm, rarely up to 3 cm) are occasionally concentrated. Sometimes, pebble-bearing fluviatile bar deposits are deflated at the top. The pebble population is concentrated as aeolian lag deposits at the base of the succeeding dune sands (Fig. 3-3). PLATE I. Sedimentary structures of aeolian dune sands in the Middle Buntsandstein (Lower Triassic) of the Eifel (Germany). 1. Large-scale cross-bedding of aeolian sandstones. Dip angle of laminae increases slightly towards the top of the set which is terminated by a horizontal erosional surface. Geologist for scale. Western Eifel, Stadtkyll area. Quarry in the Schmidtheim forest southeast of the road from Dahlem to Blankenheim (Sheet Blankenheim, r 41 600, h 86 340). 2. Syngenetic slump or tensional faults in aeolian sandstones. Cluster of slump faults with minor throws (up to some din, occasionally only a few cm). Length of hammer 29 cm. Southern Eifel. Rocks at the Genoveva cave southeast of Butzweiler (Sheet Welschbillig, r 46 320, h 18 930 to r 46 740, h 19 320). 3. Steeply dipping alpha-cross-bedded dune sands overlie horizontal-laminated aeolian sheet sands and are succeeded by omikron- to pi~ross-stratified fluviatile sediments. Length of hammer 29 cm. Southern Eifel. Rocks at the southern side of the Steinkopf southeast of Kordel (Sheet Welschbillig, r 46 970, h 22 220 to r 46 760, h 22 330). 4. An isolated alpha~ross-bedded dune sand is intercalated into a succession of horizontal-laminated interdune sheet sands alternating with lacustrine deposits. At the top of the figure, pebble-bearing fluviatile sediments of the basal Upper Buntsandstein overlie the horizontal-laminated sands with very low-relief erosional lower boundary. Diameter of the figure about 6 m. Western Eifel, northern Trier area. Rocks at the eastern side of Enz river valley south of Sinspelt (Sheet Mettendorf, r 23 500, h 36 600). 5. A steeply dipping cross-bed set is intercalated into a succession of low-angle crossbedded and horizontal-laminated aeolian sands. Length of hammer 29 cm. Northern Eifel. Rocks east of the road from Mechernich to Satzvey, near Katzvey (Sheet Euskirchen, r 48 740, h 07 720).

200 Gravel lags on erosional lower bounding surfaces are also reported by Kiersch (1950), Walker and Harms (1972) and McKee and Bigarella (1979b). Deflated residual gravel sheets are also described by Dachroth (1980). Wind-blown pebbles which have carved o u t tiny grooves on the sediment surface (Gradzinski et al., 1979) have not been observed. garely, very small pebbles up to 5 mm diameter are concentrated on individual foreset planes (Mader, 1981d). Granule ripples (common in serif environments; Glennie, 1970) which have been reported from British Permian aeolianites (Clemmensen and Abrahamsen, 1981) have n o t been observed in the Eifel Middle Buntsandstein. Due to the absence of larger pebbles and cobbles in the fluviatile sediments associated with the aeolian sands, the general rarity of gravel lags on erosional lower boundaries and unusually small to very small size of concentrated pebbles, it is difficult to record ventifacts with certainty. Facets on pebbles have only occasionally been observed in the Eifel Middle Buntsandstein. Widespread ventifacts have been especially reported from the Eastern Bavarian Buntsandstein (Goller, 1935). Faceted pebbles originated b y deflation in-situ are d o c u m e n t e d by Dachroth (1976, 1980) from the Pfalz and Saar Middle Buntsandstein. (9) Absence of dispersed pebbles in the aeolian units. The sandstones are completely free of pebbles (except for basal deflation layers). (10) Absence of intraformational reworking horizons, contrary to their widespread occurrence in fluviatile sediments. (11) Absence of channel-fill structures. The lower erosional boundaries of the foreset sequences are planar or slightly curved concave-upwards. Scoured surfaces occur only at the base of aquatically relald dune sands (Fig. 3-4) and fluviatile bar deposits (Fig. 3-3), either as symmetrical channels or most differently cutting into the substratum with local relief. Flute casts, c o m m o n on bedding planes in fluviatile sequences, are absent on depositional surfaces of aeolian sands. (12) Intercalation of aquatically relaid dune sands. They are present as thin (10 c m - 1 m), often unstratified sandstones, sometimes irregularly truncated into the aeolian foreset sequence or into older redeposited dune sands (Fig. 3-4). Similar structureless sandstones are reported b y Gradzinski et al. (1979). (13) Intercalation of thin clayey lacustrine sediments. These quiet-water deposits occur as clay laminae of 1 mm to 4 cm thickness in horizontallaminated aeolian sands or cover planar erosional boundaries at the t o p of cross-bedded dune sands. They settled o u t between the dunes in shallow lakes which originated by ephemeral rainfall, either forming as intercalations into the sheet sand sequence or draping the erosional t o p of an older dune complex cropping out in the interdune depression. Thin m u d drapes in aeolian sequences are also interpreted as deposits of stagnant waters in interdune ponds by Gradzinski et al. (1979). Similar lenses of clayey or carbonate interdune deposits are described b y McKee (1934), Thompson (1969),

201 Hanley and Steidtmann (1973), Picard (1977), Fryberger et al. (1979), Fryberger (1979), Ahlbrandt and Fryberger (1980) and Doe and Dott (1980). The lenticular deposits intertongue with dune sands (McKee and Moiola, 1975; Ahlbrandt and Fryberger, 1981). (14) Occurrence of isolated cross-bedded dune sand bodies in successions of horizontal-laminated aeolian sands (Plate I-4), or intercalation of solitary steeply dipping sets into low-angle cross-bedded aeolian sediments (Plate I-5). In contrast, fluviatile sequences are characterized by continuous diminution of set thickness and angle towards the top of the cyclothems, and by unidirectional transition from cross-bedding to horizontal-lamination or vice versa rather than random alternation. (15) Occurrence of adhesion-ripple-lamination in parts of horizontalbedded aeolian sands, especially in association with lacustrine interdune deposits. Adhesion-ripple-lamination originates from catchment of sand which is blown over a damp surface (Reineck, 1955; Hunter, 1977) and it has often been described from dune sand sequences (Glennie, 1972; Nemec and Porebski, 1977, Plein, 1978; Clemmensen, 1979; Hunter, 1981). Sometimes, thick sequences of generally horizontal-laminated sand consist of alternating beds of micro-laminated sands, adhesion-rippled sand and micro-scale interbedding of sandy and silty-clayey sediments. Current- and wave-ripple trains which are abundant in alluvial successions, are restricted in aeolian sequences to lacustrine and fluviatile intercalations. (16) Occurrence of curled m u d flakes. According to Glennie (1970), fiat pebble conglomerates of clayey and silty intraformational deposit fragments generally occur in water-laid sediments, whereas curled m u d flakes could only be preserved in aeolian sands (compare Turner, 1980). Curled m u d flakes also occur in Lower Triassic dune sands of the Holy Cross Mountains (Gradzinski et al., 1979). Contrary to the literature, however, this criterion taken alone may be misleading, because preservation of curled m u d flakes has been recently recognized in sandy fluviatile topstratum deposits in the Eastern Bavarian Buntsandstein (Mader, unpubl.). Further evidence is given from Rotliegendes (Strack, 1978). (17) Occurrence of thin aeolian sands at the top of fluviatile cyclothems. Apart from thick dune sand complexes, in parts of the sequence aeolian sediments also occur as thin layers or veneers capping alluvial cyclothems. (18) Distribution of palaeowind directions. The aeolian dunes generally migrated to the north. The range of variation of dip directions of the dune foresets is narrow compared to greater angular deviation of dips of fluviatile bar deposit foresets (Mader, 1980c). Identical palaeowind directions as described from the western margin have been found at the eastern margin of the Mid-European Triassic Basin (Gradzinski et al., 1979). Concordant palaeowinds throughout, the Middle Buntsandstein sequence in the Eifel are in marked contrast to occasionally divergent palaeoflows in different cyclothems or due to random variations. (19) Textural and mineralogical maturity. The aeolian sands are generally

