PLAYA AND PERENNIAL LAKE SEQUENCE STRATIGRAPHY OF THE EAST BERLIN FORMATION (JURASSIC, HARTFORD RIFT BASIN, CONNECTICUT) – RECOGNITION OF MULTIPLE SCALES OF CLIMATIC AND ACCOMMODATION CYCLICITY

Lacustrine sediments of the Early Jurassic East Berlin Formation (Hartford Basin, Connecticut) were interpreted within a sequence stratigraphic framework. Two types of parasequences were identified, including playa parasequences and perennial lake parasequences. These parasequence types stack vertically into two distinct sequence types reflecting deposition under different climatic and accommodation controls.

An ideal playa parasequence consists of thin, white, cross-bedded sandstone beds with erosional bases (interpreted as sheet flood deposits) overlain by red siltstone beds containing oscillatory ripples (interpreted as playa lake deposits). These in turn are overlain by red siltstone beds containing desiccation cracks and possible paleosols (interpreted as dry playa deposits). The base of an ideal perennial lake parasequence consists of laminated black shale overlain by laminated or massive gray mudstone (both interpreted as perennial lake facies). Perennial lake parasequences are capped by red siltstones and paleosols representing dry playa environments similar to those in the playa parasequences.

Parasequences within the top 77 meters of the East Berlin Formation are vertically stacked into 5 depositional sequences. Three sequences, termed perennial lake sequences, contain both playa and perennial lake parasequences. Maximum accommodation (maximum flooding surface) occurs within a perennial lake parasequence. Two sequences, termed playa sequences, contain only playa parasequences. Beds containing oscillatory ripples (playa lake deposits) are used to identify times of maximum accommodation in playa sequences.

The high frequency cyclicity defined by parasequences is most likely related to changes in the rate of water plus sediment supply which is interpreted to be controlled by climatic variability. Lower frequency cycles recorded as the transitions between sets of perennial lake sequences and sets of playa sequences appear to be present throughout the entire East Berlin Formation (~110m) and in the basal 500m of the overlying Portland Formation. Fluctuations between these sequence sets could be attributed to lower frequency climate variability or to accommodation changes linked to tectonic movement along the Eastern Border Fault which controls subsidence of the basin.