Lateral distribution of detrital clay minerals in Lower Cretaceous sediment, western North Atlantic and its continental margin.

摘要

Fine grained sediment from Lower Cretaceous turbidites (DSDP Site 603) and their correlative deltaic deposits (Baltimore Canyon Trough) were compared mineralogically and texturally. The shelf sediment is coarser grained than deep sea sediment and both are poorly sorted. Deltaic sediment is dominated by illite and kaolinite while deep sea sediment is dominated by illite and smectite. Some kaolinite in deltaic units is authigenic and fills pore space, but at least part of it, particularly in fine-grained sediment, is detrital. Deltaic illite appears to be largely detrital in the SEM. In some deltaic samples where heavy minerals appear partly dissolved, authigenic corrensite fills pore space.; In deep-sea turbidites, smectite dominates the {dollar}<{dollar}0.5 micrometer fraction and illite is most abundant in coarser clay fractions. Smectite level is highest in clay-rich sand beds and in clay rip-up clasts, indicating it has been reworked from deep sea fan levee deposits. Illite levels increase uphole, indicating an increase from the Valanginian to the Albian. Smectite in potassium feldspar-rich samples exhibits delicate fibrous overgrowths interpreted as illite.; Previous studies of the clay mineral composition of Lower Cretaceous western North Atlantic sediment hypothesized extensive vertisols along the eastern North American margin to account for high smectite levels, but the results of this study indicate high levels may result from pericontinental fractionation as a result of differential clay mineral flocculation and settling velocities. The greater volume of shelfal sediment (several kilometers thick) compared to abyssal sediment (tens to two hundred meters) further indicates that smectite was probably not the dominant clay weathered from the Early Cretaceous eastern North America. From field and literature studies, Lower Cretaceous paleovertisols of the Salisbury Embayment comprise a minor component of paleosols.; Possible sources for pelagic smectite include Early Cretaceous volcanic ash derived from the New England-Quebec volcanic event, from andesitic volcanism in Mexico, and possibly more extensive exposure of syn-rift diabase and associated basalt. A high iron content of the smectite (over 7% Fe*) supports the latter source.