A STABLE ISOTOPE STUDY OF CARBONATE COMPONENTS FROM LACUSTRINE STRATA OF THE TURNERS FALLS FORMATION (NEWARK SUPERGROUP), MASSACHUSETTS

The Turners Falls Formation (Early Jurassic) at the Turners Falls locality in the Deerfield Rift Basin contains five successions of dark lacustrine mudrocks interbedded with playa and fluvial clastics. These successions have been described as lake beds 0-4, and interpreted as climate-controlled cycles of development and evaporation of perennial lakes. Carbonate components from beds 0, 2 and 3 were analyzed for stable isotopes to better understand their origin.

Dolomicrite from carbonate layers near the top of bed 0 (d¹⁸O=-7.1 to -3.3; d¹³C=-6.6 to -5.2; all in ‰ VPDB; reacted at 75°C), was compared to that from irregular nodules associated with deep mudcracks in the upper part of bed 3 (d¹⁸O=-11.0 to -4.3; d¹³C=-5.7 to -2.3). The data form two distinct fields on the d¹⁸O/d¹³C plot, reflecting differing conditions during the deposition of lacustrine layers and the formation of nodules related to a prolonged subaerial exposure. The compositions of carbonate layers, lenses, and nodules from the upper part of bed 2 (d¹⁸O=-12.7 to -4.1; d¹³C=-5.7 to -2.6), overlap with bed 3 nodules, but not bed 0 layers. This comparison provides information on the differences in the chemistry of lake waters and in the degree of subaerial and later diagenetic modifications. The lower parts of the lake successions contain elliptical to circular, mixed calcite/dolomite nodules. Compared to the irregular nodules and carbonate layers, these nodules are in general more depleted in ¹⁸O and ¹³C (d¹⁸O=-12.6 to -8.9; d¹³C=-9.2 to -4.9), which suggests their formation and alteration during burial of lacustrine mudrocks. This is supported by the presence of common pyrite and fractures with ferroan carbonate cements, and by the comparison with fracture-occluding dolomite and calcite cements (d¹⁸O=-13.2 to -8.9; d¹³C=-13.1 to -4.6). Such comparisons of various carbonate components reveal unique insights into the complex depositional and diagenetic history of the lacustrine strata.