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A depositional model for spherulitic carbonates associated with alkaline, volcanic lakes

Mercedes-Martín, Ramon; Brasier, Alexander T.; Rogerson, Mike; Reijmer, John J.G.; Vonhof, Hubert; Pedley, Martyn


Ramon Mercedes-Martín

Alexander T. Brasier

Mike Rogerson

John J.G. Reijmer

Hubert Vonhof

Martyn Pedley


The South Atlantic Aptian ‘Pre-salt’ reservoirs are formed by a combination of spherulitic carbonates and Mg-rich clays accumulated in volcanic alkaline lake settings with exotic chemistries. So far, outcrop analogues characterised by metre-thick successions deposited in lacustrine scenarios are elusive so disentangling the genesis of spherulitic carbonates represents a major scientific challenge with business impact. In particular the controls on spatial distribution and the environment of spherulitic facies formation remain poorly constrained, little studied, and hotly debated. To shed light on this conundrum, a spherulitic carbonate-rich, alkaline volcanic lacustrine succession has been analysed at outcrop scale: the Carboniferous East Kirkton Limestone (Scotland). Despite clays being very scarce and limited to layers of amorphous Mg-Si minerals, a diverse array of spherulitic calcitic components were formed, including coated grains, crusts, and build-ups. This setting enables the mechanisms of spherulitic calcite development and the patterns of sediment accumulation to be explored in a geobiological and hydrochemical scenario similar to the ‘Pre-Salt’ subsurface occurrences but divorced from clay influence. The integration of logs, borehole data, outcrop photomosaics and petrographic observations collectively allowed the reconstruction of a depositional model for the East Kirkton lacustrine succession. In this model, calcite spherule nucleation took place at the sediment-water interface in the littoral zone, driven by the co-occurrence of 1) high alkalinity, 2) Ca-Mg rich hydrochemistry, and 3) microbial-derived colloidal exopolymeric substances. These environmental conditions permitted the coeval development of spherulitic cementstone build-ups and spherulitic grainstone-packstone within the wave-agitated zone, and the accumulation of floatstones and laminites of spherulitic grains in deeper lake regions by means of downslope reworking. This model is consistent with the previously documented microbial bloom occurrences and highlights the need to better understand the complex ‘microbe-solution’ interactions before any reliable facies model is envisaged.


Mercedes-Martín, R., Brasier, A. T., Rogerson, M., Reijmer, J. J., Vonhof, H., & Pedley, M. (2017). A depositional model for spherulitic carbonates associated with alkaline, volcanic lakes. Marine and Petroleum Geology, 86, 168-191.

Journal Article Type Article
Acceptance Date May 21, 2017
Online Publication Date May 25, 2017
Publication Date 2017-09
Deposit Date May 22, 2017
Publicly Available Date May 29, 2018
Journal Marine and petroleum geology
Print ISSN 0264-8172
Electronic ISSN 1873-4073
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 86
Pages 168-191
Keywords Spherule, Spherulitic, Calcite, Alkaline, Lacustrine, Volcanic, Carboniferous, Pre-salt
Public URL
Publisher URL
Additional Information This article is maintained by: Elsevier; Article Title: A depositional model for spherulitic carbonates associated with alkaline, volcanic lakes; Journal Title: Marine and Petroleum Geology; CrossRef DOI link to publisher maintained version:; Content Type: article; Copyright: © 2017 Elsevier Ltd. All rights reserved.


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