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Sediments and Depositional History of the Southeast Lobe of the Colorado River Delta, Texas
Leroy L. Manka (1), Richard Steinmetz (2)
Reconstruction of the depositional history of the Colorado River Delta's southeast lobe is made possible by combining the Delta's documented history, sedimentary and hydraulic principles, past and present depositional environments, and the present distribution of resultant sediment types.
Synthesis of these data makes possible reconstruction of the depositional history as follows:
- pre-1929 sedimentation was restricted to the slow accumulation of fine detritus in a reducing environment.
- deltation began in 1929. By 1930 the initial delta lobe extended half-way across Matagorda Bay. Progressively siltier prodelta clays were deposited in the study area. Deposition of generally finer prodelta sediments followed as a second lobe formed on the east flank.
- development of southeast lobe began in 1933 at the mouth of an artificially leveed channel that extended a half-mile beyond the southern edge of the existing delta plain. After a period of adjustment to this unnatural channel, a normal sequence of prodelta and delta-front environments and associated subenvironments developed and progaded east-southeastward.
- in 1936 the discharge channel was extended through Matagorda Peninsula, and the Colorado River began discharging directly into the Gulf of Mexico. Distributaries to the southeast lobe remained open but could divert only a fraction of the total discharge. This decrease in volume of discharge was accompanied by lower flow velocities, a slower progradation rate, and the deposition of generally finer sediments.
- the lobe entered its destructional phase in 1941, when all distributaries were closed. Wave action has removed most of the subaqueous delta front. Beaches and interdistributary bays have formed, and a flourishing marsh has developed.
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