Trace fossil assemblages reflecting stressed environments in the middle jurassic carmel seaway of Central Utah

ABSTRACT-The shallow-marine Carmel Formation (Middle Jurassic) in central Utah hosts low-diversity trace fossil assemblages, including Arenicolites, Chondrites, Gyrochorte, Lockeia, Planolites, Protovirgularia, Rosselia, Scalarituba, Skolithos, Taenidium, and Teichichnus. Non specialized ichnotaxa with a remarkably small burrow size dominate the assemblages. The amount of bioturbation is lower than expected in comparison with modern shallow-marine carbonate environments. These ichnological features also are significantly different from those of other Jurassic shallow-marine carbonates. The trace fossils represent an environmentally stressed benthic community in a marginal marine, restricted setting, with salinities above normal marine and with depletion of oxygen in pore waters.

INTRODUCTION

ESPITE THE importance of bioturbation in modern carbonate environments (e.g., Shinn, 1968; Braithwaite and Talbot, 1972; Tedesco and Wanless, 1991; Curran, 1994), trace fossils in shallow- and marginal-marine carbonate rocks have received much less attention than those in siliciclastic rocks. This situation may be a consequence of the lower degree of visual enhancement of the traces in carbonate rocks due to the reduced lithological contrast and the higher degree of diagenetic alteration (Pemberton and Jones, 1988; Curran, 1994). Some authors have tried to fill the void with the publication of ichnologic studies in shallow-marine carbonates (e.g., Farrow, 1966; Firsich, 1975; Pickerill and Forbes, 1979; Archer, 1984; Narbonne, 1984; Pickerill et al., 1984; Pemberton and Jones, 1988; Fillion et al., 1990; Curran, 1994; Gaillard et al., 1994; Gibert, 1996a). Together with Ordovician and Quaternary units, Jurassic carbonate formations have received much of the attention. Jurassic trace fossil assemblages typically are dominated by crustacean burrows, such as Thalassinoides and Rhizocorallium.

The marine Jurassic of North America has been studied less intensively by paleontologists than Jurassic strata in other parts of the world (Imlay, 1980; Tang and Bottjer, 1997). The study of the trace fossils in carbonates of the Middle Jurassic Carmel Formation in central Utah reveals significant differences with assemblages studied previously in other geographic areas (mostly in Europe). This situation also has been observed with body fossil assemblages (Tang and Bottjer, 1997), and it seems to be a reflection of the particular paleogeographic and paleoenvironmental characteristics of the Middle Jurassic sea in western North America. Jurassic trace fossils in the western part of the San Rafael Swell (Emery County, Central Utah) have contributed significant information to the interpretation of the paleoenvironments and paleoecology of the Carmel Formation.

GEOLOGICAL SETTING

The Carmel Formation extends across central and southern Utah and a small part of northern Arizona, and it grades northwards into the Twin Creek Formation in northern Utah. Despite the rarity of biostratigraphically useful fossils, paleontological and radiometric data indicate a Bajocian-Bathonian to early Callovian (Middle Jurassic) age (Imlay, 1980; Everett et al., 1989; Zhang et al., 1996).

The Carmel Formation is more than 70 m thick in the western part of the San Rafael Swell anticline. It disconformably overlies the eolian Navajo sandstone (Lower Jurassic), and it has a transitional upper boundary to the fluvial and eolian Entrada Sandstone (Middle Jurassic). During the Middle Jurassic this area was part of a shallow, restricted sea (the Carmel-Twin Creek Seaway of Imlay, 1980; Nielson, 1990), which extended from southwestern Utah to Canada (Fig. 1). This seaway was bordered to the east by an extensive and coastal plain and to the west by a foreland fold and thrust belt (Peterson, 1994).

The sections measured for this study are located north and south of Interstate 70 along the Moore Road and in the Furniture Draw about 25 Km East of Castledale.

STRATIGRAPHY AND SEDIMENTOLOGY

In the San Rafael Swell, the Carmel Formation consists of more than 70 m of carbonates, siliciclastics and evaporites (Fig. 2). The only previous stratigraphic investigation in this area was done by Bagshaw (1977), who studied the lower part of the formation exclusively. Other authors have provided detailed sedimentologic studies of the Carmel Formation in other areas of Utah (Taylor, 1981; Nielson, 1990). Our examination of the complete section in the western flank of the San Rafael Swell has revealed the presence of eleven sedimentary facies:

1) Facies A: Sandstone and siltstone. This facies consists of alternating fine-grained sandstones and siltstones. The sandstone beds range from a few centimeter to decimeter thick, and they exhibit cross-lamination and ripples on the top. The siltstones are horizontally laminated.

2) Facies B: Siltstone and claystone. These are gray to yellow siltstones and claystones. They are horizontally laminated or apparently structureless or locally nodulose, with a variable carbonate content.