Abstract: Stratigraphy and Sedimentation of Freshwater, Intermontane Delta

N. J. Hyne, W. A. Cooper

The Catatumbo delta, in the freshwater, intermontane basin of Lake Maracaibo, Venezuela, is unique as a model for some of the Tertiary basins and deltas of the Rocky Mountain area. The processes and geomorphology of the lower deltaic plain of the Catatumbo are similar to those of the Mississippi. Despite almost identical river and lake water densities on the Catatumbo delta, the river water enters Lake Maracaibo as a hyperpycnal jet. However, no lake water-wedge intrusion occurs.

The Catatumbo delta is prograding rapidly into the Lake Maracaibo basin. Fine sand and silt are deposited on the distributary-mouth bar, and redistributed down the relatively steep delta-front slope by grain flow. There is no evidence for subaqueous levees, canyons, and slumping and relatively little evidence for turbidity-current flow out onto the prodelta sediments. Although the lake is relatively fresh (3 percent), the fine-grained suspended sediments are deposited in a well-delineated prodelta area directly lakeward of the distributary. The delta morphology, effluent jet, and prodelta sediments demonstrate an asymmetry which is attributed to prevailing currents and waves.

The Catatumbo delta is depositing a characteristic, vertical stratigraphic sequence. From base to top it consists of a transgressive basal sand overlain by organic, clay-sized, prodelta sediments which contain subaqueous peat layers; distributary-mouth bar sands, silts, and organics; channel sands; fine-grained levee deposits; and, finally on top, interdistributary-swamp deposits.

The geomorphology, sedimentation, and stratigraphy of the Catatumbo delta is similar to that of the Paleocene, Fort Union Formation. Both were deposited in freshwater, intermontane basins.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA