OCCURRENCE AND FEATURES OF MICROBIAL STRUCTURES OF THE THERESA FORMATION, CAMBRO-ORDOVICIAN, NEW YORK

The Cambro-Ordovician Theresa Formation of northeastern New York has yielded an association of peritidal facies characterized by a poor body fossil assemblage but rich biogenic structures. Road-cut stratigraphy is complicated due to the patchy character of exposed sections, but a characteristic vertical sediment sequence could be recognized. The Theresa is characterized by a distinctive alternating sequence of grey, intensely bioturbated, poorly sorted calcareous sandstone and white quartz sandstone. Bioturbated facies of the grey calcareous sandstone contains a Cruziana ichnofacies of abundant deposit feeders. Scolithos ichnofacies is present in the white cross-bedded sandstone. The white sandstone in the upper Theresa is also characterized by wave ripples, herring-bone cross-stratification and horizontal lamination.

The microbial structures distinguished by wavy laminated stromatolite growth structures are common in the white quartz sandstones of the middle Theresa. Vertical sections of stromatolites exhibit predominantly space-linked hemispheroids with close-linked hemispheroids as a microstructure in the constituent laminae. Hemispheroids vary both in amplitude and in shape, i.e. from low-amplitude (5-10 cm) and gently convex, to higher-amplitude (up to 20 cm), steeply convex to slightly rectangular, vertically stacked hemispheroids. Subcircular, concentrically stacked spheroids up to 30 cm in diameter, with laminae composed of close-linked hemispheroids are observed in plan view.

The facies stacking pattern observed within the microbial structure-rich part of the Theresa Formation likely represents shallowing-upward parasequences composed of grey, intensely bioturbated, restricted subtidal facies, capped by microbial laminites of tidal flats. Some parasequences are capped by thin breccia-conglomerate horizons suggesting periodic subaerial exposure of tidal flats. The alternating vertical stacking pattern of the two facies is complicated by their common interfingering in the upper Theresa, suggesting facies mosaics.