Ev i dence for a very low-en ergy flu vial sys tem: a case study from the di no saur-bear ing Up per Tri as sic rocks of Southern Po land
Beata GRUSZKA and Tomasz ZIELIÑSKI
Gruszka B. and Zieliñski T. (2008) — Ev i dence for a very low-en ergy flu vial sys tem: a case study from the di no saur-bear ing Up per Tri - as sic rocks of South ern Po land. Geol. Quart., 52 (3): 239–252. Warszawa.
The Up per Tri as sic suc ces sion in S Po land in which di no saur bones have been found con sists pre dom i nantly of siltstones and claystones.
Three units are dis tin guished. The low er most and the up per most units re flect an al lu vial en vi ron ment, whereas the mid dle one rep re sents lac us trine fa cies. The lower al lu vial unit is in ter preted as a re cord of ephem eral, sin u ous, sus pended-load chan nels with rapid ver ti cal ac - cre tion. Chan nel barforms are lack ing. The en vi ron ment is in ter preted as a low-en ergy anastomosing flu vial sys tem. The clayey mid dle unit is in ter preted as hav ing formed in a wide long-lived lake. The top of the lac us trine de pos its shows signs of vertisol-type pedogenesis, most prob a bly un der sub trop i cal con di tions, with sea son ally-in duced wet and dry in ter vals. The up per unit re flects a low-en ergy me an - der ing river sys tem. Silty point bars were abun dant and the chan nels mi grated freely. The en ergy level of this flu vial sys tem was slightly higher than that of the ear lier one, which is in ter preted as an ef fect of base-level low er ing in com bi na tion with an in creas ingly hu mid cli - mate. The al most ex clu sively silty/clayey al lu vial de pos its rep re sent an ex cep tion ally rare fa cies. The drain age ba sin must have been an ex tremely flat low land. The pres ence of ver te brate bones within the anastomosing and me an der ing river de pos its in di cates that low-en - ergy al lu vial plains were ap par ently fa vour able hab i tats for both rep tiles and am phib i ans dur ing the Late Tri as sic: un der the sub trop i cal, sea son ally dry con di tions, the an i mals must have pre ferred moist low ar eas, i.e. the flood bas ins and aban doned chan nels on the flat val - ley floors.
Beata Gruszka and Tomasz Zieliñski, In sti tute of Ge ol ogy, Adam Mickiewicz Uni ver sity, Maków Polnych 16, PL-61-606 Poznañ, Po - land; e-mail: bgruszka@amu.edu.pl (re ceived: Sep tem ber 19, 2007; ac cepted: June 23, 2008).
Key words: Po land, Late Tri as sic, sedimentology, vertisol, me an der ing river, anastomosing river.
INTRODUCTION
The Up per Tri as sic (Keuper) de pos its of the Ger man Ba sin are de vel oped in Mid dle Eu rope as a thick, re gres sive suc ces - sion. It is widely ac cepted that the epicontinental Muschelkalk sea evolved into a brack ish ba sin, then into a con ti nen tal playa in ter rupted by some ma rine in gres sions. Ex po sures of the Keuper are rare in Po land. One of them is an aban doned open-cast mine of claystones and siltstones near Krasiejów in the Opole Low land, di rectly south of the Ma³a Panew River val ley (Fig. 1A, B). This site has be come fa mous in the last de - cade due to finds of nu mer ous ver te brates, rep re sent ing one of the most im por tant events in Pol ish ge ol ogy at the end of the 20th cen tury. These in clude am phib i ans such as capitosauroids and phytosaurs (the first find of Metoposaurus diagnosticus in Po land) and rep tiles (among oth ers the di no saur Silesaurus opolensis) (Dzik et al., 2000; Dzik, 2001, 2003; Sulej, 2002).
The Keuper am phib i ans and rep tiles were found in three
bone-bear ing ho ri zons, which are pres ent in the lower and up - per part of the ex ca vated suc ces sion (Fig. 1C, D).
A re con struc tion of the sed i men tary en vi ron ment of the de - pos its with the bone ho ri zons is the main ob jec tive of this study.
A de tailed sedimentological study of the Krasiejów site was pub lished 30 years ago (Bilan, 1975) and an other pa per has ap - peared re cently (Szulc, 2005), but our study of the depositional en vi ron ment co mes to sig nif i cantly dif fer ent con clu sions.
GEOLOGICAL SETTING AND PREVIOUS STUDIES
The Up per Tri as sic de pos its of Po land are de vel oped as a mo not o nous suc ces sion with out clear marker ho ri zons. All lithostratigraphic cor re la tions are con se quently dif fi cult and still un cer tain. Bilan (1975) sug gested that the lower part of the up per Keuper in the Opole Low land cor re lates with the “Up per Gyp - sum Se ries” (late Carnian), and the up per part (con tain ing the
bone-bear ing sec tion) with the “Lisów Brec cia”. The strati - graphic po si tion of the lat ter is still not clear; it used to be cor re - lated with the Jarkowo Beds or Zb¹szynek Beds, i.e. with the mid or late Norian (Grodzicka-Szymanko, 1971; Mader, 1997).
The fauna en coun tered so far has not pro vided new in sights into the stra tig ra phy of the Krasiejów de pos its. The rep tiles are com pa ra ble to those from the Carnian Red Sand stone For ma - tion, a Pol ish equiv a lent of the Schilfsandstein (Dzik et al., 2000) in the Ger man Tri as sic. The silty/clayey Krasiejów Beds lack sand stones and there fore must be youn ger, cor re lat ing rather with the Norian Drawno Beds. Dzik et al. (2000) and
Dzik (2001, 2003) con sid ered that the Krasiejów bone-bear ing ho ri zons cor re late with the Ger man Untere Bunte Mergel For - ma tion, i.e. with the Carnian, whereas Sulej (2002) cor re lated them with the Drawno Beds, i.e. with the Norian. The lat ter view is sup ported by the lack of gyp sum in ter ca la tions. This cor re sponds well with the opin ion of Deczkowski et al. (1997), that the sed i men ta tion of gyp sum had stopped in the Opole Low land at the Carnian–Norian bound ary. Taken to gether, the chronostratigraphy of the Krasiejów pro file is still un cer tain in de tail, but a cor re la tion with the late Carnian and/or Norian is most likely (Fig. 2).
