Sequence-stratigraphic significance of Miocene to Pliocene glauconite-richlayers, on- and offshore of the US Mid-Atlantic margin

Glauconite is generally agreed to be a reliable indicator of low sedimentat ion rare, but little systematic work has been done to specify the role of g lauconite in a sequence-stratigraphic framework. Ocean Drilling Program Leg 174A recovered a good record of late Tertiary sediments along the shelf ed ge of the New Jersey US Atlantic margin, and glauconite was present in many intervals of the cores, sometimes in vertical proximity to sequence bounda ries. Leg 174A glauconite was analyzed with binocular microscope, XRD and S EM to determine the percent of potassium and degree of maturity in order to relate occurrence to depositional environment. Seismic data were used to l ocate sequence boundaries, and percent glauconite was visually estimated. G lauconite samples from Site 1073 were found to have formed within a lowstan d systems tract (LST), and as part of a distal condensed section (CS) withi n a transgressive systems tract (TST). These results are comparable to thos e from nearby Site 903 of Leg 150, which indicate a similar depositional se tting for glauconite. Glauconites at shelf Sites 1071 and 1072 likely forme d in the TST as well. Onshore, glauconite occurs mainly in transgressive sy stems tracts. The Miocene appears to be the upper limit of glauconite forma tion onshore. As the magnitude of sea-level change decreased, present onsho re locations became too nearshore to maintain sediment-free environments, a nd the zone of glauconite deposition moved seaward. The same process did no t occur offshore until the Plio-Pleistocene. Low subsidence-rate margins su ch as the US Atlantic are subject more to the variations of sea-level than to changes in sediment supply, tectonics, or other factors influencing thei r depositional patterns. Although glauconite occurrence is widespread in th e stratigraphic record, this study demonstrates that for low subsidence-rat e margins, primary deposition of glauconite is largely restricted to the TS T. (C) 2000 Elsevier Science B.V. Al rights reserved.