GR Letter Tomographic study of Paleoproterozoic carbonates as key to understanding the formation of molar-tooth structure Darrel G.F. Long Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada P3E 2C6 Received 21 September 2006; received in revised form 16 January 2007; accepted 14 February 2007 Available online 28 February 2007 Abstract X-ray computed tomographic studies of relatively pure Paleoproterozoic limestones from the George Formation, Muskwa Assemblage, northern British Columbia, Canada indicate that molar-tooth structures developed along linked fractures in gel-like semi-plastic carbonate mud, with a high organic content. Where pore fluid and/or gas pressures matched confining loads, MT blobs developed. Where pressure exceeded loads, cracks propagated into adjacent semi-elastic sediment and were rapidly filled by clusters of uniform, equant, microcrystalline carbonate. Where abundant carbonate was not precipitated, incipient cracks and sheets collapsed leaving residual trains of microcrystalline carbonate with similar density to the molar-tooth carbonate. Tomographic studies show that the density of calcite domains within petrographically uniform sheets of MT void-filling calcite is uneven, suggesting that precipitation was not instantaneous, but was propagated from discrete centres. It is here suggested that carbonate production and sediment rheology were both strongly influenced by organic matter. During early sea-floor diagenesis microcrystalline carbonate precipitated within organic-rich sediment with high water content, possibly within decomposing mats of microbial extracellular polymeric substances (EPS). When pore pressures in the host sediment increased in response to cyclic loading by long- period waves, pore fluids containing EPS were injected into newly created fractures, allowing rapid precipitation of molar-tooth carbonate. Because tomographic studies allow detailed resolution of minor density differences, they provide a useful method of evaluating structures in relatively uniform carbonate rocks of any age. © 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Paleoproterozoic; Carbonates; Tomography; Diagenesis; Cyclic loading by storms 1. Introduction Molar-tooth structures are early diagenetic features of enig- matic origin that appear to be largely restricted to fine-grained Proterozoic carbonates deposited in mid- to upper-ramp set- tings. The term was first used in the geological literature by Bauerman (1885) to describe structures “resembling the markings on the molar-tooth of an elephant” in carbonates of the Belt Supergroup, near the Canada–USA border. This group of structures includes predominantly downward-tapered, ver- tical to inclined, micro-crystalline carbonate-filled crack-like structures, which in many examples appear to be ptygmatically folded and/or fractured due to early compaction (Fig. 1). Sub- horizontal sheets and spherical forms (blobs) are less common. All show signs of compaction, in both cross-section and plan view. Some resemble extensional vein arrays and parts of ductile shear zones that have been modified by compaction and thinning (c.f. Maltmam, 1994; Brothers et al., 1996; Grimm and Orange, 1997). The fill of molar-tooth structures is characterized by pure, uniform, equant, polygonal, blocky calcite spar 5–15 μm in diameter (Fig. 2). These crystals lack typical cement features and in some cases show evidence of injected sediment fill (Pratt, 1998) or internal fluid flow (Bishop et al., 2006; Bishop and Sumner, 2006). Associated host carbonates are typically charac- terized by wavy to crenulate lamination, suggestive of microbial mat development. The development of molar-tooth structures seems to be largely restricted to the interval between 1900 and 550 Ma (Pratt, 1998; James et al., 1998; Shields, 2002), with two examples (in siliciclastic mudstone) from the Archean (Bishop et al., 2006; Bishop and Sumner, 2006). The major decline in abundance Available online at www.sciencedirect.com Gondwana Research 12 (2007) 566 – 570 www.elsevier.com/locate/gr E-mail address: dlong@laurentian.ca. 1342-937X/$ - see front matter © 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2007.02.004