202 well to very well sorted. In the coarse layers, both light and heavy minerals (including tourmaline, rutile and zircon) are well to very well rounded. Frosted grain surfaces are common. Textural maturity is accompanied by nearly comparable mineralogical maturity, as indicated by an average quartz content of about 70% (Mader, 1981i). (20) Authigenic tourmaline and neoformed rutile, highlighting an extraordinarily well developed and diverse heavy-mineral diagenesis in the Eifel Buntsandstein sediments (Mader, 1981h), occur only rarely in aeolian sands, but are locally widespread in fluviatile sandstones. (21) Absence of mica. Aeolian sands do not contain mica flakes, whereas parts of the fluviatile succession of the Upper Buntsandstein are characterized by their mica content. Similar results are reported by Thompson {1969), Home (1975), Magraw (1975), Waugh (1978), and Brookfield (1980). (22) Diagenesis. There is considerable difference in diagenesis between aeolian and fluviatile deposits. Aeolian sands are only cemented by slender authigenie overgrowths on detriCal quartz grains, whereas fluviatile sands often show completely regrown quartz grains with polygonal texture. Aeolian sands are friable in most cases or only weakly cemented, fluviatile deposits are often hard-like quartzites and could only be split by hammer. Weakly lithified Triassic aeolian sands are also reported by Thompson (1969) and Clemmensen (1978, 1979, 1980). Magraw (1975) finds nearly uncemented dune sands in the Permian (compare also Doe and Dott, 1980 and Turner, 1980). (23) Occurrence of nest burrows of the Recent silk bee Colletes daviesanus (Hymenoptera, CoUetidae). The stratibound position depends on grain size and diagenesis of the subst~ate (Mader, 1980d) and thus the burrows are often nearly exclusivelydug into aeolian sands (Mader, 1981c). (24) Occurrenceof small~cale bedding deformations (Fig. 343). The geopetal structures are restricted to aeolian sands and are thought to have mainly originated by footprints of vertebrates, to a minor amount by impacts of coarse hailstones (Mader, 1981e). (25) Occurrence of slump deformations and contorted bedding. Some foresees show wavy deformation patterns due to slip-faceavalanching.Similar structures are reported by McKee (1945, 1966), Murphy and Schlanger (1962), Bigarella et al. (1969), Thompson (1969), Bigarella (1972), Gtadzinski and Jerzykiewicz(1974), McKeeand Bigarella (1979b) and Ahlbrandt and Frybetger (1980). Sand-flow lobes or toes near the lower bounding surface of slipfaee deposits have not been observed in aeolian sands of the Eifel Bunt~andstein. Slump deformation and contorted bedding are absent in fluviatile sediments of the Eifel Middle Buntsandstein. In contrast, injection structures, sand dikes and (rare) recumbent cross-bedding occurring in alluvial deposits have not been observed in aeolian series. The same applies for bioturbation in sedimentsand trace fossils on bedding planes which in dune sand series axe

203 restricted to fluviatile and lacustrine intercalations. (26) Occurrence of syngenetic slump or tensional faults. These small faults, of in most cases some centimeters (Plate I-2), occasionally up to 40 cm throw, intersect bedding planes. Their length is up to some meters. At the top, they are overlain by undisturbed aeolian sediments. Similar slump faults have been observed in Quaternary volcanic tufts (Sch~fer, 1977). From dune sands, they are described by McKee et al. (1971), Ahlbrandt (1975) and Ahlbrandt and Fryberger (1980). The faults today usually do not appear as joints, and small-scale bleaching does not initiate here. The planes originated in freshly deposited aeolian sands and are tightened by diagenesis. Rarely, the throw of syngenetic slump faults is up to some meters. Postfracture aeolian sands are adapted to the fault plane and dip extraordinarily steeply in the lowest laminae. With continuing sedimentation, the angles flatten more and more to the usual values (Fig. 3-5). Dissipation structures, important secondary structures in Recent to subRecent dunes (BigareUa, 1975; Ahlbrandt and Fryberger, 1980), are absent in unaltered aeolian sands in the Eifel Buntsandstein. They occur, however, at the top of quarries below the soil zone as result of superficial infiltration. (27) Different pigment intensity of aeolian and fluviatile deposits. In most cases the colour of dune sands is orange to light red, whereas the colour of river sands is often middle to dark red. Similar colour nuances in different depositional milieus, in Middle Buntsandstein sediments also present in Pfalz and Saar area (Dachroth, 1976, 1980), have recently also been observed in the Rotliegendes of the Saar-Nahe-Basin (Mader, unpubl.). Preservation of these original environmental differences is (apart from numerous other evidences) taken as a hint to primary pigment provenance (Mader, 1981g; 1982c). (28) Absence of lateral coarsening of the predominantly aeolian succession of Middle Buntsandstein. Towards the western margin of the sedimentation area, the Buntsandstein members are continuously pinching out; increasingly younger strata cover the Variscan basement. There is, however, no lateral coarsening of the upper Middle Buntsandstein, it being almost entirely built up of aeolian sands in the Western EifeLOn the contrary, the fluviatile Upper Buntsandstein members display remarkable coarsening in direction of lateral pinch out, especially well developed in the upper part of the Upper Buntsandstein which passes from sandstones free of pebbles to coarse congiomerates. (29) Position of the upper Middle Buntsandstein intertonguing aeolian and fluviatile succession in the evolution of alluvial sedimentary environment. The dune sand sequence occurs in the final phase of an evolution which is mainly characterized by continuous increase of river channel spacing and sinuosity (for details see below). (30) Large lateral extent of the aeolian sand series in the Eifel. Dune sands in the upper Middle Buntsandstein occur from the Southern Eifel via the Western Eifel to the Northern Eifel, covering a distance of approx. 100 km

204

in the direction of sediment transport. This widespread distribution is in contrast to the isolated sand bodies of the Rotliegendes Kreuznacher Sandstein that has recently been reinterpreted as fluviatile deposits (Strack, 1978; Strack and Stapf, 1980).

