Petrology and Diagenesis of Tertiary Aquifer Carbonates, North Carolina: ABSTRACT

Abstract

Superior stone quarry near New Bern, North Carolina, exposes 9.2 m of Eocene Castle Hayne Limestone that is disconformably overlain by 2 m of an unnamed Oligocene carbonate unit. Both form part of the most important aquifer system in eastern North Carolina. The Eocene Castle Hayne strata are divisible into 2 massively bedded facies: (A) a sandy, pelecypod-mold biomicrudite with microspar and pseudospar matrix, and (B) a sandy, pelecypod-mold biosparite and biosparrudite, which grades westward into calcareous quartz sand. Facies A consists of unabraded pelecypod valves (dominantly molds of aragonite Macrocallista shells) and 20% by volume of moderately well-sorted, fine quartz sand that are set in microspar and pseudospar. Facies B consists of pelecypod valves (chiefly molds of worn, fragmented, and unabraded Macrocallista) and 27% of well-sorted, fine quartz sand. Sparry calcite cement forms 41% of this facies. Subordinate allochems in both are gastropods, foraminifers, bryozoans, echinoderms, ostracods, intraclasts, peloids, glauconite, and bone. Facies A is a low-energy, shallow-marine bank that accumulated seaward of Facies B, which was deposited in a higher energy nearshore environment. The Oligocene stratum is a sandy, molluscan-mold biomicrudite consisting of pelecypods, turritellid and naticid gastropods, and scaphopods (all as molds of unabraded aragonite shells) that are set in micrite. Fine, moderately well-sorted quartz sand forms 3% of the unit. Other allochems are echinoderms, foraminifers, peloids, ostracods, and bone. This is an inner or middle-shelf deposit that accumulated below wave base. Upon subaerial exposure, the following diagenetic changes occurred: (1) high-Mg calcite skeletons, mainly echinoderms, recrystallized to low-Mg calcite, (2) most aragonite skeletons dissolved to form molds, and the carbonate either precipitated nearby as low-Mg calcite cement, or neomorphed to spar, (3) molds were reduced or filled with low-Mg spar, and (4) micrite and pelmicrite aggraded to microspar and pseudospar. End_of_Article - Last_Page 1903------------