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Jackalope Geological Ltd., Berthoud, CO

Abstract: Life and Death Roles of Red River Thalassinoides

Burrows of the ichnogenus Thalassinoides characterize the lower Red River Formation (Upper Ordovician) in the Williston Basin and beyond. These 3-D anastomosing features consist of (1) intertwined cylindrical burrows measuring 2 mm diameter, and (2) irregular burrow mottles surrounding the smaller burrows as dolomitized envelopes. Burrow mottles and enclosed cylindrical burrows have been attributed to successive deposit-feeders. However, irregularity of burrow mottles, paucity of back-fill Spreite, and the unrewarding strategy of deposit-feeding at depth argue otherwise. That the uncollapsed, small burrows are occluded by dolomite and anhydrite cement or remain unobstructed imply dwelling burrows built by filter-feeders, perhaps polychaetes or decapod crustaceans who actively maintained vertical egress for disposal of mined and infiltrated sediment and for circulation of oxygen- and plankton-bearing marine waters. By analogy with Mesozoic Thalassinoides, preserved as dolomite or chert in limestone and chalk and known to have been constructed by mole shrimp and callianassid shrimp, enveloping burrow mottles may represent the coherent walls of the small burrows, maintained by secretion or physical packing of excreta. Small quantities of ?organic mortar? alter the kinetics of carbonate reactions and could have facilitated early dolomitization of burrow mottles. Thus, one need not invoke the hypothesis that Thalassinoides served as conduits for downward flow of brines responsible for Red River dolomitization. Areal configuration of Red River dolomitization and reservoir porosity in Montana and Saskatchewan (elongate trends oriented principally NW-SE) instead suggest that regional fracture sets created avenues for downward migration of Silurian and Devonian brines.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana