Cement Fabrics of the Bahamian Platform and Its Margin Near Lee Stocking Island
WHITTLE, GREG, LINDA ROUCHE, ROBERT F. DILL, and CHRISTOPHER G. ST. C. KENDALL,* University of South Carolina, Columbia, SC
Consolidated to friable carbonate rock samples were collected on and around Lee Stocking Island to determine the distribution, fabric, and mineralogy of their cements. The rocks include: (1) beachrock rimming the islands, (2) shallow-water hardgrounds, (3) reef rock, (4) channel stromatolites, and (5) Pleistocene bedrock. Analyses by SEM, microprobe, X-ray diffraction, and petrographic microscope have revealed ten different cement fabrics. Five of these cements are varieties of fibers, all of which are aragonitic except the "whisker" fibers that form coarse networks of intertwining high-Mg calcite in a Pleistocene cave sample. Acicular fan-druse and square-tipped coarse fibers cement the beachrock, while an isopachous, needle-fiber rim is found only in the hardgrounds. A radial fibrou cement occurs in several ooids and biogenic grains, representing a replacement fabric of aragonite that has inverted to high-Mg calcite.
Two types of blades are present: a stubby variety with a length: width ratio of 2:1 and an elongated 5:1 variety, both of which are high-Mg calcite. While the 2:1 variety is rather common, the 5:1 variety only occurred in one sample. Aragonitic micrite envelopes often surround grains in beachrock and hardgrounds, but only in association with fibrous cement. An aragonitic lime mud matrix cements the crusted mud beds and low-Mg calcite equant spar cements the Pleistocene samples and occurs as void-fill in beachrock and hardgrounds.
The most common marine cementation is associated with the aragonitic fibers found in the discontinuous hardgrounds and beachrocks. The more widespread cements are the low Mg-calcite spar associated with meteoric diagenesis and cementation of the Pleistocene surface.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)