240 Beata Gruszka and Tomasz Zieliñski
Fig. 1. Geo log i cal set ting of the Krasiejów site
A — po si tion of the Krasiejów site within Po land; B — sche matic ge ol ogy of the Krasiejów area, with out Qua ter nary cover; C — sim pli fied sedimentological log of the Krasiejów sec tions (note the po si tion of the bone-bear ing ho ri zons); D — de tailed map of the out crop with lo ca tion of the stud ied pro files
All spe cies found in Krasiejów (ver te brates, bi valves, crus - ta ceans, foraminifers) lived in a fresh-wa ter en vi ron ment (Dzik et al., 2000; Olempska, 2004), but Zatoñ and Piechota (2003) do not ex clude ma rine brack ish con di tions, as sug gested by the so dium con tent in these beds.
Bilan (1975) in ves ti gated two sed i men tary units in cores from nearby boreholes. He in ter preted the lower one, built mainly of cross-strat i fied claystones and siltstones, as de pos - ited by a me an der ing river. The up per one, built of mas sive claystones and siltstones in ter ca lated with car bon ate-marly con glom er ates, was in ter preted to have been de pos ited in a brack ish sea. The Krasiejów sec tion be longs mainly to the up -
per unit. The re sults of our sedimentological anal y sis are in con flict with Bilan’s in ter pre ta tion of a ma rine or i gin these men tioned de pos its, be cause al lu vial palaeochannel bod ies are found to be abun dant.
Dzik et al. (2000) de scribed the ho ri zons with ver te brate bones as a lake de posit formed dur ing the max i mum phase of a large-scale transgressive cy cle. The trans gres sion must have started with the Schilfsandstein (Fig. 2), which is com monly con sid ered as an al lu vial de pos its. Dzik (2003), how ever, pro - posed that the same strata were formed in a floodplain (i.e. al lu - vial) en vi ron ment. Nev er the less a deltaic or i gin of the bone-bear ing de pos its were still fa voured by some re search ers
Fig. 2. Com par i son of the lithostratigraphy, sed i men tary en vi ron ments and cli ma tic con di tions of the Pol ish and Ger man Up per Tri as sic (based on: Dadlez and Kopik, 1963; Kotlicki, 1973; Haisig et al., 1979; Aigner and Bachman, 1992;
Deczkowski et al., 1997; Mader, 1997; Hornung and Aigner, 2002a, b)
(Olempska, 2004; Sulej, 2005), even though no sedimento - logical ev i dence was put for ward. Re cently, Szulc (2005) car - ried out a sedimentological anal y sis of the Krasiejów sec tion, which he in ter preted as a typ i cal sheetflood fa cies con tain ing de bris-flow ho ri zons, de pos ited as a fan close to tec toni cally- in duced el e va tions.
Any dis cus sion of the Keuper in Po land should take into ac - count the re sults of Ger man re search ers. The Keuper of Ba - varia, Thuringia and Baden–Württemberg is lithologically well doc u mented, and the palaeoenvironments and the stra tig ra phy are known in de tail. The late Carnian and Norian are rep re - sented by two interfingering fa cies: Sandstein–Keuper and Steinmergel–Keuper. The Sandstein–Keuper con tains al lu vial sand stones con sist ing of ma te rial de rived from the Bo he mian Mas sif and trans ported to the Ger man Ba sin. The rivers formed large al lu vial fans. In the hot cli mate, the flu vial sys tems changed down stream grad u ally into shal low, ephem eral sheet - flows (Hornung and Aigner, 2002a). The Steinmer gel– Keuper For ma tion rep re sents the more cen tral part of the ba sin, and is built of playa-type de pos its: siltstones, claystones, dolomites and gyp sum. Dur ing pe ri ods with a warm and hu mid cli mate, the al lu vial fans prograded into the cen tral part of the ba sin. By con trast, the fans re treated dur ing pe ri ods with a hot and dry cli mate, so that the semi-desert area with evaporatic sed i men ta - tion be came en larged (Reinhardt and Ricken, 2000; Hornung and Aigner, 2002b). This cli mat i cally con trolled fa cies mi gra - tion ex tended over an area hun dreds of kilo metres long. Five such ma jor dry/hu mid cy cles have been iden ti fied in the Late Carnian and Norian of Ger many.
PARAMETERS OF PALAEOCHANNELS AND PALAEOFLOWS — METHODS OF ESTIMATION
Lithological data en abled cal cu la tion of three ba sic phys i - cal pa ram e ters, char ac ter iz ing palaeochannels: chan nel li thol - ogy pa ram e ter (M), chan nel sin u os ity (sn), and the ra tio be - tween the width and depth (w/d). The chan nel li thol ogy pa ram - e ter was cal cu lated as the per cent age of silt and clay in the bed and banks of a palaeochannel. The chan nel sin u os ity sn (nondimensional) was cal cu lated from the Langbein and Leopold (1966) for mula, mod i fied by Miall (1975) and later by Reddy and Prasad (1988):
sn = 1/ [1 – (V1 / 252)]2
where: V1 — dif fer ence be tween the modal palaeoflow az i muths cal cu lated from dif fer ent palaeochannels.
