UNDERSTANDING ‘GRIDWORK’ TEXTURES WITHIN PROTEROZOIC EARLY DIAGENETIC CHERT

Early diagenetic chert is abundant in Proterozoic carbonate successions and provides a critical window into organic preservation. The origins of early diagenetic chert, however, remain uncertain. Although it is assumed that, prior to the evolution of silica-secreting organisms, seawater would have been supersaturated with respect to silica, we do not know whether silica precipitated directly from seawater or whether precipitation may have been associated with an initial gel phase. Additional difficulties in determining the origin of authigenic silica arise from the potential for postdepositional recrystallization that inhibits the identification and interpretation of primary depositional fabrics.

Here we present a comprehensive petrographic analysis of fabrics of early diagenetic chert from the 1.1 Ga Angmaat Formation, northern Baffin Island. Angmaat chert preserves a spectacular array of microbial mat fabrics (Knoll et al., 2013). Preliminary observations of thick sections (approximately 100 microns thick, for identification of microbial features) suggested microcrystalline quartz was the dominant fabric. Analysis of 30 micron sections, however, reveal that the primary fabric is composed of a distinct ‘gridwork’ texture composed of elongated crystals that occur at approximately 90-110 degree angles (cf. Camana et al., 2002). Differences in crystal size populations within gridwork correspond to (1) the density of organic matter within the chert, and (2) the presence of precursor carbonate, and suggest that nucleation of gridwork may be affected by the presence of precursor materials. Where best preserved, gridwork texture appears to be composed of a series of 50-150µm chalcedony spheres that, when observed under the gypsum plate consist predominantly of length-slow chalcedony, consistent with chert formation in restricted, evaporative environments of the Angmaat Formation. Variation in angles of the gridwork appears to reflect the size and packing of these micro-spherules. We suggest that gridwork texture represents crystallization from a primary gel phase that permeated microbial and porous carbonate substrates. Near complete crystallization limited water-rock interaction and later diagenetic recrystallization, and aided in the exquisite preservation of microbial features.