ORIGIN OF MULTIPLE ORDERS OF CYCLICITY IN LOWER JURASSIC LACUSTRINE STRATA OF THE HARTFORD RIFT BASIN, CT

A sequence stratigraphic analysis of Lower Jurassic lacustrine/playa strata (East Berlin Formation, Hartford Rift Basin, CT) reveals at least three orders of depositional cyclicity. Sequences (roughly equivalent to previously described Van Houten cycles) are the most obvious of these cycles, and are characterized by an alteration between gray/black highstand perennial lake shale and reddish lowstand playa siltstone. They are typically 10-15 meters thick in the East Berlin Formation. Several sequences stack into larger-scale sequence sets (50+ meters) that are distinguished primarily by variations in the degree to which perennial lake facies are developed within sequences. Sequence sets are lower frequency cycles than sequences. Sequences are subdivided into smaller-scale (1-3 meters) parasequences, the highest frequency cycles in the basin. Parasequences are characterized by progressively shallower/drier facies bounded by more abrupt shifts to deeper/wetter facies.

Interpreting the origin of cycles in continental strata is difficult, particularly for those facies deposited in environments, such as playa-related environments, that are not necessarily influenced by changes in base level. The rate of accommodation growth associated with syndepostional tectonics controlled the overall pattern of basin fill (e.g., lacustrine strata are associated with the highest rates of fault-generated basin subsidence) and the types of environments present. Tectonics may also have influenced the origin of the lowest frequency cycles (sequence sets). Milankovitch-forced climate variations in sediment+water supplied to the basin are interpreted to be the main control on the development of sequences and sequence sets.

The origin of the higher frequency parasequences is enigmatic. They are interpreted to be deposited within 2-5K years, durations well below known tectonic and Milankovitch forcing mechanisms. An embedded Markov chain analysis of playa-related facies in the East Berlin Formation suggests that the distribution of playa facies within parasequences is not random. Therefore, parasequences do not reflect random shifts in laterally contemporaneous facies. Instead, they may be the result of either intrabasinal or extrabasinal processes, not currently identified, that effected basin hydrology.