LACUSTRINE DOLOMITE FROM HYDROLOGICALLY CLOSED EARLY JURASSIC LAKES OF THE DEERFIELD RIFT BASIN IN MASSACHUSETTS: INSIGHTS FROM ISOTOPIC ANALYSIS OF VARIOUS CARBONATE COMPONENTS OF THE TURNERS FALLS FORMATION

Five dark lacustrine mudrock successions, interbedded with playa and fluvial clastics, have been previously described as lake beds 0-4 from the Turners Falls Fm. (Lower Jurassic) at the Turners Falls Dam locality of the Deerfield rift basin in Massachusetts and interpreted as climate-controlled cycles of development and evaporation of perennial lakes. Petrographic and stable isotope analyses of various carbonate components from beds 0, 2 and 3 provide invaluable insights into their origin and lake histories.

Data for dolomicrite from thin carbonate layers and associated nodules in the upper part of bed 0 and the middle to upper part of bed 2 show a strong positive correlation between their δ¹⁸O and δ¹³C values (r>0.92). The two data sets do not overlap, but their best-fit lines parallel each other. Such covariant trends suggest that dolomite formed as a primary precipitate from hydrologically closed lakes of this rather small Mesozoic rift basin. The distinct isotopic signatures may be related to smaller size and shorter existence of lake 0, which has thinner strata relative to lake 2. Along the covariant isotopic trend, dolomicritic deposits with more negative values are silty and laminated, reflecting a greater inflow and input of siliciclastic detritus to the lake. In contrast, the most positive values come from more homogenous dolomicrite, indicating reduced inflow and enhanced evaporation.

Isotopic data for dolomicrite from irregular to semicircular carbonate nodules found on top of beds 2 and 3 overlap substantially and have similar best-fit lines with a strong positive correlation (r>0.86). These data overlap with those of the dolomicritic layers and associated nodules from bed 2. This suggests that the top dolomitic nodules also represent primary lacustrine precipitate (and not pedogenic nodules, which they superficially resemble), and that lakes 2 and 3 had a similar history. Some of the top nodules have more positive isotopic compositions relative to the dolomicritic layers. This reflects nodule formation during the final stages of lake evaporation as is also supported by their association with deep mudcracks and evaporite molds. Finally, the lacustrine dolomicrite is isotopically distinct from other carbonate components present and interpreted as formed and/or altered during burial conditions.