The dy nam ics of chan nel flows de pos it ing coarse silt in a rip pled bed con fig u ra tion was in ter preted on the base of palaeohydraulic for mu las. The unit stream power w (W m–2) was es ti mated from equa tions by Bridge and Jarvis (1982) and Allen (1982). Then the value of stream power was con verted to an av er age flow ve loc ity v (m s–1) (cf. Rees, 1966). Due to the small size of the quarry and the poor ex po sure of the de pos its, an es ti ma tion of the chan nel slope was, un for tu nately, im pos si - ble. Thus, other palaeohydraulic equa tions have not been used in this study.
DESCRIPTION OF THE DEPOSITS
Three sed i men tary unit are dis tin guished in the Krasiejów exposure. Their lithological char ac ter is tics are as fol lows.
LOWER UNIT
The lower units ex posed over a ver ti cal dis tance of about 6 m; its base is be low the bot tom of the quarry. Nu mer ous chan - nels char ac ter ize this unit. Four sin u ous chan nels have been mapped (Fig. 3A). The depth of the chan nels ranges from 2 to 4.5 m, and they are 125–150 m wide. The chan nel-fills them - selves are built of coarser de pos its, and are em bed ded within the claystones.
The chan nels are infilled with siltstone beds 15–25 cm thick that al ter nate with thin ner lay ers of claystones. Claystones within the chan nels are of sec ond ary im por tance, al though they make up to 16% of the de pos its (Fig. 4A). The siltstones (38%) and sandy siltstones (45%) com prise the ma jor ity of the de pos - its, and show rip ple cross-lam i na tion (also climb ing-rip ple cross-lam i na tion), hor i zon tal lam i na tion, or are mas sive (Fig. 5A). The claystones and sand stones are al ways mas sive.
Coarse-grained sand stone/con glom er ate basal beds are rare.
Sand stones and con glom er ates make up to 1% of the de pos its only (Fig. 4), and grains >1 mm are ex tremely rare (Fig. 6A).
Suc ces sions of graded beds av er ag ing 20–30 cm thick each have been noted within the chan nel-fills. These beds are sim ple cou plets in which a thicker layer of coarse-grained sandy siltstone with rip ple cross-lam i na tion (TSr) is over lain by a thin layer of claystone with a mas sive struc ture (Mm) or by siltstone with hor i zon tal lam i na tion (Tl) (Fig. 7A). Re verse graded cou plets (Tm ® TSrc) are pres ent as well (Fig. 5A).
Ero sional sur faces are very rare in the chan nel-fills. De pos its with a mas sive struc ture or hor i zon tal lam i na tion dom i nate the up per parts of the chan nel-fills. Claystones are more com - mon in these top parts, and the thick ness of the in di vid ual lay - ers de creases to the top.
The lay ers within the chan nels dip slightly (up to 9°), fol - low ing their basal, large-scale ero sional sur faces, but they are asym met ri cal with re spect to the chan nel axis (Fig. 8A). Rhyth - mic col our al ter na tions be tween beds from red dark brown to sil ver grey are com mon, but these changes do not co in cide with lithological changes or with and sed i men tary struc tures (Fig. 7B). A ho ri zon that is rich in ver te brate bones is lo cated in the basal layer of the low er most palaeochannel.
MIDDLE UNIT
This unit is dom i nated by claystones (Fig. 6A). Its thick ness reaches 6.5 m. It is a mo not o nous in ter val of dark brown mas - sive claystones. No sys tem atic ten den cies in grain-size have been found in this part of the suc ces sion. The fre quency of siltstone in ter ca la tions in creases slightly in the up per most part of the unit.
The up per part of the unit shows nu mer ous de for ma tion fea tures. These are found in a layer 0.8–1.2 m thick. In the lower part of the de formed layer slick en sides are com mon.
These microfaults are up to 1 m long and dip at an gles of
242 Beata Gruszka and Tomasz Zieliñski
25–45° in two com ple men tary sets of planes with op pos ing di - rec tions (Fig. 9A, B, C). Their sur faces are formed by thin laminae of grey clay. Shear striae are pres ent on the sur faces of well-de vel oped slick en sides (Fig. 9D). This de formed zone gradationally passes up wards into mas sive blu ish clay with cal -
car e ous nod ules, that are 3–5 cm in di am e ter (Fig. 9B). The main palaeontological finds of ver te brate bones at the Krasiejów site have been made in the up per most bed of this unit, im me di ately above the de formed layer.
Fig. 3. Palaeochannel gen er a tions of the lower and up per flu vial units
A — four palaeochannels iden ti fied in the lower unit; B — two palaeochannels iden ti fied in the up per unit;
palaeochannel axes marked by ar rows
Fig. 4. Fre quency of lithofacies within the palaeochannels of the lower (A) and up per (B) al lu vial units
Grain-size: M — claystone (mudstone), T — siltstone, TS — sandy siltstone, S — sand stone, SG — grav elly sand stone; struc ture: m — mas sive struc ture, l — hor i zon tal and low-an gle cross-lam i na tion, r — rip ple cross-lam i na tion, rc — climb ing rip ple cross-lam i na tion
UPPER UNIT
The up per unit is 4 m thick. Its li thol ogy is sim i lar to that of the lower unit. Palaeochannels filled mainly by siltstones have been also found within the mas sive or lam i nated claystones.
The main dif fer ence is that the up per unit con tains a sig nif i cant num ber of coarse-grained (sand stone/con glom er ate) de pos its (Figs. 4B and 6A). The coarse-grained beds are most com mon in the basal parts of palaeochannel-fills (Figs. 10 and 11A).
These sand stone/con glom er ate lithofacies con sist of re worked soil-de rived cal car e ous nod ules up to 1 cm in di am e ter (cf.
