Dolomitization Interrupted by Silicification

Abstract

ABSTRACT Relationships among dolomite, chert, and quartz sand grains within the Cambro-Ordovician Knox Group the vicinity of Blacksburg, Virginia, bear directly on the manner, time, and environment of dolomite and chert formation. Of particular interest is the fact that dolomitization was at times and places interrupted by the introduction of silica. The original carbonate sediment was CaCO3, typically pelletiferous, much of which was chemically precipitated. Initial dolomitization is represented by dolomite rhombs within chert and within overgrowths on quartz sand grains which were deposited along with the original CaCO3 sediment. Silica, the deposition of which interrupted the dolomitization process, is now represented by white to gray chert and by the overgrowths on quartz sand grains. The chert occurs chiefly nodules with their long axes essentially parallel to bedding. Some of the chert shows well defined relict oolitic and/or pellet texture. Bedding laminations extend through many of the nodules. Some nodules are skeletal and some show concentric banding. A few nodules exhibit shrinkage cracks and locally the resulting fragments were imbricated prior to deposition of the overlying carbonate sediment. Dolomite crystals within chert nodules are euhedral and commonly show zoning whereas nearly all dolomite grains at the same stratigraphic horizons but outside the nodules are anhedral to subhedral and unzoned. Although there is a general uniformity size, structure, and distribution of dolomite crystals along individual stratigraphic horizons within the nodules, there is a general lack of such uniformity between adjacent horizons. The entry of silica apparently caught the dolomitization process in the act by selectively replacing CaCO3, but not dolomite, and preserved the features of the dolomite developed up to that time. In some places the silicification prevented further dolomitization. In others, dolomitization was resumed after deposition of the silica and resulted partial to complete conversion of remaining, unsilicified CaCO3 sediment to dolomite. Typically the dolomite grains of the mosaic, which encloses the chert nodules, are appreciably smaller than those at the same horizons but within the nodules. Possibly solution at contacts of the dolomite grains outside the chert nodules--probably concomitant with expulsion of water during compaction--reduced grain size and produced the mosaic texture typical of most Knox dolomite. Other features of the dolomite include relict crossbedding, scour-and-fill, ripple marks, mud cracks, intraformational breccia, relict pellet texture, and small supratenuous folds--produced by differential compaction--over chert nodules and displaced chert fragments. Nearly all quartz sand grains enclosed wholly by chert have retained all or most of their original clastic shapes whereas many of the grains contact with dolomite have been slightly to markedly etched. It is concluded that: the areally extensive sediments of the Knox Group were deposited relatively quiet, shallow, marine waters; some were intermittently exposed to the atmosphere; initial dolomitization and deposition of silica, at least, occurred essentially at the sediment-water interface; most silica was probably deposited under oxidizing conditions; and, dolomitization occurred under hypersaline, probably abnormally alkaline, conditions or water relatively low silica content.