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An Integrated Approach to the Sedimentological Analysis of Some Lower Cretaceous Shoreface and Delta Front Sandstone Sequences
An integrated process sedimentological approach to the analysis and description of some Lower Cretaceous (Albian) sandstones in core, has provided the basis for differentiation of shoreface from delta front facies sequences. While these sequences look almost identical on gamma-ray well log signatures, they contain numerous distinct and markedly different internal sedimentary characteristics. Recognition of these facies, however, would be extremely difficult if not impossible without the combined utilization of physical and biogenic sedimentary structures. Sedimentary structures in distal and proximal delta front facies suggest a dominance of suspension, traction and gravity-induced processes of deposition. The presence of rhythmically bedded, laminated siltstones in the distal delta front facies implies a cyclicity to sedimentation patterns, perhaps as a function of seasonal fluctuations in sediment supply or fluvial discharge. In contrast, lower shoreface units display an increase in the frequency and thickness upward of laminated sandstone beds interpreted as storm-generated deposits. Syneresis cracks and a very low-diversity Cruziana ichnofacies assemblage in the distal delta front facies, suggest fluctuating water salinites and harsh environmental conditions due to high water turbidity and fluctuating sedimentation rates.
Physical sedimentary structures in upper shoreface sandstones are produced by high energy, traction deposition and oscillatory flow produced by waves. A very high-diversity Skolithos ichnofacies assemblage implies more uniform bottom water conditions and sedimentation rates. In contrast, sedimentary characteristics of the proximal delta front facies suggest scour, differential loading of sediment, and wave and current reworking or erosion. Burrow traces are correspondingly rare to nonexistent.
In at least one core, shoreface and delta front facies are interbedded and bounded by abrupt contacts. Such a sequence suggests very rapid alterations in fluvial input to the marine environment probably as a function of delta lobe abandonment or channel avulsion. The recognition of delta front facies within shallow marine sequences can have a very important impact on regional paleogeographic reconstructions and determination of facies relationships in subsurface investigations. These reconstructions include identification or prediction of the following: 1) proximity to, or precise location of, terrigenous clastic depocenters; 2) thick accumulations of woody organics such as coals; 3) paleostrandline orientation; and 4) sand body “geometries”.
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