ROLE OF GROUNDWATER DURING DEPOSITION OF THE JURASSIC EAST BERLIN AND LOWER PORTLAND FORMATIONS, HARTFORD BASIN, CONNECTICUT

Depositional models for lacustrine systems are based on the relative balance of accommodation and sediment + water supply to the basin which control lake characteristics and distribution. Lake systems range from those that have little sediment + water relative to accommodation (commonly called underfilled systems) and typically form under arid conditions, to those that have high rates of water + sediment entering the basin compared to accommodation (commonly called overfilled) and typically form under more humid conditions. Sedimentological and stratigraphic criteria for distinguishing lake types in ancient systems have been widely published. Until recently, however, there has been limited attempts to incorporate the role of groundwater flow into these classification systems even though groundwater can have a significant impact on the sedimentological record that is used to interpret lakes.

The Jurassic East Berlin and lower Portland Formations in the Hartford Basin (Connecticut and Massachusetts, USA) have been interpreted by different authors to represent both underfilled and balanced-fill lacustrine systems. These formations record many sedimentological and stratigraphic aspects expected for underfilled systems but preserve only limited occurrences of evaporite minerals that are common in underfilled lakes. Application of Michael Rosen’s 1994 playa basin groundwater classification system may provide a solution to this dilemma. Lacustrine deposition in the Hartford Basin records cyclic fluctuation between perennial lakes characterized by organic-rich, microlaminated mudstone and playas characterized by current- and wave-ripple cross-laminated mudstone containing mudcracks and evidence of pedogenic alteration. Evaporites in the playa strata are limited and occur as pedogenic carbonate nodules and less commonly as intervals with mm-scale evaporite molds (now empty or filled with calcite). This can be explained by interpreting the groundwater system in the basin as a through-flow system at the time of deposition. Under this scenario, groundwater would pass through the basin with little evaporation, resulting in only limited precipitation of evaporite minerals. Furthermore, any evaporite minerals that may form can be dissolved in the groundwater and removed from the system.