A New Depositional Model of Spherulitic Carbonate Settings: Insights From a Carboniferous Hyper-Alkaline, Volcanic-Influenced Lake

Abstract

Although carbonate spherulites are well known to form in a wide range of carbonate environments, the depositional conditions that underpin their origin and facies distribution is still enigmatic. To better predict the facies heterogeneities and depositional models associated to spherulitic lacustrine carbonates the specific hydrologic, biogeochemical, and the process-product paradigm need to be unravelled. Detailed stratigraphic sections, bore-hole logs, outcrop photomosaics, and new petrographic and mineralogic data of the Carboniferous East Kirkton Limestone (Scotland) allowed the facies architecture of a spherulitic lacustrine carbonate setting to be reconstructed. The East Kirkton Limestone was a volcanic-influenced, hyper-alkaline, saline lake system which provided an interesting array of calcium carbonate spherulitic textures (coated grains, intraclasts, crusts, and stacked fans). Some of these precipitates occur intermingled within organic-to mudstone-rich/Al-Mg-rich smectite matrices, primary fine-grained chert, and volcaniclastic-igneous tuffs. The carbonate factory of spherulitic textures was located in the shallow lake region. There, a combination of high alkalinity, metal-rich hydrochemistry and organic acid complexation likely promoted by colloidal-EPS substances of microbial origin collectively stimulated widespread spherulitic calcite nucleation in the sediment-water interface. Metre-thick deepening-upwards cycles were identified and built two sequences bounded by an erosional surface (unconformity) representing long-term lake contraction-expansion episodes. Three depositional settings were recognised: shallow-water, margin-slope, and deep water. The shallow-water setting generated primary, syn-depositional and para-autochthonous spherulitic components (coated grains, intraclasts and crusts) producing spherulitic grainstone to packstone textures in an agitated environment, and autochthonous components such as carbonate mounds made up of ‘shrub-like’ calcite textures. The margin-slope is characterised by metric scale, slumped beds, and sediment gravity deposition triggered by slope gradients. Spherulitic floatstone and spherulitic-to organic/smectite-rich laminites were predominant. The deeper water setting displayed laminites and evidence of water stagnation and oxygen depletion. Down-slope re-sedimentation of near-shore spherulitic grains produced spherulitic/ clay-rich floatstone and spherulitic laminites in a margin-to-slope position.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016