Style contrast between forced regressive and lowstand/transgressive wedges in the Campanian of south-central Wyoming (Hatfield Member of the Haystack Mountains Formation)
Donatella Mellere, Ronald Steel, 2000. "Style contrast between forced regressive and lowstand/transgressive wedges in the Campanian of south-central Wyoming (Hatfield Member of the Haystack Mountains Formation)", Sedimentary Responses to Forced Regressions, D. Hunt, R. L. Gawthorpe
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The Campanian Hatfield Member of the Haystack Mountains Formation is composed of two well-exposed marine sandstone tongues that extend up to 35 km basinward from their earliest shoreline position into the Western Interior Seaway. Each tongue (HI and H2) is comprised of two parts that have characteristic architecture, external geometry and facies assemblages. Together, the tongues form a stratigraphic sequence that is formed of four systems tracts and bounded by erosional unconformities. The sequence is interpreted to have been generated over an interval of less than 1 Ma during a fall-to-rise cycle of relative sea level. The earliest and latest systems tracts of the sequence, interpreted as lowstand prograding deltaic wedge and forced regressive shoreface respectively, are distinguished on the basis of their position with respect to the sequence-bounding unconformities, reconstructed shoreline trajectories, and by their component facies that indicate the dominant depositional regime.
The mapped basinward shift of the Hatfield 1 lowstand prograding wedge from the previous shoreline deposits and erosional relief on the sequence boundary, indicates a relative sea-level fall prior to its deposition. The lowstand prograding wedge consists of parasequences that are dominated by tidally influenced cross-stratified sandstones and step for more than 30 km basinward, and are readily distinguished from the underlying highstand shoreface facies. Distal aggradational stacking of the lowstand produced a slightly rising shoreline trajectory that in combination with proximal onlap against the underlying erosional unconformity indicates accumulation under conditions of rising relative sea-level with abundant sediment supply. The domination of tidally influenced facies and an estimated relief of at least 20 m in proximal reaches of the underlying sequence boundary suggests that the lowstand wedge was a tidally dominated deltaic system localized and fed through an incised valley. This systems tract resembles other cross-stratified Mancos-type sandstone bodies of the Western Interior Seaway which have been under debate. However, unlike most of these, the Hatfield 1 has great outcrop extent and the updip relationship of the lowstand wedge with the older shoreline deposits can be traced. The overlying retrogradational Hatfield 1 transgressive systems tract has comparable facies to the lowstand wedge and also shows proximal onlap of the sequence boundary, suggesting that it developed within a tidally influenced estuary. As such, the lowstand and transgressive systems tracts form a distinctive cross-bedded tidally influenced lithosome that is readily distinguished from the wave-dominated lithosomes of the preceding Hatfield 1 highstand systems tract and the overlying Hatfield 2 highstand and forced regressive systems tracts.
The Hatfield 2 forced regressive systems tract is a wave-dominated shoreface that like the preceding Hatfield 2 highstand shoreface is strongly progradational. However, in contrast to the highstand shoreface from which it builds, the forced regressive shoreface is relatively thin, lacks shaley offshore transitional facies at its base, and displays a downstepping trajectory relative to the underlying MFS. The basal surface of the forced regressive shoreline also has an enrichment of coarse glauconitic grains derived from erosion of the underlying condensed section whereas the upper bounding surface of the systems tract is an erosional unconformity, documenting the maximum fall in relative sea level. There is a clear sedimentological distinction of the lowstand and forced regressive systems tracts because whereas the former has a tidally influenced facies association, forced regressive facies tend to be wave-dominated. Such facies partitioning and style contrast are thought to reflect the less-confined nature of the highstand and forced regressive shorelines in comparison to the incised or embayed nature of the lowstand and transgressive shorelines.
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An increasing number of studies in recent years have demonstrated that significant progradation of shallow marine systems occurs under conditions of base-level fall. These new data are forcing many sedimentary geologists to critically re-evaluate many aspects of sequence stratigraphy relating to erosion and deposition during base-level (lake- or relative sea-level) fall, and the intrinsic link made between stratal geometries and base-level change. For the first time, this volume brings together a collection of articles that focus solely on forced regressions, providing a more complete picture of the development, formation, variability and preservation of the surfaces and deposits generated during base-level fall.
The results of the studies published here will be of interest to all geologists attempting to understand the relationship between changes in base-level and stratigraphy, and to all who use sequence stratigraphy as a method of stratigraphic correlation and interpretation at outcrop and in the subsurface.