--> ABSTRACT: The 1.8 Ga Stark Megabreccia: Facies Association and Reconstruction of a Major Paleoproterozoic Evaporite, by M. C. Pope and J. P. Grotzinger; #91021 (2010)

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The 1.8 Ga Stark Megabreccia: Facies Association and Reconstruction of a Major Paleoproterozoic Evaporite 

POPE, MIKE C., and JOHN P. GROTZINGER

Paleoproterozoic (aprox. 1.8 Ga) sedimentary rocks of the Pethei Group and overlying Stark Formation, East Arm of Great Slave Lake, Canada record a transition from normal- to restricted-marine and terrestrial conditions. The Lower Pethei Group contains two shallowing-upward sequences of normal marine ramp carbonates. Inner ramp facies of the uppermost Pethei consist of chemically-precipitated carbonates, including dendritic tufas and fine, isopachously laminated stromatolites. Mid-ramp carbonates are domal stromatolites with secondary voids filled with isopachous marine cements (including herringbone calcite), breccia and stromatolites(?). Outer ramp rhythmites contain interbedded silicified halite molds. The overlying Stark Formation, consists of interbedded red mudstone, siltstone, sandstone with abundant halite casts/molds and a few shallow-water limestone/dolomite units. The Stark Formation is commonly chaotically brecciated and lies conformably upon the uppermost Pethei Group with little evidence for subaerial exposure. Sub-vertical, laminated, silicified breccias are interpreted as solution pipes, and similar sub-horizontal laminated carbonate breccias may represent travertine-like solution cavity fill.

The abundance of halite molds/casts in the uppermost Pethei Group and Stark Formation, as well as chaotic bedding and solution pipes suggest that Stark brecciation resulted from dissolution of halite, collapse of overlying units and associated fluid flow during dissolution and deformation. The abundance of halite molds/casts, chemically-precipitated carbonates and cements, and lack of gypsum pseudomorphs in these units suggests that Paleoproterozoic oceans were likely highly oversaturated with respect to calcium carbonate and possibly had elevated alkalinity. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.