Interpretation The above-reported criteria permit one to interpret these deposits as aeolian sands (compare Allen, 1970; Glennie, 1970; Selley, 1970; Collinson, 1978; McKee and Bigarella, 1979b; and Fryberger, 1980). According to Wilson (1972) and Brookfield (1977, 1978), dunes are the predominant aeolian bedforms in the Eifel Middle Buntsandstein. Questionable aeolian ripples have only rarely been observed, draas could not be recognized. Most erosional boundaries between individual sets represent second-order surfaces after Brookfield (1977); internal reactivation planes are to be identified as third-order surfaces. Predominance of second-order surfaces is also reported by Hubert and Mertz (1980). Cross-bedded sands were mainly deposited by grainfall {Hunter, 1977)on lee slopes of barchanoid-type dunes (McKee, 1979), as indicated by the narrow spread of foreset dips. Mean angular deviations between 30 ° and 60 ° point to both barchan dunes with curved slipfaces and transverse-ridge dunes with relatively straight slipfaces (a similar interpretation is given for Lower Triassic dune sands in the Holy Cross Mountains; Gradzinski et al., 1979). Probably transverse ridges of barchan dunes (Inman et al., 1966) may sometimes also have existed. Due to the supply of large amounts of sand, as is also reflected by fluviatile deposits intertonguing with aeolian sands, transverseridge dunes are thought to have been the main constituents of the Eifel sand sea. There are no hints of linear seif dunes. Blowout and parabolic dunes may not develop slipface deposits and may be composed almost entirely of topset deposits (Ahlbrandt and Fryberger, 1980}; the predominance of steeply dipping foreset deposits leads to the conclusion that these dune types might only occur rarely and locally, if at all. Concave
205

complexes of 10--20 m in height and at least 80--100 m in lateral extent. In these sequences, foresets are occasionally cut by internal erosion surfaces, dipping at low angles (compare excavations of Recent dunes by McKee, 1966 and Bigarella et al., 1969). Internal erosion surfaces in aeolian sands are comparable to reactivation surfaces of fluviatile bars and probably are the products of reversed or transverse wind episodes (Turner, 1980), or they originated by eddying winds which carried sand along the shape of the dunes (Walker and Harms, 1972). It is assumed that most dune sand sequences in the Eifel Middle Buntsandstein are built up of such complexes of cosets; however, there are only a few outcrops exposing aeolian sands in such a lateral extent that the complete complex could be observed. Therefore, it is not possible to determine whether only basic barchanoid-type dunes formed during the Middle Buntsandstein in the Eifel, or if they were partly combined into compound or complex dune types (after McKee, 1979). The predominant amount of cross-bedded aeolian sands consists of steeply dipping foreset beds. Gently windward inclined backset beds are only occasionally preserved; this is due to their susceptibility to reworking (McKee, 1934; Reiche, 1938). Backset beds originate when the available sand load exceeds the amount of sand that the wind can move up the dune slope (according to McKee and Bigarella, 1979a). Foresets usually dip up to 25--30 ° , occasionally up to 40 ° (maximum angle in aeolian foresets is 42°; Land, 1964). The angle of repose (maximum dip) in dry sand, however, is 34 ° (Bagnold, 1941; Burkalow, 1945; McKee and Bigarella, 1979b). Therefore, some moisture (at least some dew) should have been present to conserve foresets dipping with angles exceeding 34 ° (compare Burkalow, 1945; McBride and Hayes, 1962). In most cases, however, dip angles are about 25--30 ° . In ancient dune deposits, a maximum angle of dip consistently lower than in m o d e m dry sand has been attributed to compaction (Glennie, 1972; Walker and Harms, 1972; Hubert and Mertz, 1980), to removal of unstable grains or to differential preservation of cross-stratification sets (that is, the upper parts of foresets being relatively higher angled, they are commonly removed by erosion and planation; Poole, 1962; Walker and Harms, 1972). According to widespread erosion, the latter possibility (besides a certain influence of compaction) is considered to be mainly the reason why dip angles in Middle Buntsandstein dune sands are often not as steep as is usual in Recent aeolian sediments. Foreset beds with moderate dip angles (less than 20 ° ) might be compared with low-angle aeolian deposits described by McKee and Tibbitts (1964), McKee and Moiola (1975) and Fryberger et al. (1979), or represent downwind topsets (Ahlbrandt and Fryberger, 1980). High-angle and low-angle sands could be interpreted as sediments from the base and the top of the dunes (Nagtegaal, 1979). Because of widespread reworking, complete dune sequences including slip-

206 face deposits, downwind topsets and upwind topsets {Ahlbrandt and Fryberger, 1980) are only rarely preserved. Horizontal-laminated aeolian sands originated as sand sheets in interdune areas (Ahlbrandt, 1975; Fryberger et al., 1979; Clemmensen, 1978, 1979; Ahlbrandt and Fryberger, 1980), probably at greater wind velocities. Thus, as in aquatic environments {Allen, 1964; Simons and Richardson, 1962), horizontal-bedding represents at least partially higher energies than crosslamination. Interdune areas range from long narrow corridors between barchanoid ridges to very wide, flat surfaces. Narrow spacing of barchanoid ridges and thus rapid change from dune to interdune areas in the Eifel Buntsandstein is reflected by close intertonguing of cross-bedded dune sands and horizontallaminated aeolian sheet sands in many outcrops. Alternating horizontal laminae, high-angle and low-angie foresets and occasionally preserved topset deposits reflect the variation of wind velocities during deposition of the Middle Buntsandstein. These variations are in most cases of random distribution and do not give rise to cyclicity in aeolian sands. Some interdune sediments probably were originally deposited by the same wind activity that built up the dunes (McKee and Moiola, 1975; compare also Lupe and Ahlbrandt, 1979) and are commonly reworked by water and wind (Glennie, 1970). In interdune areas between dune complexes of considerable height (10-20 m), small-scale, solitary-set dunelets occasionally accumulated. The intercalation of isolated cross-bedded dune sand bodies into successions of horizontal-laminated aeolian sheet sands {Plate I-4) reflects episodical paroxysms of wind velocity. Horizontal, planar upper erosional surfaces (Plate I-1 and I-4) indicate water table rise after dune accumulation, resulting in preservation of the water-saturated lower part and later deflation of the dry upper part of the dune {Stokes, 1968), a process responsible for the origin of almost all erosional boundaries between individual cross-bed sets. Small-scale oscillations of water-table level in interdune areas sometimes led to alternation of horizontal-laminated aeolian sheet sands deposited on dry surfaces, adhesion-rippled layers which originated when sand was blown across damp surfaces, and sandy/silty-clayey lacustrine deposits which were laid down in very shallow water forming an extensive lake. Especially adhesion-ripple-lamination is often taken as reliable indication of aeolian environment and has been numerously described (Glennie, 1970, 1972; Nemec and Porebski, 1977; Plein, 1978; Clemmensen, 1979, 1980). It is formed by catchment of sand which is blown across a damp surface (Reineck, 1955; Hunter, 1977). Non~lune aeolian sands as reported by Tanner {1980) have not been found. Aeolian dust deposits, from ancient formations reported by Oesterlen (1979) from the Karroo System and assumed by various authors in the Old

