Response of a Nonmarine Coastal Plain/Fluvial Depositional System to High Frequency Relative Sea Level Fluctuations: The Judith River Formation, Wyoming, USA
Detailed facies and architectural element analysis of the regionally well exposed Late Cretaceous Judith River Formation of the Big Horn Basin, Wyoming revealed a cyclicity in both coarse and fine members. Three man in sedimentary packages were observed:- Unit 1: Carbonaceous shales with common poorly drained soils and rare, well drained soils, coals, isolated small-scale meandering, tidally-influenced fluvial channels and rare crevasse splay sandstones. Deposition in a poorly drained coastal swamp environment. Unit 2: Grey shales, well drained soils, common crevasse splay and single storey channel sandstones. Deposition in a well drained alluvial plain. Unit 3: Stacked, fluvial and fluvial/tidal channel complexes of both low and high sinuosity. Deposition in incised sand-rich fluvial/estuarine systems. The most common arrangement in a vertical profile is 3-->1-->2-->3. This cyclicity was then traced into the coeval shallow marine succession where known flooding surfaces and sequence boundaries allowed the non-marine succession to be interpreted in terms of relative sea level cycles. Sequence boundaries, when traced via continuous outcrop up-dip into nonmarine strata, are defined by regional scale erosion surfaces overlain by vertically amalgamated fluvial-tidal systems, (unit 3). increase in accommodation space in up-dip areas during the transgressive systems tract is marked by high water table deposits (unit 2). Finally the well drained alluvial plain deposits of unit 1 correlate down-dip with rogradational shorefaces characteristic of the highstand systems tract.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California