COMPARISON OF STABLE ISOTOPE COMPOSITION OF CARBONATE DEPOSITS FROM MULTIPLE LACUSTRINE SUCCESSIONS, JURASSIC TURNERS FALLS FORMATION, DEERFIELD RIFT BASIN, MASSACHUSETTS
Samples analyzed from the Barton Cove and Greenfield Road sites are muddy to silty laminated dolostone, and breccias with dolostone and siltstone clasts. Clustering of isotopic data suggests the presence of three distinct lake successions at Barton Cove. Although some of these lakes might have been hydrologically closed, as evidenced by their positively covariant δ18O and δ13C values, they do not represent the same lake successions documented previously at the Turners Falls Dam site (Glumac, 2011). Within a single set of samples the more silty and laminated carbonate deposits have more negative δ18O values compared to homogeneous dolomicrite, reflecting the relative influence of precipitation vs. evaporation. The strata exposed at Greenfield Road most likely belong to a single lake succession, which is similar to one of the Barton Cove lakes. The rest of the Greenfield Road data reflect the influence of later diagenesis (precipitation of coarse-crystalline calcite cement) and evaporation (with common evaporite molds now occluded by calcite cement).
This work supports previous stratigraphic observations about the presence of multiple lacustrine successions in the Turners Falls Formation (Olsen et al., 1992). Stable isotope analysis proved useful for distinguishing among distinct lakes, most of which were hydrologically closed and characterized by the formation of dolomite as a primary precipitate or early diagenetic replacement. All analyzed lake successions have similar later diagenetic signatures, and some samples from the new Barton Cove and Greenfield Road study sites reveal a more pronounced evaporation influence as reflected by their more positive δ18O values and the presence of common evaporite molds.