Erosional Origin of Inner Shelf Sediments--Evidence from North Florida: ABSTRACT

Abstract

Study of 194 vibratory cores (4-20 ft long) from the Atlantic inner shelf off central and northern Florida indicates that most of the Holocene shelf sediments were derived from erosion and reworking of shelf substrata, and that direct fluvial contribution attending the last rise in sea level was negligible. Erodable, unconsolidated, and semi-lithified Tertiary and Pleistocene deposits are present in localized exposures or lie at shallow depths beneath the inner shelf surface in many places. All these older sediments contain ample quantities of fine to coarse quartz sand. Selective removal of the finer constituents of these older deposits, such as small Foraminifera, silt-size dolomite rhombs, and terrigenous muds, by erosion and reworking during Holocene transgression, ca readily account for the veneer of fine to medium orthoquartzitic sand that mantles the inner shelf. Progressive upward depletion of these characteristic fine constituents within the Holocene sand body is evidence of continuity with the underlying source strata. In addition, species of large, durable Foraminifera and phosphorite grains, both typically abundant in the Tertiary substrata, are present throughout the Holocene sand body. Although ultimately derived through the large Piedmont-drainage rivers in Georgia, the present shelf assemblage is indicative of mixed local sources. An erosional origin further explains the observed characteristics of the surface sediment: low feldspar, high phosphorite, and unstable heavy mineral assemblage, and pronounced rounding of quartz grains. In contrast, direct fluvial mechanisms of deposition do not account for such characteristics. The last major rise in sea level was evidently a period of extensive erosion for the Atlantic shelf, in addition to being a transgression with discontinuous deposition as Curray has suggested. End_of_Article - Last_Page 778------------