JURASSIC LAKE EVOLUTION MODEL INFERRED FROM THE COE LIMESTONE, LOWER SHUTTLE MEADOW FORMATION, HARTFORD BASIN, CONNECTICUT

Samples of the Coe Limestone from the southeastern portion of the Hartford Basin have recently been identified as a sublacustrine seep through petrographic analysis. Textures include multiple generations of baroque dolomite, late silica filling of moldic porosity, asphalt zoning, and hematitic rims. Multiple generations of baroque dolomite occur in the form of bedded rhombs, sweeping baroque, and euhedral late stage filling spar, preserving a complex fabric that represents a fluctuating chemocline and diverse fauna indicative of deeper lake waters. Anoxic conditions are suggested by sulfate and organic rich content of the lower facies.

Siliceous organisms within bedded dolomite rhombs and void-lining chalcedony comprise some of the early textures supporting a biogenic source for silica. Silica precipitation ceases as pH and water temperature rise as vents discharges. Briny hydrothermal waters rich in Mg⁺² and Fe⁺² ions favor dolomite deposition. Degassing at elevated temperatures would cause higher concentrations of interstitial salt ions and raise the Eh, further oxygenating the stagnant lake bottom waters (Renaut & Owen, 1988).

Sweeping baroque dolomite and iron oxide staining suggest that the precipitation occurred proximal to the vent at elevated temperatures ~60˚C to ~120˚C. Deposition either transitioned away from the vent or waned with cooling and with mixing of meteoric water causing a return to neutral conditions. The drop in pH is evident by precipitation of silica in fractures and in moldic porosity, by the formation of dedolomites and by the spar filling of voids. Further shallowing and tectonic uplift are inferred from spar-filled voids, dedolomites, authogenic evaporites, brecciation, hydrocarbon flushing, and late stage dissolution.