early permian depositional environments and pond

Palaeogeogr~phy, Palaeoclimatology, Palaeoecology, 69 (1989): 1 21

1

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

EARLY PERMIAN DEPOSITIONAL ENVIRONMENTS AND POND BONEBEDS IN CENTRAL ARCHER COUNTY, TEXAS P. MARTIN SANDER Paldontologisches Institut und Museum, Universitdt Ziirich, Kiinstlergasse 16, CH-8006 Ziirich (Switzerland) (Received December 1, 1987; revised and accepted July 4, 1988)

Abstract Sander, P. M., 1989. Early Permian depositional environments and pond bonebeds in central Archer County, Texas. Palaeogeogr., Palaeoclimatol., Palaeoecol., 69:1 21. The Lower Permian redbeds of North Texas contain the oldest a b u n d a n t non-aquatic vertebrate fauna. In the past, collecting of fossils and study of t h e i r paleozoology has been emphasized. With the exception of the pioneering work of Olson (1958, ].977) little a t t e n t i o n was paid to sedimentological, paleoecological, and taphonomical aspects of the fauna. This study focuses on the sedimentary framework, paleoenvironment, fossil content, and taphonomy of the most common type of fossil occurrence - - the pond bonebed. Five sedimentary facies are recognized: meanderbelt sandstones, large straight channel fill sandstones, small straight channel fill sandstone, interbedded facies, and floodplain mudstone. These are the components of small meandering river systems with local tributaries and crevasse splays. Pond bonebeds occur as gray claystone lenses in red floodplain mudstones, deposited in oxbow lakes or swamps. The fauna is largely aquatic (Orthacanthus, Ectosteorhachis, paleoniscoids, Sagenodus) or semiaquatic (Eryops, Trimerorachis, Archeria, Edaphosaurus) with subordinate terrestrial elements (Diadectes, Dimetrodon). Fossil plants, found as impressions, petrifactions, and charcoal, are indicative of swamps (Psaronius) and well drained soils (conifers). All of the organic debris accumulated gradually by biological and physical processes. Occasional isolated but complete vertebrate skeletons are found in the floodplain mudstones.

Introduction Vertebrate fossils from the Lower Permian redbeds in North Texas were first described by Cope (1878). The fauna is of major importance because it represents the first fully terrestrial vertebrate ecosystem in the geologic record (Olson, 1976). The paleozoology of the fauna is well known (e.g. Case, 1915; Romer and Price, 1940; Romer, 1947; Langston, 1953; Olson, 1958; Reisz, 1986). However, with the exceptions of Olson, (1958, 1977), Parrish (1978), Bakker (1982), and Olson and Mead (1982), little attention has been paid to sedimentological, taphonomical and paleoecological aspects of the fossil occurrences. 0031 0182/89/$0:3.50

The original objective of this study was the deciphering of the taphonomy of the well-known Geraldine Bonebed (Sander, 1987), but soon its scope widened and incorporated the questions of the influence of the sedimentary depositional environment and ecosystem of the formation of other fossil occurrences in the region. The first step was mapping the sedimentary facies of an area, sufficiently large to allow an understanding of the facies geometries and to contain several vertebrate fossil occurrences. During this mapping, all plant fossils were noted or collected to reconstruct plant paleoecology. A detailed investigation of the facies, floral, and faunal content of every vertebrate occurrence followed. All the data were then

:¢:: 1989 Elsevier Science Publishers B.V.

synthesized to g e n e r a t e a p i c t u r e of the paleoe n v i r o n m e n t and the t a p h o n o m i c a l processes a c t i n g u p o n it. F o u r types of fossil deposit were recognized: (a) flood plain pond bonebeds, (b) isolated skeletons, (c) c a t a s t r o p h i c e v e n t bonebeds, and (d) lag bonebeds. Bonebeds a and b, the most c o m m o n types, are the topic of this paper. An example of type c was described in detail by S a n d e r (1987), and an example of type d was a n a l y z e d by P a r r i s h (1978). An i m p o r t a n t s o u r c e of i n f o r m a t i o n was the 98 ° 50'

