Abstract

Glaucony occurs in abundance in clinoform-top and clinoform-toe positions within Atlantic-margin depositional sequences (offshore New Jersey, U.S.A.; Oligocene to Pliocene). Thin section and backscatter scanning electron microscope (BSEM) analyses indicate that grains of glaucony commonly formed in situ within burrows in deep-water (600-1000 m), clinoform-toe settings; fragmentation of glaucony grains in the matrix is most likely to have occurred through animal disturbance (e.g., ingestion and displacement by burrowers). Deep-water glaucony occurrences in the distal clinoform-toe positions show a pattern of association with quartz-sand abundance: in the most distal settings quartz sand is a minor component and its importance increases with proximity to the clinoform fronts. We hypothesize that these glauconitic sands, which commonly have erosional bases, formed by sediment starvation during relative sea-level rise and highstand, when the sandy clinoform fronts (deposited during sea-level lowstand) were abandoned. During particular times of regional sediment starvation over Oligocene to Middle Miocene time, biologically mediated erosion and transport were thus dominant processes in distal clinoform-toe settings, and in situ glaucony grains were mixed with quartz sand grains derived by degradation of the clinoform front. From Middle Miocene time onwards, development of submarine canyons may have restricted redistribution of quartz sand to discrete conduits. Sedimentary fabrics exhibited by shallow-water (< 100 m) glaucony in clinoform-top settings indicate reworking through localized biological or physical means. One extremely glauconite-rich bed in the clinoform-top setting, of Pliocene age, contains the most mature glauconite, in whole grains and fragments of pellets, constituting up to 75% of the sediment. This remarkable bed likely took several million years to accumulate, a time characterized by little terrestrial sediment input. It may represent a significant horizon of at least regional extent corresponding to an extended time of overall sea-level rise during the Pliocene.

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