207

Red of England (Prof. Dr. J.R.L. Allen, written commun., 1980), are also absent in the Eifel Buntsandstein. Penecontemporaneously, aeolian bedding was deformed by slip-face avalanching of foresets and vertebrate footprints in horizontal-laminated sands. Sometimes, small faults of little throw were formed by sliding or slumping in loose aeolian sands. At the base of the dunes, aeolian lag deposits occasionally originated by deflation of the sand-sized population of pebbly fluviatile sediments. Such lag deposits occur on serifs and interdune surfaces of various modern dune fields (McKee and Tibbitts, 1964). Unusually coarse sand grains are also concentrated in layers or lenses. The latter accumulated in small outwash channels or tiny hollows that were formed by vertebrate footprints or impact of coarse hailstones. Heavy ephemeral rainfalls or channel-shifting of the braided rivers led to aquatic redeposition of dune sands. The lower erosional boundary of the massive strata without distinct bedding often irregularly truncates into the aeolian sands or older relaid dune deposits. Rapid water soaking in the substratum frequently favoured sedimentation of nearly structureless sands (Gradzinski et al., 1979). Ephemeral rainfall also led to the genesis of shallow lakes in depressions between the dunes. In quiet water, thin clayey lacustrine sediments settled out. Aeolian sands occur occasionally as thick uniform sequences of up to some tens of meters thickness. More often, however, water-laid deposits are intercalated into the dune sand succession. Besides the above-mentioned aquatically relaid aeolian sands and lacustrine clayey deposits, fluviatile sediments are present. They consist in most cases of bar deposits with or without pebbles. To a minor amount, well
208

between the modes in contrast to continuous angular spread of -+90° in some roses, compare Mader, 1980c) and reversing dunes {modes 180 ° apart; McKee, 1979; Turner, 1980) have not been found in individual dune complexes. The dune sands were deposited by northern hemisphere trade winds in a low palaeolatitude (compare Bigarella, 1972; Robinson, 1973; Clemmensen, 1978, 1979). According to the palaeowind directions, the dunes accumulated predominantly under southeasterly and southwesterly trade winds in summer when the intertropical convergence zone was shifted to the north (Mader, 1980c). Identical palaeowind directions suggest similar depositional environments for Lower Triassic dune sands at the Eastern margin of the Mid-European Triassic Basin (Gradzinski et al., 1979). The absence of bimodal palaeoroses pointing to bidirectional winds leads to the conclusion that, contrary to the results of Clemmensen {1978, 1979) only southeasterly and southwesterly trade winds might have been capable of aeolian sand transportation and thus to initiate dune migration. However, the absence of deposits which could be correlated with northwesterly and northeasterly trade winds which blew in winter when the intertropical convergence zone was shifted to the south requires further explanation. In the Eifel during Middle Buntsandstein time, aeolian sands have not formed as isolated dunes but built up large sand seas (compare McKee, 1979; Turner, 1980) which were intersected by braided to anastomosing river channels accompanied by narrow or wide (depending on channel sinuosity and spacing) floodplains. Shallow lakes originated in interdune areas of the sand seas.

Aeolian sands occur throughout the whole Eifel area in the Middle Buntsandstein. Quantitative distribution, interfingering with fluviatile deposits and composition of associated alluvial sediments, however, are different in the Northern, Western and Southern Eifel. DISTRIBUTION OF AEOLIAN MIDDLE BUNTSANDSTEIN

AND

FLUVIATILE

SEDIMENTS

IN T H E

EIFEL

Description The Middle Buntsandstein of the Eifel is most completely developed in its southern part. In the Western and Northern Eifel, the upper part of the sequence present in the Southern Eifel overlies the Variscan basement with angular unconformity.

Southern Eifel In the Southern Eifel, aeolian sands do not occur in the Trifels-Schichten. The entirely fluviatile sequence is nearly exclusively built up of stacked conglomeratic bar deposits. Cyclothems are only rarely completely preserved, but are not capped by aeolian sands. In the Rehberg-Schichten, occasionally fluviatile cyclothems finish up with thin aeolian sands.

209 In the Karistal.Schichten, contrary to the inferior members of the Middle Buntsandstein, aeolian sands occur widespread, mainly as sequences of 5-20 m thickness which are separated by fluviatile bar deposits (sections 1 and 2 in fig. 4). These alluvial successions of 5--15 m thickness are built up exclusively of stacked omikron- and pi-cross-stratified pebble-bearing bar deposits; the complexes are named Felszonen (set up by Thtirach, 1894). Maximum size of the numerous pebbles rarely exceeds 5 cm, average size is about 2 cm. The bar deposit series are occasionally truncating in channels up to 5 m deep into the aeolian sequences. The multistorey channel sedim e n t complexes have been mapped out in parts of the area by Degen et al. (1980). The intervals between fluviatile bar deposit complexes rarely consist of aeolian sands only. In most cases, some fluviatile cyclothems are intercalated which are built up of pebble-poor or pebble-free bar deposits and sandy-silty, partially bioturbated topstratum sediments. Clayey-silty topstratum deposits are only occasionally preserved. Western Eifel In Western Eifel, distinction is to be made between the northern Trier area, the Oberbettingen area and the Stadtkyll area. In the northern Trier area (section 4 in Fig. 4), Middle Buntsandstein succession is predominantly composed of aeolian sediments. Fluviatfle deposits are intercalated occasionally as pebble-free; alpha-, omikron- or pi-cross-stratified bar deposits of up to a few meters thickness. Only locally, well
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Summarizing, throughout the Western Eifel area aeolian sediments predominate and fluviatile deposits are of minor importance. In contrast in the Southern Eifel, the amounts of aeolian and fluviatile sediments in the upper Middle Buntsandstein succession are nearly equal.

