Gulas-Wroblewski, Bonnie E.1, Anton F. Wroblewski2
(1) University of Chicago, Chicago, IL
2) Northeastern Illinois University, Chicago, IL
ABSTRACT: Vertebrate Fossils as Indicators of Accommodation/Sediment Supply Ratios in Fluvial Systems: The Bird Perspective
Preservation quality of vertebrate fossils has been linked directly to sequence stratigraphic systems tracts and indirectly to changes in accommodation/sediment (A:S) supply ratios. Preservation of relatively complete and articulated specimens has been interpreted as indicative of the transgressive systems tract. Less complete, fragmentary specimens are conventionally attributed to the late highstand and falling stage systems tracts. Bird fossils are unique among vertebrate remains in being delicate, hollow, and easily destroyed by sedimentary processes. Mostly represented in quiet-water lacustrine and marine deposits, bird fossils have an extremely low preservation potential in fluvial channel deposits. In the Eocene Bridger Formation (the fluvial equivalent of the lacustrine Green River Fm.) of southwestern Wyoming, the preservation of a partially articulated galliform bird (Aves: Galliformes; megapodes, cracids, and “phasianoids”), Amitabha urbsinterdictensis, in a massive fluvial sandstone indicates rapid sedimentary infill of the fluvial channel. Secondary reworking of the sediment would have resulted in disarticulation and/or destruction of the specimen. The articulation and association of the elements of this fossil avian demonstrate the rapid abandonment of the channel by the fluvial system shortly after initial deposition. The stratigraphic succession in which this channel deposit occurs is characterized by an abundance of vertebrate fossils preserved in channel and overbank deposits. The overall good quality of preservation of these specimens, coupled with the isolate, single storey, channel deposits characterized by massive, structureless bedding indicates rapid deposition, high frequency of channel avulsion, and high rates of accommodation creation. This is supported by independent sedimentary, radiometric and biostratigraphic data. The degree of completeness of vertebrate fossils, particularly birds, proves a good proxy for estimating A:S ratios in alluvial deposits in which more quantitative data may be lacking.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.