Ultrastructure of a Middle Cambrian Primary Nonpelletal Phosphorite and its Early Transformation into Phosphate Vadoids: Georgina Basin, Australia

Abstract

ABSTRACT Mudstone phosphorites (collophane mudstones) and mini-profiles composed of phosphate-coated grains from the Duchess phosphate deposit of early Middle Cambrian age in the Georgina Basin, Australia, are described in detail. The mudstone phosphorites form thin beds and laminae, interlayered with transported peloidal phosphorites. In thin section the mudstone phosphorites are characterized by predominately homogeneous micritic phosphate; under the scanning electron microscope (SEM) they are mostly composed of globose structures of 10-25 µm diameter and tubules. These structures are interpreted as the preserved (phosphatized) remnants of solitary and colonial unicells and sheaths that once formed part of a larger microbial mat community. Intermittent subaerial exposure of the microbi lly constructed mudstone phosphorites resulted in their in situ vadose transformation into mini-profiles composed of coated phosphate grains. These profiles are characterized by poorly laminated coated phosphate grains, inverse grading, hollow rims, gravitational cements, and a fitted fabric. Transition zones between the mudstone phosphorites and the overlying mini-profiles allow the steps involved in the transformation to be understood. The mudstone phosphorites and mini-profiles are characterized by distinctive structures, identical to those found in many transported phosphate grains. As these surfaces are frequently laterally discontinuous and eroded at their margins, they are probably important contributors of grains to the deposits.