Application of Sequence Stratigraphy to Nonmarine Successions Revisitied: An Example from the Middle and Upper Williams Fork Formation, Piceance Basin CO

Wiechman, Michele; Aschoff, Jennifer L.

Sequence stratigraphy provides an invaluable tool to correlate genetically related packages of rock to predict regional trends in lithology. Unfortunately, there are numerous problems in applying sequence stratigraphy in fluvial strata due to the lack of marine influence that makes flooding surfaces difficult to identify, and a plethora of truncations that make discerning significant unconformities nearly impossible. A wide range of approaches have been proposed to correlate and subdivide fluvial successions, but no single methodology has been adopted. Popular methods for correlation in fluvial strata include (1) using channel stacking patterns to delineate regional trends in accommodation, (2) correlation of tidal influence into up-dip successions to track flooding surfaces, (3) vertical shifts in detrital composition to identify significant unconformities, (4) fluvial channel types and (5) patterns in paleosol development on the floodplain. Used alone, each of these methods is highly uncertain due to the affects of tectonics and climate; an integrated approach is thus preferred whenever possible. Presented is a basin-scale sequence-stratigraphic correlation through fluvial to marginal marine strata of the Williams Fork Formation in the Piceance Basin, northwest Colorado that integrates these common approaches to correlate in fluvial strata. The new outcrop-to-subsurface correlation consists of 12 stratigraphic profiles, 10 outcrop gamma-ray curves, 2 core descriptions, 15 point counts from core, and >100 well logs. Lithofacies assemblages include (a)high-sinuosity, meandering fluvial channels, (b)isolated, low-sinuosity anastomosed fluvial, (c)wide-ranging floodplain deposits, (d)tidally influenced fluvial, (e)estuarine, (f)regressive marine shoreface (g) transgressive marine shoreface and (h)coastal plain. Vertical net-gross patterns show an overall increase in accommodation space, vertical stacking patterns show decreases in vertical channel amalgamation to more isolated sand bodies with developed floodplains. Two major zones with tidal or marine influence relate to changes in the types of channels and floodplains deposited updip. Four major provenance changes within vertical sections can be related to regional tectonic changes and used to confirm the location of low order sequence boundaries. By examining and applying various correlation methodologies it is possible to better constrain the sequence-stratigraphic framework of fluvial successions.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013