PETROLOGY AND STRUCTURE OF THE COCKEYSVILLE FORMATION (PRE-SILURIAN) NEAR BALTIMORE, MARYLAND

Abstract

The Cockeysville formation near Baltimore, Maryland, is a sequence of intricately folded carbonate-rich metasedimentary rocks about 750 feet thick and of pre-Silurian age (Wasserburg et al. , 1957). The formation lies near the base of the Glenarm series, which mantles five elongated domes of Precambrian (?) basement gneiss at the eastern edge of the Piedmont Province. Sediments and gneissose phases of the domes were regionally metamorphosed to about the amphibolite facies as defined by Eskola et al. (1939). Crystalline metadolomite, calc-schist, and calcite marble constitute most of the Cockeysville. Local breakdown of dolomite accompanied the formation of diopsidequartz-tremolite calc-silicate marble and was marked by a “retrograde” paragenetic sequence of diopside to tremolite to talc. Calcite marble, which occurs in distinct layers and discordant masses associated with calc-schist and phlogopite-calcite metadolomite, is believed to have had a multiple origin. Mineral assemblages and rock associations suggest that some of the marble formed syntectonically from argillaceous limestone, quartzose limestone, and argillaceous dolomite (dedolomitization); other calcite marble may have recrystallized from essentially pure limestone. Discordant masses of calcite marble formed in part by replacement of dolomite or argillaceous limestone and locally dilated their enclosing rocks; some of these masses may have begun crystallizing from limestone; all show coarse nondirectional fabric suggestive of a late tectonic or post-tectonic origin. Metasomatism in the recrystallizing sediments was evidently restricted to aureoles around basic and granitic dikes. Such intrusions are rare, and in general the marbles are chemically similar to sedimentary carbonate rocks. Tourmaline, apatite, and scapolite, which are minor but widespread, can reasonably be attributed to recrystallization with little or no addition of material. Metamorphic grade remains essentially constant throughout the area mapped by the writer. Various facies of the Cockeysville marble, however, can be assigned on the basis of their mineral assemblages to any one of several metamorphic subfacies; local variations of CO 2 pressure, H 2 O pressure, and bulk composition may be largely responsible for these variations. Structural elements in the carbonate rocks—minor fold axes, lineations, cross-joints, and foliation—closely reflect corresponding structural elements of the gneiss domes, and this parallelism diminishes with distance from the domes. The minor folds show a crude variation in intensity and type with distance from the domes.