Holocene and Ancient Hardgrounds: Petrographic Comparison: ABSTRACT

Abstract

Subtle diastems or truncation surfaces in carbonate sequences can go unnoticed during core inspection when mineralized coatings or encrusting fauna are absent. These End_Page 975------------------------------ diastems are commonly characterized by a hardground which ideally has a bored or abraded surface; holdfasts of sessile organisms or encrusting fauna; and intraclasts derived from the hardground in overlying sediments. Petrographic examination of Holocene hardground samples from the Persian Gulf and the Bahamas establishes the morphologic habit of contemporaneous interparticle submarine cements. Micritic high-magnesium calcite, fibrous aragonite, fan druses of aragonite, and a graded calcite mosaic (crystal size increases away from nucleation site and crystals exhibit sweeping extinction) are characteristic cements. Inspection of relict cement morphologies in ancient hardgrounds suggests at least five petrographic criteria for the recognition of synsedimentary cementation: (1) a graded calcite mosaic in which crystals exhibit sweeping extinction; (2) impurities incorporated in blocky calcite which occur in fibrous bundles perpendicular to the site of nucleation; (3) a graded clastic mosaic with incorporated layers of micrite (only on the up side) that parallel the nucleation site; (4) radiaxial fibrous cement; and (5) micritization present only on the upper surface of the hardground horizon with no intergranular penetration of micrite below. Further observations include borings which cut grains; truncated grains at apparent breaks in sedimentation; absence of overlying sediment in grain interstic s; intraclasts exhibiting borings and/or encrusting fauna; and obvious absence of compaction of grains. Because permeability and porosity are greatly reduced along hardgrounds, they may provide effective seals within carbonate reservoirs acting not only as hydrocarbon traps but also as inhibitors to vertical migration of potentially cementing fluids. End_of_Article - Last_Page 976------------