Regional and Fine-Scale Strata Formation in a Major Accretionary Environment: Amazon Continental Shelf: ABSTRACT

Investigation of Amazon shelf sedimentation demonstrates that strata is presently accumulating on the inner shelf (< 60 m water depth) in the form of a subaqueous delta (composed primarily of silt and clay) which is prograding over relict basal sands. Seismic reflection records (3.5 kHz) reveal regions of topset, foreset, and bottomset stratigraphy. The formation of fine-scale (< 50 cm) strata was examined using x-radiographic and radiochemical (e.g., ²¹⁰Pb, ¹³⁷Cs, ²³⁴Th) techniques on more than 60 box cores from the deltaic deposit. On the basis of sedimentary structure, three distinct sedimentary environments have been delineated within the topset region: (1) interbedded mud and sand in a corridor extending from the river mouth across the inner shelf, (2) faintly laminated muds to both flanks of this corridor, and (3) bioturbated muds comprising the remainder of the deltaic deposit. The interbedding of mud and sand may result from short-term fluctuations in a turbulent jet emanating from the Amazon River mouth. Faintly laminated muds are deposited from suspension on the flanks of the turbulent jet and have the highest accumulation rates found on the inner shelf (> 2 cm/year). Where accumulation rates are reduced, bioturbated muds are present. ²³⁴Th data from the bioturbated muds indicate rapid mixing down to 5 cm with a mixing coefficient of about 30 cm²/year. Observations of sedimentation on the foreset and bottomset regions are not as comprehensive because of their relatively small areal extent. However, accumulation rates are found to decrease from topset (> 2 cm/year) to bottomset (< 0.5 cm/year) regions. With decreasing sediment accumulation rate the effects of biological mixing on preserved strata become more pronounced.