Szulc, 2005). Be cause of the lim ited ex po sure in the Krasiejów quarry, only two palaeochannel-fills have been stud ied in de tail (Fig. 3B). The depth of the larger one is 3 m. The chan nel bod - ies of the up per unit are sub stan tially wider than those in the lower unit: the width of one of them ex ceeds the length of the wall of the quarry; its to tal width may reach some 300 m. The chan nel base is al most flat (Fig. 8B). A sec ond sub stan tial dif - fer ence be tween the chan nels of the lower and up per units is formed by the shape and dip of the in di vid ual beds. The lay ers fill ing the chan nels of the up per unit dip at steeper an gles (7–13°). They do not par al lel the chan nel bases but con verge with them. All the lay ers dip in the same di rec tion, al most per - pen dic u lar to the chan nel axis (Fig. 11B). The thick nesses of the in clined lay ers are usu ally larger than those in the chan nels of the lower unit; siltstone beds with rip ple cross-lam i na tion are over 0.5 m thick. Re peated suc ces sions of nor mally or re verse graded beds have not been found in the up per unit. Ero sional sur faces be tween in fill ing beds and small-scale de for ma tion
struc tures due to load ing are com mon in the lower parts of the chan nels (Fig. 5B). The youn gest Krasiejów bone-rich ho ri zon lies within the mas sive claystone cap ping the up per chan nel (Fig. 1C).
INTERPRETATION OF THE SEDIMENTARY ENVIRONMENTS
LOWER UNIT
The lower unit was de pos ited in a low-en ergy flu vial sys - tem, typ i cally con sist ing of sus pended-load chan nels, where the ma jor ity of the grains were de pos ited from uni form or graded sus pen sion (Fig. 6B). The chan nel li thol ogy pa ram e ter (i.e. the con tent of silt and clay in the bed and banks of a chan - nel) is es ti mated to be 90%, whereas the ra tio be tween the width and depth (w/d) of the chan nels ranges from 27 to 60.
244 Beata Gruszka and Tomasz Zieliñski
Fig. 5. Sed i men tary logs of the lower (A) and up per (B) units For lithofacies code see Fig ure 4
Fig. 6. Ar eas cov ered by cu mu la tive curves (A) from de pos its of the three units stud ied; Passega di a gram (B) with
the de po si tion types: sus pen sion and sal ta tion, counted for each unit
Based on the tex tural anal y sis of 30 sam ples (10 for each se ries)
The value ob tained for chan nel sin u os ity is sn = 1.27, which puts the en vi ron ment of the unit into the class of mod er ately sin u ous, al though not yet me an der ing rivers (Teisseyre, 1985;
Bluck, 1987).
The most im por tant point re gard ing pre cise in ter pre ta tion of the al lu vial en vi ron ment is the ar range ment of the beds within the chan nels. Ar range ments of the beds that fol low the palaeochannel bases ex cludes the pres ence of point bars as the pri mary chan nel forms. There fore ver ti cal and not lat eral ac cre - tion must have been the main depositional pro cess within the al lu vial chan nels. The lack of ero sional sur faces be tween the beds within the chan nel bod ies shows that the chan nels were rel a tively quickly infilled with sed i ment. This in di cates that the chan nels of the lower unit had an aggradational char ac ter.
The lithofacies of the chan nel-fills in di cates that low-en - ergy bedload de po si tion in the form of rip pled beds (the or i gin of the rip ple cross-lam i nated sandy siltstones TSr) al ter nated with set tling from sus pen sion (the or i gin of the mas sive claystones Mm). The sandy siltstones with climb ing rip ples
TSrc in di cate a high con cen tra tion of fine-grained sus pen sion load in the chan nels. Con sid er ing these lithofacies, the de pos its ana lysed are sim i lar to chan nel al lu vium of the low est reach of the Yel low River, a clas sic sus pended-load river (Van Gelder et al., 1994). In our opin ion, the Krasiejów chan nels had even lower en ergy than the Yel low River, which re sulted in a smaller num ber of fine-grained sand stone in ter ca la tions and an ab sence of rib bons built of fin ing-up wards suc ces sions.
The chan nel flows that de pos ited the coarse-grained siltstones, TSr and TSrc, were char ac ter ized by very low stream power, close to w = 1 W m–2. An av er age flow ve loc ity was v @ 0.1 m s–1. It must be noted, how ever, that the cal cu lated pa ram - e ters char ac ter ize the flood-flow con di tions, which even more clearly stresses the ex tremely low en ergy level of this flu vial en vi ron ment. The lack of any kind of depositional forms larger than rip ples sup ports this palaeohydraulic anal y sis. The char ac - ter is tic al ter na tion of siltstone and claystone beds most prob a - bly re flects a spe cific cli ma tic fea ture: the rhythm of hu mid and dry sea sons. Dur ing the dry sea sons, the chan nel flows un -
Fig. 7. De tail of the lower al lu vial unit
A — graded cou plet of rip ple cross-lam i nated sandy siltstone over lain by hor i zon tally lam i nated siltstone; B — rhyth mic al ter na tion of lay ers from red/dark brown to sil ver/grey in the top part of a palaeochannel infill;
the colours do not cor re spond to the shapes of the sed i men tary struc tures
Fig. 8. Shape and char ac ter is tic fea tures of the in fill ing of the chan nels
A — lower unit: the bed bound aries are nearly par al lel to the ero sional base of the palaeochannel;
B — up per unit: al most flat base of a large palaeochannel-fill with dis tinctly in clined beds
doubt edly de cayed. The al ter na tion of colours from red/dark brown to sil ver/grey in the up per most parts of the rib bons in di - cates that the bed of shal low, al most com pletely sed i - ment-choked chan nels emerged pe ri od i cally and was sub jected to ox i da tion (the red/dark brown col our), whereas the un der ly - ing, still wet stra tum was per ma nently un der gley re duc tion con di tions (sil ver/grey col our) (see Daniels et al., 1971). Ad di - tion ally, the pres ence of mud cracks (Wojewoda, pers. comm.) sup ports the as sump tion that the chan nels be came com pletely dried-up dur ing hot and dry sea sons.
