FORAMINIFERAL COMMUNITY DYNAMICS OF THE CHESAPEAKE GROUP, CALVERT CLIFFS, MARYLAND, USA

Abstract

The long-term impact of climate and sea-level change on marine communities can be documented using the fossil record of well-studied, and widely distributed organisms such as benthic foraminifera. The Calvert and Choptank formations of the Calvert Cliffs, MD were deposited during the Middle Miocene Climatic Optimum (MMCO) when global temperatures were ~3–6°C higher than present and the subsequent Middle Miocene Climatic Transition (MMCT) when global temperature dropped ~6 °C from peak temperatures of the MMCO. These strata also record five sea-level cycles separated by regionally continuous disconformities during these climate events. Previous studies on the benthic foraminifera of the Calvert Cliffs have been mostly taxonomic in scope. This study investigates the response of benthic foraminiferal communities in shelf sediments of the Calvert and Choptank formations from the Calvert Cliffs, Maryland.to the MMCO, the MMCT, and sea-level cycles. To accomplish these objectives, 21 laterally equivalent samples in each of three vertical, composite transects were collected from beds 4–18 of the Calvert Cliffs, MD for foraminiferal and grain-size analyses. Stratigraphic ranges of the foraminifera in the Calvert Cliffs dataset and relative abundance of all species were documented over the ~4-million-year period represented by the sampling scheme. The majority of abundant species occurred, with little change in relative abundance, over several sea-level cycles. Many rare species, however, occurred intermittently, indicating that these species were either present but not recorded due to their rareness or that these species had populations (a species pool) outside of the study area; that they migrated to and from over time. Cluster analysis of foraminiferal relative abundance data defined two groups, one composed of beds from the MMCO, characterized by assemblages of low energy, inner to mid-shelf environments and the other composed of samples vertically bracketing the first group, from the beginning of the MMCO and from the MMCT, characterizing higher energy, inner shelf environments. The diversity of each group and the taxa present were similar and the two groups and subgroups within them, the latter generally restricted to particular beds, were distinguished based on variations of the relative abundance and rank order of dominant species. Sediment grain-size data were used in conjunction with foraminiferal data to interpret the paleoenvironments of the strata. In summary, the data indicate warmer, deeper mid-shelf environments during the MMCO compared to cooler, shallower, inner shelf environments during the MMCT. The pattern of species distributions over five sea-level cycles indicates the need for a species pool from which species are recruited during transgressions.