Northern Eifel In the Northern Eifel, the composition of the Middle Buntsandstein succession is strikingly different from the more southerly regions. Aeolian sediments are of minor importance in the Northern Eifel. They are restricted to the southeastern part of the Mechernich area (section 7 in Fig. 4), and occur as a single sequence of 5--20 m thickness. Sometimes, they contain smallscale fluviatile intercalations. The Middle Buntsandstein succession in Northern Eifel is characterized by coarse massive conglomerates which are interpreted as channel deposits in braided rivers (Hendriks and Schrader, 1980; Mader, 1982a). The conglomerates contain numerous cobbles and pebbles up to 15 cm size; boulders of 20--30 cm diameter are also present. Such coarse conglomerates are u n k n o w n from the Middle Buntsandstein of b o t h the Western and Northern Eifel; similar rocks only occur in the lower part of the Upper Buntsandstein in the Stadtkyll area of Western Eifel (Mader, 1981a). In the Northern Eifel, these coarse conglomerates are widespread and build up successions of some tens of meters thickness. In other cases, cyclothems are preserved which consist of coarse conglomeratic channel deposits; pebble-bearing, pebble-poor or pebble-free, omikron- or pi-cross-stratified bar deposits and sandy and/or silty~clayey topstratum deposits. F i n e , r a i n e d topstratum deposits are in most cases eroded, sandy bar deposits are more often conserved. In the southeastern part of the Mechernich area, occasionally aeolian sands of various thickness (30 cm to 8 m) terminate the cyclothems.

Fig. 4. Distribution o f aeolian sands in the u p p e r part (Karistal-Schichten) o f the Middle Buntsandstein in the Eifel (Germany). Representative, idealized sections have been synthesized f r o m all field observations and are inserted into the map. L o c a t i o n o f sections: 1 and 2 = S o u t h e r n Eifel, southeastern part o f S o u t h e r n Trier area; 3 = Western Eifel, s o u t h w e s t e r n margin o f N o r t h e r n Trier area; 4 = Western Eifel, central part o f N o r t h e r n Trier area; 5 ffi Western Eifel, O b e r b e t t i n g e n area; 6 = Western Eifel, Stadtkyll area; 7 ffi N o r t h e r n Eifel, southeastern part o f Mechernich a r e a ; 8 ffi N o r t h e r n Eifel, western part o f Mechernich area. Legend o f sections 1 to 8 . 1 = coarse conglomerates, massive o r crudely cross-bedded (facies o n l y d e v e l o p e d in the N o r t h e r n Eifel); fluviatile channel deposits. 2 = pebble-bearing sandstones, o m i k r o n - or pi-cross-stratified; fluviatile bar sediments. Pebble c o n t e n t ranging f r o m solitary to n u m e r o u s . 3 = pebble-free sandstones; alpha-, o m i k r o n or (rarely) pi-cross-bedded; fluviatile bar deposits. 4 = aeolian sand, horizontal-laminated o r cross,stratified. B a r c h a n o i d - t y p e dune deposits or i n t e r d u n e sand sheets. 5 = aeolian sands with intercalations o f thin clayey sediments (lacustrine i n t e r d u n e deposits). 6 = aquatically relaid aeolian sands (by heavy e p h e m e r a l rainfall or fluviatile incursion). 7 = fiuviatile t o p s t r a t u m deposits, sandy a n d / o r silty-clayey, horizontal-laminated or lowangle cross-bedded, rarely ripple-laminated. Legend o f m a p : thick d o t s = Buntsandstein, white = older and y o u n g e r formations.

212

Outside the southeastern part of the Mechernich area, aeolian sands are completely absent. The Middle Buntsandstein consists exclusively of fluviatile deposits (section 8 in Fig. 4). Thus, quantitatively, the Middle Buntsandstein sequence in Northern Eifel is built up predominantly of fluviatile sediments.

Interpretation Aeolian sands occur in various amounts and in different associations with fluviatile deposits in the Southern, Western and Northern Eifel. Additionally to regional diversification, distinction is to be made stratigraphicaUy in the Southern Eifel. The composition of the Middle Buntsandstein sequence in the Southern Eifel reflects an evolution of fluviatile depositional environment (Mader, 1981f) at the Western margin of the Mid-European Basin also incorporating the occurrence and distribution of aeolian sediments. Dune sands are not distributed randomly in the Middle Buntsandstein succession, but occur at a distinct stage of evolution of fluviatile deposition. Aeolian sediments appear in the final phase of an evolution which is characterized by increasing channel spacing and sinuosity, diminishing supply of coarse detritus from the source areas, and decreasing braiding of the river systems. Evolution led in the Middle Buntsandstein from local alluvial fans via cobbly and pebbly braided rivers to sandy braided streams and finally to an intertonguing of aeolian dunes with braided to anastomosing rivers matching with the climax of regional diversification of depositional environment. Following a recurrence in fluviatile style at the boundary Middle/Upper Buntsandstein, mainly caused by tectonic activity in the source areas and change from arid to semiarid climate, in the Upper Buntsandstein again an evolution from pebbly braided rivers to sandy braided and anastomosing streams took place. In the Upper Buntsandstein, however, no dune field developed, but increasing sinuosity of river channels, once more interrupted by a last minor recurrence, finally led to meandering streams, passing into deltaic environment which graded into shallow marine milieu at the end of the Buntsandstein. An evolution of the fluviatile depositional environment in parts of the Mid-European Buntsandstein sequence is also reported by Durand (1978) and Tietze (1981). A similar evolution as outlined above from the Eifel has been recognized in Spanish Buntsandstein (Dr. A. Arche, Dr. M. Marzo, Dr. A. Ramos and Dr. A. Sopena, pers. commun., 1981).

Trifels-Schichten In the Trifels-Schichten, the southern source areas supplied large amounts of coarse detritus which accumulated in low-sinuosity, multi~hannel, highlybraided river systems. Strong erosion during rapid shifting of narrowly spaced channels, accompanied by slow subsidence, often resulted in considerable reworking of fine-grained topstratum deposits laid down in narrow floodplains. As a result, thick sequences of multistorey conglomeratic bar deposits were piled up. Fluviatile cyclothems are only rarely completely preserved, but do not finish up with aeolian sands.

