Denver Basin Isolated Sandbodies: Signature of Dynamic Subsidence, Laramide Uplifts and Shoreline Transitions

Plink-Bjorklund, Piret; Kiteley, Louise

A set of six vertically stacked late Campanian isolated sandbodies, previously interpreted as offshore (shelf) bars or shoreface sandstones was studied in outcrops in the Denver Basin. Based on detailed facies analyses we reinterpret the Hygiene, Terry, Rocky Ridge and Laramie Sandbodies as deltaic and the Richards sandbody as estuarine. The riverine origin of these sandbodies is conformed by petrographic observations that indicate relatively coarse grain size (up to medium sand) and relatively immature composition including lithic clasts and feldspar. Several of the sandbodies, especially Hygiene, Terry and Richards indicate strongly tide-influenced to tide-dominated deltaic and estuarine settings. This interpretation is based on ubiquitous occurrence of tidal depositional features like mud-drapes, bidirectional palaeocurrent indicators, compound cross stratification, cyclic changes in cross-strata thickness (neap-spring cyclicity), etc. Rocky Ridge and Larimer Sandstones indicate a stronger (storm) wave influence on deposition. Architecture and stacking pattern of these sandstones is variable and include progradation with flat to falling as well as rising trajectory. In some sandstones transgressive ravinement surfaces and lag deposits occur at the tops of the deltaic units. Other sandstones are based by landward-stepping transgressive lag deposits.

The deltaic and estuarine nature of these sandbodies indicates episodic shoreline progradation to the Denver Basin area during late Campanian. A general eastward migration of WIS depocenter was predicted by dynamic subsidence models proposed to accompany the change to flat-slab subduction and initiation of the Laramide Orogeny. The Hygiene, Terry, Rocky Ridge and Larimer Sandstones each reflect eastward migration of shorelines. In contrast, the estuarine Richards Sandstone occurs above a regressive-transgressive turnaround and indicates the beginning of the Almond landward-stepping trend and Lewis Sea transgression. We assign the significant tidal influence also to the initiation of the differential Laramide subsidence that created local irregularities in the WIS basin that have been suggested to be especially significant during relative sea-level lowstands.

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