Seismic Stratigraphic Implications of ODP Leg 101 Site Surveys

Abstract

Prior to ODP Leg 101, high-resolution geophysical site surveys were conducted in three parts of the Bahamas: north of Little Bahama Bank on the southern Blake Plateau, in the central Straits of Florida southeast of Miami, and in the southeastern part of Exuma Sound. A total of approximately 2200 km of 24-trace, 12-fold water-gun reflection profiles were collected, along with 26 sonobuoy profiles, bathymetry, and magnetics. For each region, a seismic stratigraphic framework was developed, and both structure and isopach maps of prominent acoustic horizons and seismic sequences were constructed. Some of the seismic stratigraphy has been corroborated by Leg 101 drilling results, but even where such substantiation is lacking, these geophysical data provide important insights into the structural and stratigraphic evolution of the Bahamas.North of Little Bahama Bank, the mid-Cretaceous to Holocene section sampled at Site 627 is represented by seven seismic stratigraphic sequences exhibiting a distinctive vertical succession of seismic facies. Both thrust and normal faults offset the section, which thickens to the east. Total sediment thicknesses above sequence G, a late Albian shallowwater carbonate platform, exceed 1.2 km near the modern bank edge. Thickness trends in the Cenomanian-Holocene section suggest long-term line source input from Little Bahama Bank and the smoothing effects of contour-following bottom currents.In the Straits of Florida, 11 seismic sequences are identified above a "target" horizon believed to represent a buried platform top perhaps coeval with the top of sequence G north of Little Bahama Bank. This "target" exhibits more than 400 m of relief within the area surveyed, potentially a result of erosion, karstification, and/or the presence of patch reefs on top of a drowned "megaplatform" surface. These patch reefs may now be represented by a hummocky seismic facies immediately underlying parts of the "target" surface. Total sediment thicknesses above the "target" range from 1.0 to 1.4 km.In southeastern Exuma Sound, site-survey profiles are interpreted within the context of a preexisting seismic stratigraphic framework for this part of the Bahamas. Another inferred mid-Cretaceous shallow-water carbonate platform top, P, exhibits a refraciion velocity in excess of 6.0 km/s. P dips to the southwest, perhaps in response to continuing compression between the Bahamas and the northern Caribbean. This hypothesis is supported by the presence of normal faults that offset the entire post-P section. Total sediment thicknesses above P range from 1.3 to 2.6 km. Distribution of Neogene sediments is controlled by long-term bank input from the south and continuing erosion along the axis of Exuma Sound.

Fluid flow through carbonate platforms: constraints from 234U/238U and Cl− in Bahamas pore-waters

“…The Bahamas are a series of Mesozoic basement features swathed in a thick sequence of carbonatedominated sediment [14]. These sediments have responded sensitively to sea-level change, prograding rapidly basinward during episodes of high sea-level.…”

Interstitial-Water Chemistry and Diagenesis of Periplatform Sediments from the Bahamas, ODP Leg 101

“…ANALYTICAL METHODS Pore waters were squeezed from 10-cm-long, whole-round samples on board ship immediately after coring (Austin, Schlager, et al, 1986). Methods and equipment for pore-water extraction were described by Sayles and Mannheim (1975).…”

Geophysical evidence for Mesozoic syn-rift salt diapirism in Southeast Bahamas

“…That was an effort to understand the evolution of the Bahamas (Austin et al 1988), in particular, to map structure in a carbonate mega-platform that was drowned in the middle Cretaceous on the opening of the Atlantic Ocean. A 400 cubic inch water gun was the seismic source.…”

“…• a compilation of all available gravity measurements in the Bahamas including marine surveys conducted by the University of Texas Institute for Geophysics (UTIG) in 1984 (Austin et al 1988) and in 1982 by the Lamont-Doherty geological observatory (LDGO) (Ladd and Sheridan 1987) and pre-1960 data compiled by Talwani (1960), • an aeromagnetic survey of Long Island, This paper intends to examine the geological implications of this geophysical data, in particular the evidence for a salt diaper.…”