FORMATION OF NON-SEISMIC SAND PILLOWS ON A LACUSTRINE DELTA SLOPE IN THE LOWER JURASSIC TURNERS FALLS FORMATION, DEERFIELD BASIN, MASSACHUSETTS

A river flowed from the western-hinged margin of the Deerfield rift basin into a large lake, constructing delta foresets that prograded northeast. During monsoonal floods, currents laden with sediment plunged down the upper slope, depositing 3-22-cm-thick layers made of medium to coarse silt: lithofacies Sh to Sr; Sh to Sh-finer; St to Sr; St to Sh-finer; and Sr to Sh-finer. As the flows traversed the lower slope, mm to cm-thick layers were deposited of Sr, Sh-finer, and Sr to Sh-finer.

Despite being 3 km from the seismically active border fault, sheets of sand pillows are not seismites, but formed syndepositionally on the upper and lower slopes. Sand was rapidly deposited on the delta front, triggering local collapse and liquefaction of the metastable, loosely packed, micaceous, fine and very fine sand. The sand sank to form elongate pillows with axes perpendicular to dip of the foresets and azimuth of current flow. Formation of pillows was accompanied by minor sliding downslope of the pillow sheets along decollement surfaces. Fluidization produced sand dikes, water-escape structures, and blurred laminae. Sand continued to be carried downslope in the currents, building cross-beds in some of the newly formed pillow synclines. Increasing flood velocities beveled the sediment-water interface, truncating dikes, tops of pillows, and the undeformed cross-beds in the pillow synclines. As the flows waned, finer sediment mantled the eroded pillows.