ABSTRACT

Dolostones within the terrigenous clastic middle Proterozoic Kanuyak Formation are interpreted to be dolomitized calcretes (dolocretes) which formed on a landscape largely devoid of macroscopic organic life. The dolocretes up to 5 m thick form beds or masses which either parallel or cross-cut host sedimentary rocks, and are composed of five elements: 1) curvilinear and rectilinear networks of cracks which are filled with detrital terrigenous clastic and carbonate sediments, ooids, pisoids and cements (sediments and cements are laminated and bedded, generally parallel to the margins of cracks); 2) micritic, fibrous, columnar and plumose dolomitic cements that were precipitated as calcareous speleothems, ooids and pisoids, and masses; 3) ooids and pisoids that are concentrically-laminat d, homogenous or coated grains occurring in reversely-graded beds and as isolated particles throughout dolocretes (it appears that accretionary particles formed in situ based upon polygonal fitting, vadose and grapestone fabrics, and gradational contacts between ooids/pisoids and host sediments); 4) deformation structures that include folded and brecciated detrital sediments within cracks, brecciated pisoids and minor teepees; and 5) surface karren that locally truncate the dolocretes.

While missing the features commonly associated with higher plants in post-Silurian calcretes (rhizoconcretions, root-mouldic porosity, etc.), the presence of an array of other common calcrete fabrics (e.g., cracks, cements, accretionary grains and deformation structures) suggests that physical and chemical processes were probably largely responsible for the carbonate precipitates within Kanuyak profiles. Microbiota most likely also contributed to calcretization; however, if not significantly altered by dolomitization, stable carbon and oxygen isotope values do not indicate any significant influence by organic matter.