The char ac ter is tics of this lower al lu vial unit are in many re spects con sis tent with those of anastomosing-river de pos its.
The pres ence of nu mer ous chan nel-fills in a rel a tively small area of the ex posed sed i men tary suc ces sion sug gests that the flu vial sys tem was of an anabranching type. The chan nels were rap idly filled with sed i ment, which must have re sulted in fre quent avulsions. The semi-con cen tric chan nel-fills in di cate an ab sence of lat eral mi gra tion. Sed i men ta tion was not con - trolled by the de vel op ment of side bars (such as accretionary benches, point benches or point bars), but was pre dom i nantly ver ti cal, with a slight lat eral com po nent only. The chan nels were sta ble el e ments within the flu vial sed i men tary en vi ron - ment. Lat eral sta bil ity was cer tainly also due to the pres ence of clayey, co he sive banks, which were re sis tant to ero sion by the weak cur rents.
Sim i lar chan nel-fills, lack ing lat er ally accreted bars, are described by Smith and Smith (1980), Rust and Legun
(1983), and Makaske (1998) as char ac ter is tic of anastomosing sys tems. How ever, these “clas si cal” chan nel de pos its are rep re sented mainly by sand stones. The much finer-grained char ac ter of the Krasiejów se ries sup ports the idea of an ex tremely low-en ergy flu vial sys tem. Anal o gous siltstone-dom i nated fa cies of anastomosing chan nels with a scar city of sand beds were noted by Eberth and Miall (1991) in the Perm ian Cut ler For ma tion (New Mex ico, USA).
In some stud ies, typ i cal anastomosing chan nels have been de scribed as highly sin u ous (sn > 1.5) (Schumm, 1968; Rust, 1978, 1981; Mc Car thy et al., 1991; Smith et al., 1997; Gibling et al., 1998). On the other hand, Smith and Smith (1980), King and Mar tini (1984), Smith (1983), Makaske (1998, 2001), and Gradziñski et al. (2000) con sidered anastomosing rivers as mod er ately sin u ous (sn < 1.5). In this sense, the sin u os ity of the Krasiejów chan nels (sn = 1.27) can be in ter preted as in agree - ment with the char ac ter is tics of an anastomosing sys tem. It must be em pha sized, how ever, that the sin u os ity of chan nels is only an ad di tional, though not the most es sen tial cri te rion for iden ti fi ca tion of the river pat tern (cf. Knighton and Nanson, 1993; Makaske, 2001).
The al lu vial plain rep re sented by the lower unit at the Krasiejów site had un doubt edly a very low slope. Low stream power (w @ 1 W m–2) is con sis tent the hydrologic data re cog - nised as typ i cal of anastomosing rivers (w < 8 W m–2) (Nanson and Knighton, 1996). The dom i nance of siltstones and claystones al lows a re con struc tion of the sed i men tary en vi ron -
246 Beata Gruszka and Tomasz Zieliñski
Fig. 9. Pedogenesis-re lated struc tures in claystones of the mid dle se ries
A — one side of a slickenside set with microfault planes dip ping to wards the axis of the struc ture; B — ho ri zon of calcic nod ules marked by an ar row above the sys tem of slick en sides; C — typ i cal com ple men tary set of slick en sides in di cated by sil ver/grey clay; scale bar is 0.6 m; D — shear striae (grooves) along a well-de vel oped slickenside sur face
ment of the Krasiejów lower unit as a mud-dom i nated class of the anabranching river group in Nanson and Knighton (1996) clas si fi ca tion. Such low-en ergy anastomosing rivers (such as the Dia man tina River or Coo per Creek) are typ i cal of semi-arid low lands of Aus tra lia (see Rust and Nanson, 1986; Nanson et al., 1988). This anal ogy is appropriate as the Late Tri as sic cli - mate of Pol ish and Ger man area is com monly in ter preted as semi-arid (Deczkowski et al., 1997; Mader, 1997; Reinhardt and Ricken, 2000; Hornung and Aigner, 2002b).
Not with stand ing the above ev i dence, strongly sug gest ing an or i gin of these de pos its in anastomosing chan nels, there are also some lithological fea tures which can not be con sid ered as typ i cal of com monly used mod els of anastomosing flu vial sys - tems. Within the overbank de pos its, no re cord of nat u ral lev ees or cre vasse splays has been found, al though these el e ments are con sid ered as char ac ter is tic of the anastomosing sys tem (Smith and Smith, 1980; Schumm et al., 1996; Gibling et al., 1998;
Wende and Nanson, 1998; Makaske, 2001). On the other hand, it is usu ally emphasised that rivers in semi-arid ar eas tend to have their own char ac ter is tics, in clud ing a com mon lack of cre - vasses and cre vasse splay de pos its (Rust and Legun, 1983;
Nanson and Croke, 1992; Törnqvist et al., 1993).
The sec ond im por tant dif fer ence lies in the cross-sec tional ge om e try of the chan nels. It is com monly ac cepted that the chan - nels of anastomosing rivers tend to be deep and nar row. The width to depth ra tio of typ i cal anastomosing chan nels of ten falls within the range 5 < w/d < 35 (Miall, 1977; Smith and Smith, 1980; Smith, 1986; Makaske, 1998; Tooth and Nanson, 1999).
The same pa ram e ter has val ues of 27–60 for the Krasiejów palaeochannels. We think that the Krasiejów chan nels were wide and shal low, be cause the chan nel bed ero sion dur ing avul - sion was very lim ited due to low flow en ergy, a re sis tant clayey sub strate, and a high con cen tra tion of sus pended load.
