MORPHOLOGY OF MOLAR-TOOTH STRUCTURE IN PRECAMBRIAN CARBONATES: IMPORTANCE OF SUBSTRATE RHEOLOGY

Molar-tooth structure (MT) is an unusual Meso- to Neoproterozoic carbonate fabric which is characterized by isolated or interconnecting, often complexly folded, voids and cracks filled with a uniform, equant microspar. Compaction of sediment around MT and inclusion of MT intraclasts in storm scour deposits suggests cementation of voids occurred penecontemporaneously with crack formation, early in diagenesis. MT origin, however, is enigmatic, and has been variously attributed to evaporite replacement, algal growth, synaeresis, earthquake induced dewatering and/or diastasis, and gas expansion. Mechanisms of MT genesis may be constrained by determining parameters controlling MT morphology -- the complexity of MT morphologies implies an intimate relationship between mechanism of void formation, substrate rheology, rapidity of lithification, and extent of post-depositional compaction. In particular, variation in substrate rheology (resulting from changes in grain size, lithology, early cementation, and compaction) should affect crack morphology, propagation, and deformation.

The ~1.4 Ga Belt Supergroup, northwestern Montana contains a large diversity of MT forms, ranging from spherical blobs to sheet cracks, which may be straight, ptygmatically folded, faulted, variously oriented, and form interconnecting frameworks. MT structures typically show a banded appearance in outcrop, reflecting concentration of specific MT morphologies. Furthermore, MT sheets often terminate into, fold around, or change width when crossing laminae of differing grain size. These observations suggest that substrate rheology is an important control on MT formation. Whereas more cohesive substrates inhibit MT crack propagation, resulting in lateral deflection of MT cracks, low-cohesion substrates show evidence of loss of crack-forming energy resulting in fewer MT structures in coarse-grained sediments. MT forms readily in substrates of moderate cohesion. In this way, initial shape of MT ribbons appears to result, in part, from substrate effects on MT crack growth. Continuing petrographic analysis of crack and substrate microstructure may reveal further constraints on MT morphologies and provide a better understanding of MT genesis.