Diagenetic Fabric and Structures in Ordovician Slope Limestones: ABSTRACT

Abstract

The very thin continuous bedding characteristic of the calcareous Ordovician slope sediments from western Newfoundland and Virginia is not a primary depositional fabric but a severe diagenetic modification caused by extensive physical compaction and pervasive pressure solution. The very thin-bedded (0.1 to 0.5 cm) units commonly are only 20 to 30% carbonate, but occur in a sequence with thicker (1 to 20 cm) limestone turbidite layers. Some groups of very thin layers thicken laterally into elongate limestone lenses composed of quartzose, peloidal, and radiolarian-sponge spicule packstone to wackestone. Layers in the lenses have a primary depositional fabric. Each layer thins away from the lens by 50 to 80%, but is generally traceable for over tens of meters or from lens to lens where they are repetitive along bedding. As a layer thins away from a lens, fine carbonate is lost and peloids and most other carbonate grains are partly to completely pressure solved against more resistant grains or along fine solution seams. Radiolaria become crushed and spicules reoriented from essentially random to parallel with layering. Individual layers are commonly traceable from lens to lens with no change in amount of insoluble quar z, spicules, or other resistant grains. The thickened layers associated with limestone lenses are interpreted to be remnants of once-continuous layers that have been dramatically thinned through pervasive pressure solution. Further, there are numerous very thin layers that cannot be traced into a thicker limestone-rich zone. These layers are very similar to those which pass into thicker lenses. It is very probable that each of these layers is a remnant of a once much thicker carbonate-rich layer. End_of_Article - Last_Page 640------------