I

field notes o f R . V. W i t t e r who w o r k e d in N o r t h Texas as a c o l l e c t o r for the M u s e u m of C o m p a r a t i v e Zoology of H a r v a r d U n i v e r s i t y (MCZ) from 1936 to 1942. Regional geology Regional geologic setting

T h e s t u d y a r e a (Fig.l) is located in n o r t h T e x a s at the l a n d w a r d m a r g i n of the E a s t e r n 98 ° 40'

980 35'

I

I

I

I

I

33 ° 40'

33 ° 35'

I

Alluvium Seymour Formation I ~

Petrolia Formation 5 km N o c o n a Formation

NI

A r c h e r City Formation

Fig.1. Geologic map of the study area in central Archer County, Texas, based on Hentz and Brown (1987).Dots indicate fossil localities investigated in the field. Diamonds indicate approximate position of localities that could not be relocated. Al=Archer City Bonebed 1, A2=Archer City Bonebed 2, A3=Archer City Bonebed 3, A4=Archer City Bonebed 4, CBB=Coprolite Bonebed, LBB=Loftin Bonebed, RBB=Rattlesnake Canyon Bonebed 2, S=Locality of Diadectes skeleton TMM 42424-1. Rectangle indicates area of map in Fig.3.

Shelf of the small Paleozoic intercratonic Midland Basin. Pennsylvanian to Middle Permian rocks are exposed in the region today. Presumably only a few hundred meters of overlying, younger Permian and Cretaceous strata were eroded away. The depositional history and tectonic evolution of the Midland Basin and its Eastern Shelf are well known through the work of Brown and coworkers (Brown, 1969; Galloway and Brown, 1972, 1973; Brown et al., 1973). Most recently, stratigraphic work by Hentz (1985, 1988) and Hentz and Brown (1987) has provided a definitive lithostratigraphy and geologic map of the area. By Early Permian times tectonic activity in the eastern Fort Worth Basin and Ouachita fold-belt had ceased and the paleogradient was low. Sediment supply from the east was reduced, and mainly older (Late Mississippian to Early Pennsylvanian) sedimentary rocks, the Fort Worth Basin fill, were eroded. Clastic sedimentation decreased and was replaced by carbonate deposition. No major structural deformation has affected north Texas since Permian times. The present regional dip is about 9 m/km to the northwest (Hentz, 1985). The same values were obtained on meanderbelt sandstone in the study area. During Wolfcampian time, Archer County laid about 800 km south of the paleoequator according to paleogeographic reconstructions of Tarling (1980). The climate was warm and humid with seasonality increasing in subsequent Leonardian times (Olson and Vaughn, 1970; Mapes and Gastald 0, 1986).

Permian vertebrate localities in the area (e.g., Romer, 1974; Clark and Carroll, 1973) (Fig.2), Thus the central Archer County fossil localities are currently placed in the upper Archer City and the Nocona Formations (Figs.1 and 2). The " P u t n a m Formation" is no longer recognized, and the Admiral Formation is restricted to a marine limestone and shale sequence to the southwest. Hentz (1985) and Hentz and Brown (1987) were also able to further subdivide their formations by mapping individual sandstones. For the Archer City Formation eight such sandstones (SS 1-SS 8) and for the Nocona Formation 11 such sandstones (SS 1-SS 11) were recognized. These sandstones are discontinuous. In the Archer City Formation of the study area only sandstones SS 5, SS 6, and SS 8 are developed, and in the Nocona Formation only the sandstones SS 1 and SS 6 to SS 9 are developed. For a detailed correlation chart of the old and the new stratigraphic terminologies, see Hentz (1985). For a map and detailed stratigraphic section of the Archer City and Nocona Formations, see Hentz and Brown (1987). The biostratigraphic position of the "Putnam Formation" and ~Admiral Formation"

Romer, 1974

mo