Introduction

Barrier reefs establish themselves on the edge of siliciclastic continental shelves, usually on topographic highs of the lowstand paleoshore line, when the sea transgresses and submerges the former fluvial plain. Because longshore currents usually trap the siliciclastic sediments and fresh water input from fiver discharge along the newly established coastal zones, the new shelf edge becomes favorable to coral growth. If those favorable conditions to coral growth persisted, the barrier reefs would have continued to flourish during most of the sea level highstand interval and possibly extend laterally as long as the accommodation space remain available. The reefal material, deposited during this phase of the reef maturity, usually entombs the early transgressive establishment phase of the barrier reef. Short lived drowned coral reefs along modern shelf edges, such as the reef of Southern Bank offshore Corpus Christi (Fig. 1), become small windows of opportunities to study the processes involved in the initial reef establishment and premature demise. Moreover, these reefs encapsulate in great details unique information on the early phase of the ocean transgression, in terms of its timing, amplitude, and rates, in addition to a partial record of environmental changes from annual to millennium time scales.

Methods

In Fall 1996 and Spring 1997, we have acquired, processed, and interpreted three very high resolution (VHR) seismic grids over Southern Bank, a “drowned”, 1 km-wide coralgal reef, still partially exposed on the sea floor of the Texas shelf, 55 km offshore Corpus Christi (Fig. 1). A 2D seismic grid (8 individual 24 channels seismic profiles), a 2.5D grid (40 seismic profiles), and a true 3D seismic grid (12 seismic profiles) were acquired during the survey of Southern Bank. This seismic survey images well the external morphology and partially internal architecture of this fossil coralgal reef. The survey displays also the geometrical relationship between the reef itself and the sedimentary deposits on top of which the reef has grown, and within which it was surrounded and finally partially buried (Fig. 2). This project has been funded by Amoco, Conoco, and Total.

Results

The Fall 1996 high resolution seismic survey clearly images Southern Bank. Most of the dip lines display the expected asymmetric morphology of a true reef growing in a high energy environment, with a forereef slope steeper than the slope on the back reef (Fig. 2). The deepest part of the bank exposed on the sea floor is today at - 80 m. Its highest peak, at - 60 m, has been dated in an earlier study by radiocarbon at about 11 Ky. Our seismic profiles clearly show that the basis of the bank is buried at least 20 in below the sea floor (about -100 in below sea level) (Fig. 2). Several identical reefs have been observed along the edge of the southern Texas shelf and some coralgal material collected at the base of one of these reefs has been dated by radiocarbon at about 18 Ky (Fig. 1). Southern Bank, therefore, appears to have tracked rather well the last sea level transgression prior to the Younger Dryas.

An unconformity on the seismic profiles harks the top of the coralgal bank itself and the adjacent thinner deposits, contemporaneous to the reef edifice, which were partially eroded especially in the back reef area (Fig. 2). A younger, 20 to 25 m-thick, sedimentary package, referred to as the mud blanket on the southern Texas shelf, was clearly deposited on onlaps around the lower part of the coralgal bank and dated by radiocarbon as 7.3 Ky at 19 in below sea floor. Such an unconformity associated with truncation (erosion) and onlaps can be considered to be a classic sequence boundary and should have, therefore, been developed during an interval of falling sea level and/or sea level lowstand. Our observation would then point out that sea level had to have fallen during the Younger Dryas and as a result exposed at least the top, perhaps even more of the coralgal bank. The bank was subsequently drowned during the second part of the deglaciation, once sea level started to rise again. The short-lived partial exposure of the reef during the Younger Dryas might explain its demise during the subsequent sea level transgression, characterized by rates of sea level rise in the order of m/century.

Southern Bank is part of a larger coral reef complex which briefly flourished during the early part of the late Pleistocene/Holocene sea level transgression on the edge the siliciclastic southern Texas shelf, in an 140 km-long band within an embayment between the Rio Grande and Brazos/Colorado shelf margin lowstand (Fig. 1). If coral growth had been reinitiated on the coralgal banks during the second part of this last sea level transgression, subsequently to the brief sea level pause or even perhaps sea level fall of the Younger Dryas, one can imagine that a fully developed barrier reef, 100 km in length, would occur today 50 km offshore along the edge of the southern Texas Shelf! The short lived drowned coralgal reefs, such as the reef of Southern Bank offshore Corpus Christi, can be considered to represent the roots of fully developed modem barrier reefs. These drowned reefs, therefore, become small windows of opportunities to study the processes involved in the initial reef establishment and premature demise. Moreover, these reefs encapsulate in great details unique information on the early phase of the ocean transgression, in terms of its timing, amplitude, and rates of the sea level rise.