Ancient Shorelines and Shelves
Marginal marine Trout Creek Sandstone and related Mount Harris Fork Member coastal-plain deposits (Campanian Mesaverde Group) are the regressive portion of a third-order clastic wedge that prograded from west to east into the Western Interior Seaway, Colorado, U. S. A. Progradation of this wedge was punctuated by seven higher-frequency (fourth-order) transgressions that formed as the wedge aggraded. Relationships between changes in shoreface and adjacent coastal-plain environments and stacking patterns of the Trout Creek Sandstone were analyzed using the shoreline trajectory concept.
Fourth-order shoreface tongues prograded up to 50 kilometers into the basin and are characterized by descending and flat shoreline trajectory trends that are symptomatic of falling or stable relative-sea-level conditions. Normal regression is interpreted to occur only in the most basinward portions of these tongues, prior to transgression. Time-equivalent coastal-plain facies associations either record periods of sediment bypass, consist of amalgamated channel complexes, or form a thin succession of lower-delta-plain sediments accumulated behind aggrading shoreface deposits during the terminal sea-level lowstand. Fourth-order transgressive events possess shoreline trajectory trends that become progressively more accretionary as the transgression proceeded. In a landward direction, offshore shale grades into aggrading barrier complexes that are laterally equivalent to thick coastal-plain successions (consisting of stacked lagoon/bay- fill and floodplain facies associations). Tidal-channel sandstones and isolated, fluvial sandstones are also recognized in transgressive settings.
Regression and transgression of the coastline is respectively associated with increases in the storage of sediment within shoreface and coastal-plain environments. Differing styles of shoreface and coastal-plain stacking patterns reflect varying shoreline-trajectory trends. Observed relationships between shoreline trajectories and the character of paralic depositional environments have important implications for predicting shoreface thickness and the presence of extensive fluvial channel-belt reservoirs in adjacent areas.
Figures & Tables
Siliciclastic shallow-marine deposits record the interface between land and sea, and its response to a variety of forcing mechanisms: physical process regime, the internal dynamics of coastal and shelfal depositional systems, relative sea level, sediment flux, tectonic setting, and climate. These deposits have long been the subject of conceptual stratigraphic models that seek to explain the interplay between these various forcing mechanisms, and their preservation in the stratigraphic record. This volume arose from an SEPM research conference on shoreline–shelf stratigraphy that was held in Grand Junction, Colorado, on August 24–28, 2004. The aim of the resulting volume is to highlight the development over the last 15 years of the stratigraphic concepts and models that are used to interpret siliciclastic marginal-marine, shallow-marine, and shelf deposits.