213

Rehberg-Schich ten In the Rehberg-Schichten, the supply of coarse detritus from the southern source areas ceased. Large floodplains developed due to increasing channel spacing and sinuosity and decreasing braiding. Cyclothems were often completely preserved due to weaker erosion and more continuous and faster subsidence. Occasionally, emergence of parts of the floodplains and extraordinary low water levels in the channels resulted in local deflation and accumulation of thin aeolian sands at the top of the cyclothems. Karlstal-Schichten In the Karlstal-Schichten, Buntsandstein deposition was no longer restricted to the Southern Eifel, but continuing onlap on the Devonian basement finally led to an extension of sedimentation throughout the Eifel, accompanied by an extraordinarily well
214

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215 resulted in complete reworking of fine-grained topstratum deposits, thus piling up thick sequences of stacked bar sediments. Aeolian sands only were deposited in a restricted area near a swell of the pre-Triassic continent surface. Thus, the final Phase of evolution of the Middle Buntsandstein fluviatile sedimentary environment is highlighted by a climax of regional diversification of depositional milieu reflecting the influences of basement morphology on fluviatile style and permitting one to recognize the distribution of aeolian sands in the Eifel and, resulting from this, to reconstruct a palaeogeographical model. Aeolian sands in the Karlstal-Schichten have not been deposited as local dunes, but built up a large sand sea at the western margin of the Mid-European Basin. The dune belt reached from the Southern Eifel via the Western Eifel to the southeastern part of the Northern Eifel, covering an area of nearly 2500 km 2 at the time of its largest extension. To conclude from the widespread occurrence of aeolian sands also in the Pfalz and Saar areas (Prof. Dr. W. Dachroth, pets. commun., 1978; compare also Dachroth, 1980), aprobable connection of the Eifel sand sea with the Saar-Pfalz dune belt (if not separated by an entirely alluvial zone surrounding the Sierck Swell) would imply an extension of at least 5000 km 2. The palaeogeographical model (Fig. 6) for t h e Karlstal-Schichten (the only member present throughout the Eifel) is mainly influenced by the morphology of the pre-Triassic continental surface. Additionally to the western and eastern margins of the north--south-striking depression zone which limit the sedimentation area, several intrabasinal swells occur: the Sierck Swell (Schall, 1968) in the Saar area, the Deimlingen Swell (Weiler, 1972) in the Southern Eifel, the Kalenborn Swell (Mader, 1976) in the Western Eifel and the Kalimuth Swell (SchrSder, 1954) in the Northern Eifel. These highs were completely covered in the late Upper Buntsandstein or even in the succeeding Muschelkalk. Aeolian and fluviatile sediments contribute in different amounts to the composition of the Karlstal-Schichten sequence, but intertongue in the whole area. The dune belt ended in the Northern Eifel; the intertonguing aeolian and fluviatfle sequence passed into an entirely alluvial succession. The grain size of fluviatile deposits diminished continuously in northerly direction, thus grading via the Lower Rhine and Ems area (corn-

Fig. 5. Regional diversification of depositional environment in the upper part (KarlstalSchichten) of the Middle Buntsandstein (Lower Triassic) in the Eifel (Germany). Legend of facies models: 1 = Devonian basement; 2 = Basisbildungen (Basiskonglomerat; early Buntsandstein deposits), alluvial-fan and mud-flow sediments; 3 ffi cobbly and pebbly fluviatile bar deposits; 4 = pebbly fluviatile bar sediments; 5 = sandy fluviatile bar deposits; 6 = sandy and/or silty-clayey fluviatile topstratum (floodplain) sediments; 7 = sandy fluviatile topstratum (crevasse-splay) deposits; 8 ffi clayey lacustrine interdune sediments; 9 = aeolian dune sands (barchanoid-type dunes and interdune sand sheets). Legend of map: black = Buntsandstein; white = older and younger formations.

216

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pare also Wolburg, 1956) into the fine-grained sediments of the central basin in northwest Germany. Thus, the reconstruction of depositional environment in the Eifel Middle Buntsandstein is to some extent comparable to the palaeogeographical model of Rotliegendes sedimentation in the Northwest German Basin drawn by Plein (1978, 1979). The Permian Rotliegendes succession, characterized by widespread occurrence of aeolian sands (Glennie, 1972; Lutz et al., 1975; Marie, 1975; Van Veen, 1975; Dachroth, 1976; Nairn and Smithwick, 1976; Ellenberg et al., 1976; Plein, 1978; Glennie et al., 1978; Brookfield, 1978, 1980; Nagtegaal, 1979; Falk et al., 1979; Wijhe et al., 1980), therefore seems

217

to be a terrestrial sequence of partially similar palaeogeography, sedimentology and genesis in comparison with the Middle Buntsandstein at the western margin of the Mid-European Basin. DISTRIBUTION OF BUNTSANDSTEIN

AEOLIAN

SANDS

IN

MID-EUROPEAN

MIDDLE

Description Dune sands in the Middle Buntsandstein have been reported from several parts of the Mid-European Basin (Fig. 7). Apart from the Eifel, they occur in the Pfalz and Saar areas (Prof. Dr. W. Dachroth, pets. commun., 1978; Dachroth, 1980), at the eastern margin of the Rhenish Massif (Dr. K.W. Tietze, pets. commun., 1980), in the Hessian Depression (Wycisk, 1977, 1978, 1981), in Helgoland (Clemmensen, 1979; Wunderlich, 1979), in Jutland/ Denmark (Pedersen and Andersen, 1980) and in the Holy Cross Mountains/ Poland (Gagol and Karpiniec, 1974; Gradzinski et al., 1979). In some marginal regions of the German Basin, however, they have up to now only been found locally or not at all; examples to mention are especially the Vosges (Prof. Dr. J.C. Gall, pets. commun., 1980), Thuringia (Dr. E. Grumbt, written commun., 1981; compare also Grumbt, 1974), Eastern Bavaria (Dr. F. Leitz, pers. commun., 1981; Dipl.-Geol. T. Teyssen, pers. commun., 1981) and Sudeties/Poland (Dr. J. Mroczkowski, pets. commun., 1981). Triassic dune sands outside the Mid-European Basin (which extends in north--south-direction from Scandinavia to Southern Germany and in west-east-direction from France--Luxemburg to Poland, compare Fig. 7) occur in England (Thompson, 1969; Piper, 1970; M.A. Huque, pets. commun., 1981), Greenland (Clemmensen, 1978, 1980, 1981}, Colorado (Poole, 1962), Nova Scotia (Hubert and Mertz, 1980) and Swaziland (Minter and Turner, 1981). From Spain, however, they have n o t yet been reported (Dr. A. Arche, Dr. M. Matzo, Dr. A. Ramos and Dr. A. Sopena, pets. commun., 1981). In the Mid-European Basin, dune sands occur in both proximal (marginal) and distal (central} parts. Especially at the western margin, a large dune field originated in the Middle Buntsandstein that probably reached from the Northern Eifel via the Western Eifel, the Southern Eifel and the Saar area to the Pfalz region, perhaps only divided by the largest swell of the pre-Triassic continental surface, the Sierck Swell (Schall, 1968). The southern and eastern ends of the sand sea off the Pfalz area, however, are still unknown; anyway, the dune belt did n o t extend to the Vosges in the south and to Black Forest and Odenwald in the east (Fig. 7). Northerly, the intertonguing of aeolian and fluviatile deposits graded into an almost entirely alluvial succession in the Northern Eifel which led by continuously decreasing grain size via the Lower Rhine and Ems area (compare also Wolburg, 1956) to the central basin in Northwest Germany. Smaller sand seas are thought to have developed at the eastern margin of

