Petrology and petrography of the Campeche lithic suite, Yucatan shelf, Mexico

Abstract

The Campeche Bank, which constitutes the northwestern portion of the continental shelf of the Yucatan Peninsula of Mexico, is an area characterized by recent carbonate sedimentation. The shelf is composed of Pleistocene-Tertiary indurated limestone overlain by an unconsolidated sediment veneer, only part of which is modern sedimentary material, the bulk being relic sediments associated with sealevels of the Pleistocene-Holocene fluctuations. Over 1000 core and grab samples of this veneer were collected from the Texas A. & M. Research Vessel, Hidalgo. Fragments of lithified carbonate rocks occurred in most of these samples, and were interpreted as representing true lithoclasts, having been torn by some process from the indurated limestones beneath the veneer. The lithic fragments were removed from the samples and thin-sectioned for petrographic study. The results of their examination were used to reconstruct the more recent paleogeographic history of the area, and to investigate the early diagenetic changes that occur in carbonate rocks. Petrographic analysis resulted in the division of the lithic fragments into six textural groups: (1) biomicrites; (2) biosparites; (3) oomicrites; (4) oosparites; (5) mudstones and (6) boundstones. However, in order to obtain mappable units, some condensation of these six groups was necessary. The micrites were grouped together irrespective of their allochemical constituents. A similar grouping was made of the sparites, whereas mudstones and boundstones were not included in the major groups, being quantitatively less important. Mapping the distribution of the sparites and the micrites resulted in a pattern of broad bands across the shelf, parallel to the present shoreline and bathymetric contours. As both the textural and mineralogical aspects of the rock fragments within these bands differ, their distribution must be attributed to varying depositional environments which were prevalent during or immediately preceding the Holocene transgression..