DIVERSITY DYNAMICS IN A UNIQUE FOSSIL-RICH ZONE WITHIN THE MERCHANTVILLE FORMATION OF MONMOUTH COUNTY, NEW JERSEY

Late Cretaceous exposures at Ramsetia Creek in Matawan, New Jersey reveal a glauconitic, fossiliferous layer between the Merchantville and Woodbury Formations. This 10 cm layer is dominated by Ramsetia whitfieldii, a 3-4 cm diameter bivalve rarely found on the New Jersey Coastal Plain, and mostly preserved in life position. The “Ramsetia zone” (R-zone) is located just below a semi-indurated layer marking the boundary between the glauconitic Merchantville Fm and the overlying micaceous clayey Woodbury Fm. The presence of Scaphites hippocrepis III indicates the sequence is lower Campanian in age. The section was split into four distinct units, from oldest to youngest: the Merchantville Fm (0.5m thick), R-zone (10cm), Indurated unit (30cm), and Woodbury Formation (50cm). Fossil diversity was measured using Simpson’s Index of Diversity (1-D) from analysis of bulk sediment samples of over 40 kgs per unit. The Merchantville Fm revealed a 1-D value of 0.86, however, the overlying R-zone was found to have much lower diversity with 1-D = 0.74. The Indurated unit also showed low diversity with 1-D = 0.75. Notably, this unit demonstrated a change in species composition from the R. whitfieldii-dominated R-Zone to the gastropod-rich Indurated Unit where Turritella merchantvillensis occurred in highest abundance. Finally, the Woodbury Fm showed the highest diversity with 1-D = 0.94. Low diversity in the R-zone and the absence of R. whitfieldii in the layers above and below this zone suggest a change in paleoenvironment. The R-zone represents the most glauconite-rich part of the sequence, suggesting slower sedimentation rates related to a maximum flooding surface. Further, R. whitfieldii is known from boring traces to prefer solid or at least firm substrates. Thus, it is possible that the R-zone represents the formation of a deep water firm-ground. This would explain the lowered diversity in this layer, as most bivalve species found above and below the R-zone exhibit burrowing habits related to softer substrates.