Differential subsidence and preservation potential of shallow water Tertiary sequences, northern Gulf Coast Basin

Edwards 1995: Differential subsidence and preservation potential of shallow water Tertiary sequences, Northern Gulf Coast Basin, U.S.A. In: Sedimentary Facies Analysis: A Tribute to the Research and Teaching of Harold G. Reading. Int'l. Assoc. Sed. Spec. Publ. 22, p. 265-281.

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Influence of dip position and subsidence rate on facies preservation potential in channel environments - 160k - click to enlarge

ABSTRACT

Growth faulting, which accompanied shelf-edge progradation and filling of the northern Gulf Coast Basin, resulted in partitioning of the basin margin into fault blocks with contrasting subsidence rates. Study of correlative sections in juxtaposed fault blocks reveals that contrasting subsidence rates can result in strongly differing facies patterns in neighbouring areas. This complicates the task of predicting sandstone reservoir occurrence and properties. Shallow-water clastic sections from Eocene to Miocene in age were investigated using extensive well-log observations, supplemented with micropalaeontology and seismic profiles.

All depositional environments involve an ongoing, complex interplay between sedimentation and erosion at different time and physical scales. In certain settings, a greater subsidence rate causes the preservation of certain facies that would otherwise have been eroded at lower subsidence rates by processes inherent to the environment. The critical subsidence rate that separates preservation from non-preservation is termed the preservation potential threshold for a particular depositional facies. Examples are provided for progradational mouth-bar facies in a deltaic setting (Wilcox), and storm-deposited shoreface-shelf muds in a prograding shoreline setting (Frio).

Influence of dip position and subsidence rate on facies preservation potential in wave-dominated environments - 154k - click to enlarge

Where rates of subsidence are even greater, the growth fault may produce a topographical scarp at the surface, which will influence the disposition of depositional environments. Here, the concept of preservation thresholds is not adequate to account for the observed facies changes. Rather, the presence of the surficial scarp as the surface manifestation of the subsurface fault causes the preferential development and preservation of channel activity in the topographic lows, and progradational environments with channel bypass in the topographical highs. An example is provided for a series of prograding stacked deltas (Miocene). These concepts may help to focus attention on the role of subsidence in constraining the appearance of the sedimentary record.

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