HIERARCHY OF CYCLICITY IN ANCIENT PLAYA AND PERENNIAL LAKE SEDIMENTS OF THE UPPER EAST BERLIN FORMATION (JURASSIC, HARTFORD RIFT BASIN, CONNECTICUT)

The upper portion of the East Berlin Formation (Jurassic, Hartford Rift Basin, Connecticut) is well known for its cyclic fluctuations between perennial lake and playa deposition. Detailed sedimentological and sequence stratigraphic investigations of a classic section reveal that this is just one of three scales of cyclicity. Depositional environments have been assigned ranking values based on their relative degree of water depth and desiccation. These include, from deepest (wettest) to most desiccated (driest): (1) deep lake, (2) lake margin, (3) lake plain mudflats, and (4) playa mudflats. Paleosols and terminal splay sandstones can occur in several of these facies, and are not assigned a specific place within the hierarchy above.

These ranked environments are used to subdivide the upper East Berlin Formation into meter-scale, shallowing-upward cycles interpreted as parasequences. Perennial lake parasequences contain deep lake and/or lake margin environments at their bases, and grade up into lake plain mudflat and playa mudflat environments. Playa parasequences are similar, but lack evidence of deep lake and lake margin environments. Parasequences stack vertically into five depositional sequences. Three sequences contain both playa and perennial lake parasequences (maximum accommodation occurs within a perennial lake parasequence), and are termed perennial lake sequences. Two sequences contain only playa parasequence (maximum accommodation is identified as beds containing oscillatory ripples) and are termed playa sequences. Packages of consecutive playa sequences or consecutive perennial lake sequences are grouped into playa and perennial lake sequence sets, respectively. Sequence sets represent a larger scale of cyclicity.

These three scales of perennial lake and playa cyclicity reflect the complex interactions among climatic, tectonic, and geomorphic processes that control fluctuations between underfilled (playa) and balance-filled (perennial) lake systems in the Hartford Basin. In addition, parasequences are interpreted to represent high frequency cyclicity that is shorter in duration than even the precessional component of Milankovitch cyclicity.