MIDDLE UNIT
Sed i men ta tion of the mid dle unit took place en tirely from sus pen sion set tling in stand ing wa ter (Fig. 6B). No ev i dence of
ephem eral cur rent ac tiv ity has been found. The hy dro log i cal con di tions were sta ble. A stand ing wa ter body, prob a bly a lake, must there fore be as sumed to have been the sed i men tary en vi - ron ment. The pres ence of siltstone in ter ca la tions in the up per part of the lac us trine suc ces sion in di cates that the ba sin even tu - ally be came shal lower. Con sid er ing the depositional ra tio in bas ins fed by sus pended-load rivers (An der son, 1996; Basilici, 1997; Mulder et al., 1998), the timespan cov er ing the sed i men - ta tion from sus pen sion set tling could be some 10 ka.
Af ter the ba sin had been in filled with sed i ment, soil pro - cesses took place. The slickenside struc tures re sult from pe ri - odic shrink and swell pro cesses due to ev i dent changes of ground mois ture. Such struc tures can form rap idly; this needs only some 200 years (Yaalon and Kalmer, 1978), and there fore their pres ence can not be used to es tab lish the en tire timespan dur ing which the soil pro cesses were ac tive. The calcic nod ules are a much better in di ca tor of soil ma tu rity; their for ma tion needs a few thou sands to hun dreds of thou sands of years (Gustavson, 1991; Retallack, 1994; Mack et al., 2003). Con sid - er ing the rel a tive scarce ness of the nod ules it must be as sumed that they formed dur ing a rel a tively short timespan of some thou sands of years. The gen e sis of the calcic nod ules is re lated to the mech a nism re spon si ble for slickenside or i gin, i.e. the al - ter na tion of wet and dry in ter vals. The con cre tions grew un der con di tions of re stricted mi gra tion of dis solved cal cium car bon - ate in the near-sur face zone. They are com monly con sid ered to re flect a semi-arid cli mate (e.g. Hornung and Aigner, 2002b), al though sim i lar con cre tions can orig i nate also un der hu mid and sub-hu mid con di tions (Gustavson, 1991). The blu ish col - our of the soil ho ri zon is an ef fect of gley in a re duc ing, wet en - vi ron ment. Daniels et al. (1971) es ti mated that the de vel op - ment of a gley ho ri zon re quires soil sat u ra tion for at least 1/4–1/2 of each year.
To sum up, all struc tures in the mid dle unit are typ i cal of vertisols. Most prob a bly this soil de vel oped in a sub trop i cal and warm cli mate, in which the ground un doubt edly un der went sea sonal al ter na tions of wet ting and dry ing. Anal o gous soils have been found in the Keuper of Ger many (Mader, 1997;
Fig. 10. Up per unit — lower part of the palaeochannel-fills
A — two (ar rows) coarse-grained sandy lithofacies in ter ca lated with lay ers of mas sive siltstones and claystones;
B — thick lay er of mas sive sandy con glom er ate among mas sive and lam i nated siltstones and claystones
Hornung and Aigner, 2002b). The main bone-bear ing ho ri zon of the Krasiejów site is di rectly above the vertisol. Such a su - per po si tion of both ho ri zons is log i cal and proves that the ver - te brates fa voured the wet ar eas, cor re spond ing to the top o - graphic lows, in a semi-trop i cal warm cli mate, that sea son ally be came more arid (cf. Behrensmeyer, 1987; Therrien and Fastovsky, 2000).
UPPER UNIT
The up per unit orig i nated in a river sys tem with a slightly higher en ergy than that of the lower unit. It is clear from the shape of the cu mu la tive grain-size curves and the Passega di a - gram, that trac tion and sal ta tion were sig nif i cant modes of trans port and de po si tion, while set tling from sus pen sion load was much less im por tant (Fig. 6), al though still sig nif i cant in the al lu vial chan nels. The chan nel li thol ogy pa ram e ter is es ti - mated to be M = 85%, whereas the sin u os ity is cal cu lated to have been sn = 2.04. The highly sin u ous chan nels were char ac - ter ized by in ten sive lat eral mi gra tion, which re sulted in ex ten - sive sheet-like bod ies. Cal cu la tion of the width/depth ra tio was there fore im pos si ble. The palaeochannel-fills are char ac ter ized by large-scale ep si lon cross-strat i fi ca tion ECS, termed also in - clined heterolithic strat i fi ca tion IHS (Fig. 11A), which is com - monly con sid ered as ev i dence of point bars. The point bars were sim ple forms, i.e. con sist ing ex clu sively of bar plat forms.
Such sim ple point bars are typ i cal of low-en ergy me an der ing rivers (Miall, 1996). Sec ond ary cur rents ori ented per pen dic u - lar to the chan nel axis must have been com mon above the bars (Fig. 11B). The re sult ing silty-sandy rip ples climbed up the barform slopes to wards the in ner chan nel banks. Rip ple cross-lam i na tion, ori ented coun ter to the di rec tion of the ECS bed dip, sup ports the high sin u os ity of the al lu vial chan nels.
Sec ond ary cur rents must have been ac tive dur ing floods. De -
po si tion from sus pen sion dom i nated dur ing av er age and low dis charge stages, so that clayey silt and clay were de pos ited on the bar slopes as draped laminae. Oc ca sion ally small-scale ero - sion was ac tive on bar slopes, and de po si tion of thin coarse-grained sandy con glom er ate could also take place.
Megaripples, usu ally com mon in sand-bed me an der ing chan - nels, are com pletely ab sent from the Krasiejów chan nels.
