INTRIGUING SEDIMENTARY STRUCTURES FROM THE JURASSIC TURNERS FALLS FORMATION OF THE DEERFIELD RIFT BASIN IN MASSACHUSETTS

Every spring flood reveals new exposures at the spectacular but not easily accessible site immediately downstream of Turners Falls dam along the Connecticut River in Massachusetts. Overlying Deerfield Basalt at this site is an almost 200 m thick succession of Lower Jurassic Turners Falls Formation consisting of mainly playa red beds interbedded in the upper part with gray fluvial and alluvial clastics and several black lacustrine mudrock intervals. This Deerfield rift basin exposure is famous for its dinosaur footprints and fish fossils (Olsen et al. 1992). In addition, the extent and superb quality of this outcrop provide a unique window into sedimentary processes as reflected in the succession of lithologically diverse strata with a variety of sedimentary structures. Besides abundant desiccation cracks and burrows in playa mudrocks and ripple marks and cross-lamination in fluvial sandstones, the succession has some less common sedimentary structures. These include soft-sediment deformation features, similar in morphology but smaller in scale than “sand pillows” documented from this stratigraphic interval elsewhere in the basin (Hubert and Dutcher 2005), likely formed by sediment collapse and liquefaction due to fast sedimentation rates and possibly triggered by seismic activity. Other examples include possible “wrinkle structures” whose appearance as bumpy surfaces of sediment in troughs of low amplitude ripples is in accordance with proposed formation by oscillatory wave motion of microbial aggregates (Mariotti et al. 2014). Our most recent discovery includes impressions of dendritic pattern in red beds, which likely represent plant foliage, but differ in form and preservation from carbonized plant litter and simple, straight branches that are present in gray sandstone up section. Their unusual dendritic pattern, however, has sparked interest in evaluating potential for formation and preservation of evidence for efflorescence minerals on surfaces of playa deposits (evaporite molds are present in rare lacustrine dolostone at this site), and in the possibility of frost ice related to short-lived volcanic winters in this otherwise tropical region. This illustrates that there is yet much to learn and to be inspired by from this ever-renewing and surprising “classical” Mesozoic rift basin succession.