Platt, Brian F.1, Stephen T. Hasiotis1, Mary J. Kraus2
(1) University of Kansas, Lawrence, KS
(2) University of Colorado, Boulder, CO
ABSTRACT: Integrating Ichnofossils and Substrates to Interpret Avulsion in Floodplain Deposits in the Upper Jurassic Morrison Formation, Big Horn Basin, Wyoming
The Morrison Formation deposits can be divided into three distinct mudstone-dominated intervals, each representing periods of differing degrees of paleopedogenic modification and paleohydrologic conditions. Within each interval, significant lateral variation in the mudstones and sandstones makes high-resolution correlation difficult. In particular, the middle unit contains locally extensive sheet and ribbon sandstones up to 1.5 m thick that are interbedded with poorly- to well-developed paleosols. Sandstones are very fine- to fine-grained, ripple- to climbing ripple-laminated, sublithic quartz arenites with weak to no pedogenic modification as evidenced by a lack of bioturbation and pedoturbation, presence of relict bedding, and only faint redoximorphic coloration.
Trace fossils are preserved within both sandstones and mudstones in all three intervals. Traces occur as hyporelief and epirelief casts within and at the contacts of sandstone and mudstone. Invertebrate ichnofossils are abundant and include crayfish, soil bug, beetle, and orthopteran burrows, termite nests, bivalve and gastropod resting traces, and gastropod and annelid trails. Vertebrate traces, mainly tracks attributed to sauropod, theropod, and ornithopod dinosaurs, are also present. Rhizoliths are abundant in mudstone, and are preserved as green-grey zones depleted of iron oxides, and as tubules and concretions of carbonate.
Both rocks and trace fossils are characteristic of an alluvial environment. The thin, stacked, heterolithic intervals, weak pedogenic modification, and taphonomically undisturbed traces indicate relatively rapid episodes of sedimentation punctuated by short hiatuses. This pattern, seen in all three intervals, is interpreted as resulting from aggradational avulsion associated with increased accommodation due to a rise in local base level.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.