ORIGIN OF THE ENIGMATIC BRECCIA AND FOLDS IN THE TURNERS FALLS FORMATION, DEERFIELD BASIN, WESTERN MASSACHUSETTS

The Early Jurassic Turners Falls Formation, located in the Deerfield basin of western Massachusetts, contains several laterally extensive Van Houten cycles that represent an alternation of “wet-climate” black to gray shales and siltstones as well as “dry-climate” red siltstones and sandstones. At several locations in the basin, two gray “wet-climate” horizons within the Turner Falls Formation contain mutually associated breccias and folds. Previous studies suggested these features were caused by slump sheets (Handy, 1977) or flexural-slip folding (Olsen et al., 1992). Neither the slumping model nor the shear model satisfies all the observational data collected in this study and the previously cited studies. Our work suggests that these features represent seismites.

The most critical observations based on field and thin section analysis include: presence of stratigraphically uninterrupted horizons of either red lacustrine strata and gray lacustrine strata or breccia/folds and undeformed rock that cannot be associated with a massive slumping event, corrected slip lines showing an original horizontal or near horizontal orientation of the Turners Falls Formation during the proposed time of deformation, brecciated/folded gray units separated by undeformed units or interseismic intervals, breccia and associated folds that are correlatable over a several kilometer area, and thin section features showing that the Turners Falls Formation was unconsolidated or slightly lithified when deformed.

With these observations, we conclude that earthquakes on the Eastern Border fault produced the breccias and folds in the Turners Falls Formation. Locations where breccias and folds are present are only 3 km to 6 km from the fault, so shaking from any magnitude 6 or greater earthquake would have been severe. Breccia and fold horizons in the lacustrine Pleistocene-age Lisan Formation, located in the Dead Sea graben, have ages that correspond to major earthquakes on the Dead Sea transform and clearly have a seismic origin (Marco and Agnon, 1995). Breccia and fold relationships in the Lisan Formation are remarkably similar to those in the Turners Falls Formation and further support our contention that these Jurassic features have a seismic origin.