ABSTRACT

Relative abundances of clay species in the <2 µm fractions were determined for the three lithofacies identified in a 5 m core from the proximal portion of Cubits Gap crevasse-splay, Mississippi Delta, in order to determine the control of depositional mechanics on clay-mineral composition. Proximal splay lithofacies, defined by sedimentary structures and the relative abundances of cohesive (mud) versus non-cohesive (silt and sand) sediments, are: (1) suspension-deposited mud, (2) interlaminated current-deposited silt and suspension-deposited mud, and (3) current-deposited sand. The weight percentages of sand, silt, and clay (s:slt:cly) in these lithofacies, defining discrete fields in a ternary grain-size plot, are: (1) suspension-deposited mud (6 samples), 0-9 wt% : 40-68 wt% : 28-60 wt%; (2) interlaminated silt and mud (6 samples), 1-30 wt% : 60-98 wt% : 0-31 wt%, and (3) current-deposited sand (2 samples), 79-100 wt% : 0-17 wt% : 0-4 wt%. The range in relative peak-height percentages of illite, smectite, and kaolinite plus cholrite (I:S:K + C), calculated using modified methods of Griffin (1971), are: (1) suspension-deposited mud, 33-41% : 35-50% : 18-26%, (2) interlaminated silt and mud, 31-39% : 43-52% : 17-20%, and (3) current-deposited sand, 43% : 38% : 19%. These results overlap with the observed range of illite, smectite, and kaolinite in Mississippi River suspended sediment reported by Johnson and Kelley (1984) but are richer in smectite and depleted in kaolinite relative to clay abundances in sediment on the Louisiana continental shelf (Brooks et al, 1976).

No systematic variation in the relative abundance of illite, smectite, and kaolinite plus chlorite is present in the <2 µm size fractions of the three lithofacies at Cubits Gap. In the crevasse-splay studied, physical processes that operated during sedimentation were incapable of fractionating significantly the clay species in the <2 µm fraction. These results suggest that differing clay compositions in comparable, ancient, deltaic, depositional systems reported elsewhere are the result of diagenetic processes and are not due to physical segregation in the depositional environment.