Genesis of Recent Lime Mud in Southern British Honduras

Abstract

ABSTRACT In view of the abundance of in the geologic record, it is paradoxical that studies of Recent carbonate sediments generally have concentrated on the origin of sand-size carbonate particles rather than on the origin of the finer-grained constituents. Only on the Great Bahama Bank has the genesis of received paramount attention; so much attention, in fact, that the words needles have become virtually synonymous with lime mud in the minds of many geologists. Preliminary investigation of Recent from Southern British Honduras, however, revealed a paucity of aragonite needles. An investigation was therefore undertaken to ascertain the nature and origin of the in this area. Mineralogical, chemical, and petrographic point-count data were gathered. The strontium content of the carbonate fraction of lagoon samples increases systematically toward carbonate shoals. The mineralogical composition of the carbonate fraction of lagoon samples averages 25 percent high-strontium aragonite, 24 percent low-strontium aragonite, 44 percent high-magnesium calcite, and 7 percent low-magnesium calcite. Petrographic data suggest that the high-strontium aragonite is primarily coral debris admixed with lesser amounts of Halimeda debris. Similarly, low-strontium aragonite consists primarily of mollusc debris; and high-magnesium calcite, of Foraminifera debris. The data suggest that the Shelf Lagoon consists of transported shoal-derived debris and in situ-produced mollusc debris and hyaline Formainifera debris. Physical breakage and abrasion in agitated environments are considered the dominant processes of production on the carbonate shoals; whereas the major factors in the in situ production of lagoonal appear to be: (1) the inherently fragile nature of the shells of molluscs and tests of hyaline Foraminifera of the lagoon environment, (2) the removal of binding organic matter from mollusc shells, (3) the weakening of larger skeletal particles by the activity of boring micro-organisms, and (4) the mastication, ingestion, and perhaps even simple movement of sediment by the vagrant benthos. The results of this study indicate that may originate in a variety of ways. While it may be difficult or impossible in the geologic record to recognize ancient analogues of the various types of that can be recognized in the Recent, an awareness of the possible multiple origins of serves to increase our understanding of the genesis of ancient mudstones.