Quantitative petrographic criteria for recognition of environments of deposition

Abstract

Petrographic analysis can be a powerful tool in the recognition and delination of depositional environments of detrital sediments. Major environmental complexes (barrier, fluvial, and deltaic) can be differentiated in Recent sediments of the Gulf Coast on the basis of mineral composition and quartz grain size data. In addition discriminant function analysis of data from samples taken from individual environments within each major complex reveals that distinction is possible between environments or groups of environments characterized by distinctly different sedimentary processes. In the Galveston Island barrier complex lower shoreface, middle shoreface, upper shoreface-beach-dune, and lagoon environments can be segregated. In Recent Mississippi River sediments point bar, natural levee-silted channel, lake, and swamp environments can be segregated. Finally, in Recent Mississippi Delta sediments transgressive beach, natural leveechannel-delta front, open bay, and marsh-closed bay prodelta environments can be segregated. Similar results are obtained from samples in Wilcox barrier, fluvial, and deltaic environments in central Texas. Differences between Recent and ancient examples are primarily a result of diagenesis. Environmental segregation is maximized through the use of mean values of mineral composition and quartz grain size over intervals of several feet within each sampling locality. Separation is best in environments characterized by sediments of very fine sand grade and larger. Thus in basins of known lithologic character, it is possible to supplement the more qualitative and traditional sedimentary structural analysis of depositional environments with quantitative analysis of petrographic data.