218

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Fig. 7. Occurrence of aeolian dune sands in the Mid-European Triassic Basin (map redrawn after Grumbt, 1974, and Hoppe, 1976). Legend: 1 = Pre-Buntsandstein basement (in most cases folded sedimentary or igneous and metamorphic Variscan or pre-Variscan rocks); 2 = Buntsandstein depositional area; 3 = occurrences of dune sands; 4 = areas where aeolian sediments have up to now only been found locally or seem to be missing completely. Areas specified in the map. 1 = Eifel (Mader, 1980b, 1981d, 1982a); thick dots = area of proven dune belt extension; thin dots = area of possible continuation of aeolian sand sea to Pfalz and Saar; 2 = Pfalz and Saar area (Prof. Dr. W. Dachroth, pers. commun., 1978; Dachroth, 1980); 3 = eastern margin of the Rhenish Massif around Marburg (Dr. K.W. Tietze, pets. commun., 1980); 4 = Hessian Depression (Wycisk, 1977, 1978, 1981); 5 = Helgoland (Clemmensen, 1979; Wunderlich, 1979); 6 = Jutland/Denmark (Pedersen and Andersen, 1980); 7 = Holy Cross Mountains/Poland (Gagol and Karpiniec, 1974; Gradzinski et al., 1979). A: Vosges (Prof. Dr. J.C. Gall, pers. commun., 1980); B = Eastern Bavaria (Dr. F. Leitz, pets. commun., 1981; Dipl.-Geol. T. Teyssen, pers. commun., 1981); C: Thuringia (Dr. E. Grumbt, written commun., 1981; compare also Grumbt, 1974); D: Sudeties (Dr. J. Mroczkowski, pets. commun., 1981).

the Rhenish Massif which represents an area largely not having been covered with Buntsandstein sediments. Basinwards, this dune field probably extended to the middle or northern part of the Hessian Depression, where finally the alternating aeolian and fluviatile sequence graded into a completely water-laid succession similarly to the Northern Eifel at the western margin of the Mid-European Basin. Aeolian sands not only accumulated at the western margin but also at the eastern margin of the Mid-European Basin, as is proved by the occurrence

219

in the Holy Cross Mountains in Poland (compare Gradzinski et al., 1979). The basinwards extension of these dune sands, however, is as yet unknown. The absence or rarity of aeolian sediments in the proximal fluviatile Buntsandstein sequence of Sudeties (Dr. J. Mroczkowski, pets. commun., 1981) probably indicates that the origin of dune sands depended on certain favourable conditions in the depositional area. The same applies for the entirely fluviatilesuccession in Eastern Bavaria. Thin dune sands in fluviatile successions are the only aeolian sediments reported up to n o w from the Danish E m b a y m e n t (Pedersen and Andersen, 1980), the basin south of the Fennoscandian Shield (from which the detritus was transported southwards by rivers contrary to northern palaeocurrents in the German Basin), divided from the larger Mid-European Basin by the Ringk~bing-Fyn-High (compare Larsen and Friis, 1975; Ziegler, 1979). In this area, Triassic rocks do not crop out at the surface, but are buried under thick younger strata. Accessibility only in deep boreholes, however, renders facies interpretation more difficult. Contrary to the relatively widespread occurrenee of dune sands in marginal areas, aeolian sediments have up to n o w only been found locally in the fine-grained playa/lacustrine to marine sequence in the central part of the basin. The Kater sands from Helgoland (Clemmensen, 1979) represent thin intercalations of aeolian sands into the silty~layey shallow-water succession.

Interpretation The distribution of aeolian sands in the Mid-European Basin may be preliminarily summarized by the following palae0geographical model: In the more or less coarse~lastic alluvial successions at the margins of the sedimentation area and near large intrabasinal swells, aeolian sands often occur either as thin intercalations capping fluviatile cyclothems or as thick complexes formed as a dune field. Aeolian sand seas were built up of barchanoid-type dunes accumulated in unimodal southwesterly to southeasterly trade winds at a low northern palaeolatitude in summer when the intertropical convergence zone was shifted to the north. Thin dune sands capping fluviatile cyclothems have been deflated during low-water stages from river channels and blown into emergent floodplains. Towards the central part of the basin, the intertonguing of aeolian and fluviatile sediments passed into an almost entirely alluvial succession. Continuous grain~ize decrease of waterlaid sediments finally led from proximal fluviatile to distal playa/lacustrine to marine sequences. Thin intercalations of aeolian sands into the finegrained, silty~layey lacustrine/marine succession have been blown basinwards in times of emergence of the shallow aquatic depositional environment. Given favoumble conditions, relatively slow shifting of moderately to widely spaced fluviatile channels, emergence of floodplains, low water stages in the channels and a certain subsidence, aeolian sands may have accumulated in numerous marginal parts of the Mid-European Basin. If, however,

220

rapid shifting of narrowly spaced channels resulted in considerable reworking of fine-grained topstratum deposits and probably also aeolian sands of greater or lesser thickness, thus piling up thick sequences of stacked, multistorey bar deposits, either aeolian sands have been completely eroded after deposition or never could accumulate due to dominant fluviatile sedimentation. This may explain the absence of widespread dune sands especially in proximal multistorey channel sequences (particularly applying for the successions in Eastern Bavaria and the Sudeties), as is also highlighted by the restricted distribution of aeolian sands in the Northern Eifel. A concurrent explanation is considered for palaeosol formation and distribution in the Upper Buntsandstein. Formations of palaeosols capping fluviatile cyclothems similarly required emergence of floodplains and low water stages in the channels, that are the criteria necessary to give rise to deflation and accumulation of aeolian sands. The problem of the occurrence and distribution of aeolian sands in the Mid-European Basin to be solved by future investigations may probably lead to further palaeogeographical similarity with Rotliegendes deposition in Northwest Germany, as it is possible to imagine that in at least some marginal areas of the Mid-European Triassic Basin, the basement was surrounded by a fringe of entirely alluvial sediments distally passing into a dune belt or an intertonguing of aeolian and fluviatile deposits. The palaeogeographical model constructed for the Eifel, which represents only a small section of the western margin of the Mid-European depositional area, preliminarily also fits well for the entire basin. This concurrence most clearly delineates the key position of the Eifel for reconstruction of Buntsandstein sedimentary environment and palaeogeography in Middle Europe. CONCLUSIONS