The de pos its closely re sem ble those from the Cre ta ceous Wealden Group of South ern Eng land de scribed by Stew art (1983), who in ter preted this de posit as formed in a sus - pended-load river of high sin u os ity. De pos its of the up per Krasiejów Unit are also very sim i lar to the 7th type of river in the clas si fi ca tion of Miall (1985), which is con sid ered as a river that is typ i cal of an estuarine en vi ron ment. Hornung and Aigner (1999) found al lu vial chan nel-fills dom i nated by claystones in the Keuper Stubensandstein of SW Ger many. In gen eral, the suc ces sions of the Ger man and Pol ish Keuper have much in com mon, but the Württemberg al lu vium shows many sandy rib bons, which are ab sent from the Krasiejów quarry. The Up per Tri as sic Chinle For ma tion in Ar i zona, where nu mer ous theropods have been found (Therrien and Fastovsky, 2000) is an other an a logue of the Krasiejów de po - s its be cause its chan nels are also filled with siltstones with ECS struc tures. There are many more ex am ples of low-en ergy me an der ing rivers dom i nated by de po si tion of silt from sus - pen sion (see, among oth ers: Ed wards et al., 1983; Kraus and Middle ton, 1987; Wood, 1989; Mack et al., 2003). Not with - stand ing the close sim i lar i ties be tween all these suc ces sions and the Krasiejów al lu vium, they are all char ac ter ized by a higher en ergy level, as shown by the higher fre quency of sand beds, and the pres ence of cross-strat i fi ca tion de rived from dunes and scroll bars. On the ba sis of com par i sons with all these de pos its, the flu vial en vi ron ment of the Krasiejów up per unit is in ter preted as a me an der ing river of low en ergy.
248 Beata Gruszka and Tomasz Zieliñski
Fig. 11A — Up per unit — mar ginal zone of palaeochannel in cised in overbank claystones (Ml); chan nel-fill large-scale ep si lon cross-stratification (ECS) con tain ing in clined beds with a rhyth mic al ter na tion of lam i nated siltstone (Tl) and rip pled siltstone (Tr); B — op po site ori en ta tions of Tl and Tr lami na tions within ECS struc ture, in di cat ing a point bar or i gin; N — num ber of measurements
PALAEOGEOGRAPHIC DEVELOPMENT
The re sults of our study show that the whole Krasiejów suc - ces sion was formed un der con ti nen tal con di tions. Our re sults are not consistent the hy poth e sis put for ward by Zatoñ et al.
(2005) of saline wa ter, based on a charophyte de cline above the mid dle bone-bear ing ho ri zon. We in cline rather to the sec ond pos si bil ity men tioned by Zatoñ et al. (2005) that stranger ocurrents may have been re spon si ble for palaeo eco logi cal changes, when the me an der ing pat tern was established.
Two unit of the Krasiejów suc ces sion were formed by ex - tremely low-en ergy rivers flow ing over a plain at a great dis - tance from higher ground. Our in ter pre ta tion is in con trast to the opin ion of Szulc (2005). His anal y sis of the Krasiejów suc - ces sion con cluded that the main sed i men tary fac tors were sheetfloods and de bris-flow events. Such depositional pro - cesses need steeper gradiends, typ i cal of the im ma ture to pog ra - phy of tec toni cally ac tive re gions. This is con fus ing, as Szulc did not ex clude a flu vial or i gin for the de pos its, but did not de - scribe any of the al lu vial chan nels that we found to be abun dant in the Krasiejów suc ces sion. In con trast to Szulc (2005) we con sider that the style of flu vial de po si tion, dur ing the for ma - tion of both the lower and the up per units, was con trolled by down stream fac tors, mainly base-level fluc tu a tions.
The in flu ence of down stream fac tors on the pat tern of the flu vial chan nel sys tem is par tic u larly clear in the lower al lu vial unit. The anastomosing char ac ter of the river, the high aggradation ra tio, and the lack of lat eral ac cre tion sup port a con stant rise in base level (see Woodyer et al., 1979; Smith and Putnam, 1980; Smith, 1986; McMarthy, 1993; Morozova and Smith, 1999; Makaske, 2001). Törnqvist et al. (1993) showed that an anastomosing flu vial sys tem can evolve if the base level rises an nu ally by at least 1.5 mm. In our opin ion, sed i men ta tion of the lower al lu vial unit pro ceeded un der con di tions of a con - stant lo cal base-level rise, most prob a bly con trolled by sub si - dence (Ta ble 1).
The Keuper al lu vial chan nel pat tern stud ied may have de - pended only slightly on cli mate. This con clu sion is sup ported by the fact that anastomosing river sys tems are well known from dif fer ent cli mates: hu mid tem per ate, con ti nen tal tem per - ate, semi-arid, and po lar (cf. Smith and Putnam, 1980; Smith, 1986; McMarthy, 1993; Nanson and Knighton, 1996; Gibling et al., 1998; Makaske, 1998; Jones and Schumm, 1999). The anastomosing river ran to wards the NE at Krasiejów (Fig.
12A), which is con sis tent with the in ter pre ta tion of the Late Tri as sic palaeo ge ogra phy in this part of Eu rope (cf.
Deczkowski et al., 1997; Mader, 1997).
T a b l e 1 Li thol ogy and en vi ron men tal in ter pre ta tion of the three unit stud ied
The sed i men tary con di tions of the mid dle lac us trine unit might be in ter preted in two ways. The first pos si ble in ter pre ta - tion is that the aggradation of the river did not keep up with the re gional sub si dence, so that the study area be came a lake. The sec ond pos si bil ity is that the chan nel belt of the anastomosing river shifted to wards an other part of al lu vial plain, while an inter-chan nel stand ing wa ter body (i.e. a lake) de vel oped in the Krasiejów area.
Af ter the lake had been en tirely infilled with sed i ment, subaerial soil pro cesses took place. Sub si dence was re strained, and aggradation and ero sion were nearly in bal ance. The soil char ac ter is tics are gen er ally in agree ment with what is known about the cli mate in Po land and Ger many dur ing the Keuper. It was warm, with dis tinct sea sonal changes from arid to hu mid (see, among oth ers, Deczkowski et al., 1997; Mader, 1997;
Reinhardt and Ricken, 2000; Hornung and Aigner, 2002b).
