An Organic Control on Shoreface Stacking Patterns--Fine Tuning Sequence Stratigraphic Controls

MCCABE, PETER J., U.S. Geological Survey, Denver, CO, and KEITH W. SHANLEY, Colorado School of Mines, Golden, CO

A rise in base level of several meters can cause inundation or coastal transgression of tens or even hundreds of kilometers because of low topographic gradients of coastal and lower alluvial plains. In ever wet climates, however, raised mires may cover significant portions of coastal plains. These mires are elevated several meters above flood levels of adjacent rivers and littoral environments. Raised mires have relatively flat central regions and steep sides with gradients that may exceed 4 cm/m. The elevated nature of raised mires may significantly reduce the areal extent of marine transgressions resulting from base level rise. Furthermore, thick peat deposits are difficult to erode and so retard shoreface erosion. Raised mires may also deprive the lower stretches of a river of a tr e floodplain and create an upstream area where fluvial sediments can be ponded. Under favorable climatic conditions raised mires may continue to accumulate peat and maintain their topography relative to base level rise.

In the Cretaceous of the Western Interior thick, low ash coals occur immediately landward of vertically stacked shoreface sandstones. These stacking patterns reflect a balance between sediment supply and rate of production of accommodation space. We suggest the development of raised mires may have stabilized the position of shorelines for long periods of time by modulating this balance. Because of the raised nature of the mires, accommodation space on the coastal plain would be filled by peat rather than clastic sediment. In addition, mires may regulate the supply of sediment to the shoreline by trapping sediment upstream. Major transgressions, therefore, may occur only when rates of base level rise exceed peat accumulation rates. Major regressions, however, may occur during slow rate of base level rise when sediment is able to bypass the mire.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)