A tectogenetic origin for the deep subsurface microorganisms of Taylorsville Basin: thermal and fluid flow model constraints

Abstract

Thermophilic anaerobic bacteria extracted from Triassic, 230 million years old (Ma), sandstone and shale at 2800 m below the land surface in the Taylorsville Basin, Virginia, appear to be indigenous to the rock strata. The observed maximum growth temperatures of these bacteria, 65–75°C, are compatible with the current formation temperatures. Paleogeothermometers, however, indicate that the strata were exposed to temperatures of 160–200°C at 200 million years ago. This implies that the bacteria migrated to their current depth after this thermal pulse. A 2D paleofluid flow and heat transport model indicates that during uplift and erosion in the Jurassic, 200–140 Ma, topography drove groundwater deep into the subsurface at a rate of 1–100 mm/year. The estimated minimum time required for water to move from the surface to the microbially sampled horizons during this time interval ranges from 1 to 20 million years. The time required to migrate from a depth compatible with the growth range of microorganisms, approximately 1.3 km, to the microbially sampled horizons would be even less. The present-day groundwater flow rates, however, are 10–100 times less than those during the Jurassic and the minimum time required for water to reach the microbially sampled zones from the surface is 50–180 million years. Colonization of the deep subsurface, therefore, probably occurred during the basin's last major tectonic event in the Jurassic rather than today. For many regions of the earth's crust, the age of deep subsurface microbial communities may be equal to that of the last tectonic upheaval.