New Models for Fluvial Architecture and Nonmarine Sequence Stratigraphy
John C. Van Wagoner
The Desert Member of the Blackhawk Formation and the Castlegate Sandstone were studied to develop models of nonmarine sequence stratigraphy. Two major sequence boundaries were identified in these rocks, dividing the strata into 3 sequences. Sequence boundaries are marked-by regional incision, basinward shift in facies, and abnormal subaerial exposure. Highstand strata contain foreshore and shoreface sandstones stacked in progradational packages. Thick, amalgamated, sharp-based, hummocky-bedded, lower shoreface strata with gutter casts are found in the late highstand systems tracts. Sharp bases and amalgamation are a function of normal depositional processes during progradation. Detailed outcrop correlation clearly shows that sequence boundaries sit above these stratal uni s, not below. This observation contravenes a model of forced regression for these deposits.
Lowstand strata are multi-story braided-stream sandstones composed of downstream accreting bars, up to 10m thick, filling incised valleys. These braided-stream deposits form a fluvial sheet sandstone, or megafan, at least 100km wide, the minimum width of the valleys. The lowstand fluvial deposits die out basinward forming a terminal fan culminating in anastamosing streams and splay complexes, without reaching the sea. No time-equivalent lowstand shoreline sandstones are present. Seaward of the terminal fan, the sequence boundaries are marked by subaerial exposure, thin lacustrine deposits, and oolites resting on offshore strata.
Lowstand geometry is a function of two processes. Knickpoints migrate up alluvial channels from the highstand shoreline in response to baselevel drop. Lowstand deposition is contemporaneous with this migration. As the shoreline retreats rapidly during baselevel fall, rivers anastamose. Anastamosing splay complexes "prograde" across the subaerially exposed shelf, forming the distal ends of the lowstand system.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California