STABLE ISOTOPE ANALYSIS OF CARBONATE COMPONENTS FROM LACUSTRINE DEPOSITS OF THE JURASSIC TURNERS FALLS FORMATION, DEERFIELD RIFT BASIN, MASSACHUSETTS: A VALUABLE HYDROLOGIC, DIAGENETIC AND STRATIGRAPHIC TOOL (Invited Presentation)

Stable isotope compositions of various carbonate components sampled from multiple lacustrine successions of the Jurassic Turners Falls Formation (Deerfield rift basin, Massachusetts) exposed at the Turners Falls Dam site were compared to data from nearby small and isolated outcrops at Barton Cove and Greenfield Road sites. Such comparisons provided valuable information for: 1) determining whether the lakes within this rift basin were hydrologically open or closed; 2) reconstructing diagenetic history of these lacustrine deposits; and 3) enhancing stratigraphic correlation of isolated outcrops within the basin.

Stable isotope analysis proved useful for distinguishing among deposits formed in distinct lakes, previously interpreted as climate-controlled cycles of development and evaporation of perennial lakes. Most of the lakes were characterized by the formation of dolomite as a primary precipitate or early diagenetic replacement and were hydrologically closed, as evidenced by the positively covariant δ¹⁸O and δ¹³C values of lacustrine dolostone. Additionally, silty and laminated dolostone deposits have more negative δ¹⁸O values compared to homogeneous dolomicrite, reflecting the relative influence of precipitation vs. evaporation.

Breccias with dolostone clasts were also analyzed from stratigraphically poorly constrained outcrops at Barton Cove and Greenfield Road. Clustering of isotopic data suggests the presence of three distinct lake successions at Barton Cove. Some of these lakes were likely hydrologically closed, but have different isotopic signatures from the three stratigraphically well-defined lake successions examined at the Turners Falls Dam site. The exposures at Greenfield Road most likely belong to a single lake succession, which is similar to one of the Barton Cove lakes. All analyzed lake successions have experienced similar later diagenesis and precipitation of coarse-crystalline calcite cement. Some samples from Barton Cove and Greenfield Road reveal a more pronounced influence of evaporation, as reflected by their more positive δ¹⁸O values and the presence of common evaporite molds. These results illustrate the value of stable isotopes in hydrologic, diagenetic and stratigraphic studies of lacustrine successions from Mesozoic rift basins.