A new phase of in creased sub si dence is sug gested by the ac cu mu la tion of the up per al lu vial unit. The base level was ris - ing slower than dur ing the ac cu mu la tion of the lower al lu vial unit (Ta ble 1). The al lu vial plain was not so ex tremely flat as dur ing the de vel op ment of the anastomosing flu vial sys tem, so that the river ac quired a me an der ing pat tern. Ver ti cal ac cre tion of the chan nel belt was re stricted, while lat eral ac cre tion of point bars de vel oped. The chan nel-belt aggradation rate was lim ited due to chan nel mi gra tion across the al lu vial plain.
The same river re ac tion can be re lated to the in flu ence of cli mate on the hydrologic con di tions. A more hu mid cli mate must have re sulted in some changes of flu vial trans port char - ac ter. The sed i ments were trans ported and de pos ited as bedload, while clayey sus pended load di min ished. This is why the up per al lu vial unit is mainly rep re sented by point-bar fa cies within the palaeochannels. The cur rent di rec tions of the lower anastomosing and the up per me an der ing sys tems re - mained the same, as the me an der ing river also flowed to wards the NE (Fig. 12B).
CONCLUSIONS
Al though the Krasiejów suc ces sion has a fairly uni form li - thol ogy, con sist ing mainly of siltstones and claystones, it was
pos si ble to dis tin guish three sed i men tary units: two al lu vial units sep a rated by a lac us trine one. Ver te brate bone ho ri zons are pres ent in each of these unit. Both al lu vial units are char ac - ter ized by well-de vel oped palaeochannels, mostly filled with siltstones. The lac us trine unit con tains pre dom i nantly mas sive claystones. The most im por tant clues to the in ter pre ta tion of the sed i men tary en vi ron ments are sum ma rized here.
1. Both al lu vial units rep re sent low-en ergy, silt-bed river sys tems. It seems that this part of Eu rope formed a fairly well-de vel oped ex ten sive low land dur ing the Late Tri as sic.
2. The lower al lu vial unit is in ter preted as rep re sent ing an anastomosing river sys tem be cause of the nu mer ous rib bons re - flect ing an anabranching chan nel pat tern, the mod er ately sin u - ous char ac ter of the chan nels (sn @ 1.27), the high value of the chan nel li thol ogy pa ram e ter (M = 90%), the ex tremely low power of flood flows (w @ 1 W m–2), the slightly asym met ric con cen tric fills of the palaeochannels due to ver ti cal ac cre tion as the most com mon sed i men tary pro cess, the lack of side bars re sult ing from high chan nel sta bil ity.
The anastomosing river sys tem was con trolled mainly by tec toni cally in duced base-level rise (i.e. per ma nent sub si - dence), whereas cli ma tic fac tors were of less im por tance. The hy poth e sis of an anastomosing char ac ter of the Late Tri as sic rivers in Cen tral Eu rope has been put for ward ear lier (cf.
Mader, 1997), but the pres ent con tri bu tion is the first that, on the ba sis of a sedimentological anal y sis, sup ports this pos tu late.
The up per al lu vial unit is in ter preted as hav ing formed in a me an der ing river sys tem be cause the chan nels were in ferred to have been highly sin u ous (sn = 2.04) and the chan nel li thol ogy pa ram e ter was high (M = 85%), the palaeochannel-fills with abun dant ECS/IHS struc tures in di cat ing point bars formed by trans verse sec ond ary cur rents within me an ders.
3. The rivers of both sys tems flowed in the same NE di - rec tion.
4. The mid dle, clayey lac us trine unit re cords an ex ten sive lake where sus pen sion set tling was not dis turbed by any cur - rents. Pro gres sive shallowing re sulted in fi nal emer gence. The lac us trine de pos its were then af fected by soil pro cesses, and a vertisol de vel oped in a sub trop i cal warm cli mate with an ev i - dent arid-to-wet sea sonal rhythm.
5. Base level changed dur ing sed i men ta tion from a lower to a higher po si tion, and later changed a lower po si tion again.
This en abled evo lu tion of en vi ron ments from an anastomosing river sys tem in the lower base level po si tion, through lake de - vel op ment dur ing the high est base level, to a me an der ing sys - tem when the base level was low est. How ever, these base level fluc tu a tions were not great enough to change the grain-size of de pos its greatly.
6. Sea sonal arid ity of the cli mate meant that the Late Tri as - sic ver te brates pre ferred spec i fied wet hab i tats. Pre sum ably, these were de pres sions in the gently un du lat ing al lu vial plain and re sid ual ponds af ter the lake had dried.
Ac knowl edge ments. We thank G. Racki, who en cour aged us to un der take the study of the Krasiejów sed i men tary en vi - ron ment. W. Bardziñski, G. Bzowska, A. Piechota and M.
Racka were of much help dur ing the field stud ies, and our dis - cus sions with them were most fruit ful. We thank T. Van Loon for im prov ing the Eng lish lan guage of the text. We re gard
250 Beata Gruszka and Tomasz Zieliñski
Fig. 12. Di rec tional dis tri bu tion of cross-strat i fi ca tion in lower (A) and up per (B) flu vial units
Gen eral di rec tions of the chan nels are ar rowed; V — mean az i muth, N — num ber of read ings
highly all the com ments by B. Makaske on the pre vi ous ver sion of the manu script. We ap pre ci ate the com ments on the manu - script by A. Becker and J. Hornung. We are grate ful for all an - no ta tions which much im proved the manu script. Fi nan cial sup -
port by the Uni ver sity of Silesia, pro ject BW/KGP/2004, en - abled the field work.
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252 Beata Gruszka and Tomasz Zieliñski
