ABSTRACT: Storm Bed Formation and Facies Evolution in the Shallow Marine Environment

D. J. P. Swift, J. A. Thorne, A. W. Niedoroda

Beds in the shallow marine environment are records of the progressive sorting events that drive facies differentiation. Their thickness can be directly related to the areal bed-load concentration, c², during peak storm wave height, and can be computed by means of the Grant-Madsen algorithm. Synthetic stratigraphic columns can be built be parameterizing the storm climate as a Gumbel distribution (maximum wave height vs. return period), then computing the thickness, [EQUATION], where t is storm return period, and p is the return period power. The return period intercept, a, describes the minimum (t = 1 yr) bed thickness. The intercept can be used to determine the reworking ration, r = a/a, where a is the amount of new sediment deposited per storm, and determines he degree to which the section is expanded or condensed. If the Gumbel coefficients ^agr and ß can be estimated, then the amalgamation depth can also be calculated.

These parameters yield as facies boundaries the breaker zone source, the amalgation depth, and the sand transport limit. The boundaries define a proximal condensed facies, an expanded facies of sand tempestites, and a distal expanded facies of mud tempestites. As parameters vary, the facies assemblage deforms allometrically, but key boundaries are retained. Simulations show that as r increases, beds become patchy. Larger beds tend to grow basinward through successive events.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990