Distribution and Genesis of Unusual Carbonate Fabrics (Atar Group, Mauritania) - Understanding the Evolution of the Proterozoic Carbonate Factory

University Of West Georgia, Carrollton GA

Investigators

Abstract

ABSTRACT DISTRIBUTION AND GENESIS OF UNUSUAL CARBONATE FABRICS (ATAR GROUP, MAURITANIA) - UNDERSTANDING THE EVOLUTION OF THE PROTEROZOIC CARBONATE FACTORY JULIE K. BARTLEY EAR-0106089 Recent studies of Archean and Proterozoic rocks indicate broad temporal trends in carbonate depositional style. Abiotic, macroscopically crystalline carbonate, for example, is abundant in both subtidal and peritidal environments in the late Archean and Early Paleoproterozoic. By the late Neoproterozoic seafloor precipitates are virtually absent, supplanted by micritic carbonate textures, except where associated with highly evaporitic conditions or Neoproterozoic glacial deposits. Carbonate textures in the Mesoproterozoic display features characteristic of both older and younger Proterozoic carbonate successions. Furthermore, Mesoproterozoic to early Neoproterozoic carbonate platforms record a diversity of fabrics unknown elsewhere in the geologic record. A Mesoproterozoic peak in the diversity of stromatolite morphologies suggests a dynamic interplay among mat growth and decomposition, carbonate precipitation, and micrite deposition. Similarly, a Mesoproterozoic peak in the abundance of molar-tooth fabric - an enigmatic carbonate fabric that contains rapidly lithified carbonate cement juxtaposed with slowly lithified matrix - suggests a dynamic interface between two contrasting carbonate precipitation regimes. Together, these lines of evidence serve to underscore a recurrent theme: The Mesoproterozoic represents an interval that lay at a critical threshold with respect to the kinetic ease of carbonate nucleation and precipitation. In such a system, relatively minor modification of the physiochemical environment may have inhibited or enhanced carbonate precipitation, resulting in marked changes in the style of carbonate deposition. These inferred environmental changes should therefore be sedimentologically predictable and their consequences geochemically testable. This project aims to systematically examine a wide array of carbonate fabrics within a single basin and delineate the environmental parameters, observable in the field, that control carbonate nucleation and growth. In exploring these relationships it may be possible to explicitly address the complex mechanisms governing Precambrian carbonate production. The late Mesoproterozoic to earliest Neoproterozoic Atar Group, Mauritania, is an ideal succession in which to explore these relationships. Carbonates of the Atar Group contain diverse stromatolites, deep- and shallow-water precipitates, herringbone carbonate cement, and numerous horizons of molar-tooth fabric. As the first step in a multi-faceted approach utilizing field, petrographic, and geochemical datasets, we will conduct field research in the Atar Group, Mauritania and examine the field relationships among the observable carbonate fabrics and the facies in which they occur. This approach provides the most effective method to decipher the relative roles of individual variables in controlling carbonate deposition and, in this way, will address fundamental questions regarding the mechanisms underlying changing carbonate regimes throughout the Precambrian.