UNUSUAL CARBONATE MICROSPAR ASSOCIATED WITH “FLUIDIZED” BEDS: A GEOLOGIC SNAPSHOT OF SPONTANEOUS WATER-COLUMN NUCLEATION AND FORMATION OF A VISCOUS COLLOID

The Blue Beds of the Proterozoic Atar Group, Mauritania, consist of 4-6 meters of amalgamated breccia deposits. Thick beds (>50 cm) of matrix-supported, contorted to graded breccia has led to the interpretation of formation by fluidization of a semiconsolidated substrate. Previous analysis of clasts within the matrix-supported breccia (Stagner et al. 2004) demonstrates their similarity to Precambrian molar-tooth (MT) structures. MT structure consists of a complex network of voids that develop in the shallow substrate and are immediately filled with an unusual microspar that consists of two distinct phases – spheroidal cores and secondary overgrowths. Petrographic analysis of MT microspar (Crawford 2005) indicates nucleation, rapid growth, and modification via Ostwald ripening, followed by precipitation of overgrowth cements – all of which occurred while the initial spheroidal cores remained in suspension within the precipitating fluid.

In the Blue Beds, both clasts and the “fluidized” matrix are composed of this unusual microspar. Microspar within clasts is typically fine-grained (5-7 μM cores) and contains thin (1-2 μM) isopachous rims. Bending and breakage of clasts indicates both brittle and plastic behavior during deformation. By contrast, matrix microspar is significantly more variable in grain size (2-10 μM cores), shows a greater degree of recrystallization, and contains variable amounts of secondary cement. Flow structures within the matrix suggest viscous behavior during deformation. Interestingly, recent experiments (Goodman 2007) suggest spontaneous nucleation of spheroidal carbonate (calcite, vaterite, and amorphous phases) is most likely under high saturation, high pH conditions, and that rapid nucleation in fluids containing abundant hydrated cations (Ca, Mg) can result in formation of a viscous colloid. We suggest that the Blue Beds may represent a scenario wherein physical disturbance of the marine substrate released dissolved organics from substrate pore spaces, which acted as a nucleation catalyst for precipitation within the overlying marine water column. Rather than physical fluidization, Blue Bed matrix represents in situ carbonate nucleation and precipitation-induced viscosity changes that exerted a primary control on style of clast deposition.