Abstract

The Neoproterozoic lower Beck Spring Dolomite exhibits a prominent, millimeter-scale, light–dark, planar lamination commonly interpreted as a primary depositional fabric. Microscopic examination reveals that the dark laminae are composed of micrite and contain a micrometer-scale, wavy fabric, whereas the light laminae are composed of relatively coarse-crystalline pseudospar that engulfs, surrounds, and crosscuts the dark laminae. Nearly all of the light laminae surround, or occur in proximity to, elongate (long axis fabric-parallel), spar-filled cavity structures. The average δ18O values for dark and light laminae are −1.8‰ (std. dev. 1.6) and −3.0‰ (std. dev. 1.3) VPDB, respectively. Cavity-filling cements are even more depleted in 18O, with an average isotopic value of −6.2‰ (std. dev. 1.1). These characteristics suggest that the coarse-crystalline, light laminae are the result of the preferential, aggrading neomorphism of a thinly laminated, micritic host rock and that the (now cemented) cavity structures acted as conduits for the fluid responsible for recrystallization. The oxygen isotope distributions are consistent with progressive recrystallization by an 18O-depleted fluid, likely of meteoric origin. Therefore, we interpret the prominent millimeter-scale, light–dark layering in the lower Beck Spring Dolomite to result from widespread, laminae-parallel, diagenetic processes—perhaps controlled by fine-scale layer-parallel differences in initial porosity and permeability—and not from primary sedimentary processes.

Extensive, layer-parallel diagenesis on such a fine scale is rare in Phanerozoic shallow marine deposits, due to disruption by bioturbation. However, before the advent of metazoan burrowing, fine-scale, layer-parallel fluid flow would have been more common. Thus, the diagenetic fabric of the lower Beck Spring Dolomite may represent a mode of carbonate diagenesis restricted to the Precambrian. Similar diagenesis may also occur in Phanerozoic environments devoid of vertical bioturbation.

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