(1) Aeolian sands of widespread occurrence in the Middle Buntsandstein (Lower Triassic) continental red beds are first reported from the Mid-European Basin. (2) Aeolian sands in the Eifel are associated with fluviatile deposits and, to a minor extent, also with water-relaid dune sands and lacustrine sediments. (3) The dunes built up a large sand sea at the western margin of the MidEuropean Basin covering an area of nearly 2500 km 2 at the time of its largest extension. Probable connection of the Eifel sand sea with the dune belt of the Saar-Pfalz area would imply an extension of at least 5000 km 2. (4) Aeolian sands originated as barchanoid-type dunes in unimodal southeasterly to southwesterly trade winds in low northern palaeolatitudes in summer when the intertropical convergence zone was shifted to the north. Additionally to cross-bedded dune sands, horizontal-laminated sediments originated as aeolian sheet sands in interdune areas. Cross-bedded sands were mainly deposited b y grainfall on lee slopes of dunes, horizontal-laminated sands are partially built up of subcritically climbing translatent stratification pointing to wind ripple migration (Hunter, 1977).

221

(5) The Lower Triassic continental Buntsandstein sequence in the Eifel is to be classified as an arid~limate aeolian-fluviatile association according to Turner (1979). (6} Aeolian sands often occur widespread at the margins of the Mid-European Basin and near larger intrabasinal swells forming dune fields which intertongue with braided to anastomosing river systems. Basinwards, the alternating sequence grades into an entirely alluvial succession, which leads by continuously diminishing grain size to the central playa/lacustrine to marine sediment series. Thin aeolian sands sometimes occur as intercalations into the fine-grained, silty~layey lacustrine to marine sequence representing the most distal aeolian sediments in the Mid-European Basin. (7) The palaeogeographical model for the Eifel which represents only a small section of the western margin of the Mid-European sedimentation area also fits well for the entire basin. Additionally, the Eifel Buntsandstein sequence displays an evolution of fluviatile sedimentary environment also including the occurrence and distribution of dune sands. (8) Aeolian sands in the Eifel occur in the final phase of an evolution of fluviatile sedimentary environment, leading from local alluvial fans via cobbly and pebbly braided rivers to sandy river systems and finally to an intertonguing of aeolian dunes and braided to anastomosing streams, matching the climax of regional diversification of depositional milieu. (9) Palaeogeographic models of the Upper Rotliegendes in the Northwest European Basin and Middle Buntsandstein in the Mid-European Basin are partially similar. Lateral facies differentiation from basement at the margin of the sedimentation areas to the central part of the basin is in both formations from a proximal coarse alluvial fringe and/or dune belt via a zone of aeolian and fluviatile deposits to the distal fine-grained playa/lacustrine to marine sequence sometimes containing evaporites. (10) Concurrence of the Middle Buntsandstein palaeogeographic models for both the Eifel and the whole German Basin most clearly delineates the key position of the Eifel for interpretation of the Buntsandstein sedimentary environment and the palaeogeography in Middle Europe. ACKNOWLEDGEMENTS

This paper sums up all my investigations on the Middle Buntsandstein aeolian sands both in course of preparation of my Ph.D. thesis carried out at the Department of Geology and Palaeontology of Heidelberg University, Federal Republic of Germany, under supervision by Prof. Dr. G. Fuchs (Landessammlungen fiir Naturkunde, Karlsruhe) and Prof. Dr. W. Dachroth (Department of Geology and Palaeontology of Heidelberg University), and in subsequent time outside business. Field observations were made in the years 1977--1981. I am especially indebted to Prof. Dr. W. Dachroth, who, being the first to recognize aeolian dune sands in German Middle Buntsandstein (Dachroth,

222

unpubl.) introduced me to Middle Buntsandstein aeolian and fluviatile facies and their intertonguing, and discussed with me various problems concerning Buntsandstein genesis also by showing me his own unpublished data. Without his guidance, this study would not have been possible. I am also indebted to Prof. Dr. G. Fuchs who proposed the interesting Eifel area; Prof. Dr. G. Frenzel (Department of Mineralogy and Petrography of Heidelberg University), Prof. Dr. D. Heling and Prof. Dr. G. Miiller (both Department for Sediment Research of Heidelberg University) who helped me in examining thin sections; and Prof. Dr. M. Kirchmayer (Department of Mineralogy and Petrography of Heidelberg University) who gave hints concerning interpretation of palaeowind and palaeocurrent data. During my comparative studies of Middle Buntsandstein sediments in other parts of the Mid-European Basin, I profited from guidance in the field by Dr. K.W. Tietze (Department of Geology and Palaeontology of Marburg University) at the eastern margin of the Rhenish Massif, and Dr. J. Mroczkowski (Department of Geological Sciences of the Polish Academy of Sciences, Wroclaw/Poland) in the Sudeties, which I gratefully acknowledge. Further, I benefited from discussing my model of the Eifel Buntsandstein evolution of fluviatile sedimentary environment with Dr. A. Arche (Department of Economic Geology of the Madrid University), Dr. M. Matzo (Department of Stratigraphy and Historical Geology of the Barcelona University), Dr. A. Ramos (Department of Stratigraphy of the Madrid University) and Dr. A. Sopefia (Department of Economic Geology of the Madrid University) who outlined me a comparable evolution in the Spanish Buntsandstein not having led to aeolian sand genesis. Additionally, I welcomed discussion with Dr. L. Clemmensen (Department of General Geology of the Copenhagen University) on Lower Triassic palaeowind systems on the northern hemisphere. Finally, I sincerely thank Prof. Dr. R.E. Chapman (Department of Geology and Mineralogy of Queensland University, Australia) and Dr. K.A.W. Crook (Department ~)f Geology of Australian National University, Canberra/ Australia) for critical reading of an earlier draft of the manuscript. The Ph.D. thesis research was supported by a stipendium of the Heidelberg University according to the GraduiertenfSrderungsgesetz (GFG). REFERENCES Ahlbrandt, T.S., 1973. Sand Dunes, Geomorphology and Geology, Killpecker Creek Area, Northern Sweetwater County, Wyoming. Ph.D. thesis,W y o m i n g Univ., 174 pp.; Xerox University Microfilms, no. 73-25,539, A n n Arbor/Michigan. Ahlbrandt, T.S., 1975. Comparison of textures and structures to distinguish eolian environments, Killpecker dune field,Wyoming. Mt. Geol., 12: 61--73. Ahlbrandt, T.S. and Fryberger, S.G., 1980. Eolian deposits in the Nebraska Sand Hills. U.S. Geol. Surv. Prof. Pap., 1120-A: 1--24.

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