Paper No. 173-2
Presentation Time: 8:15 AM
BARRIER-ISLAND-LAGOON COMPLEXES AND MARINE TRANSGRESSIONS: AL FISHER’S BARNEGAT BAY STORY AND THE CONTRASTING HISTORY OF THE PLIOCENE PINECREST FORMATION OF FLORIDA
In 1961, during my sophomore year at Princeton, I was enrolled in a Stratigraphy course, half of which Al Fischer taught. In his typical innovative teaching style, he took us to the ocean (Barnegatt Bay) to learn about relative sea level change. He showed us how blackened bay scallop shells were washing up on the outer beach of the barrier island. He had deduced that these had been buried in lagoonal muds that were washing out seaward of the barrier island because the latter was moving landward and was bordered by a steep shore face. The next year Al published a paper on these conclusions. He was the first person to recognize that a relative sea level rise was occurring along the New Jersey shore. We now know that his is largely a result of subsidence of the peripheral bulge that formed in this region at the margin of the Laurentide ice sheet. Years later, what I had learned here proved invaluable for my study of the Pliocene Pinecrest Formation of Sarasota, Florida. Based on a variety of evidence, including my knowledge of the ecology of bivalve mollusks, I recognized that this unit represents a transgressive barrier-island-lagoon complex. The central portion represents the lagoon, much of which, the bivalve fauna shows, was floored by seagrass. Below low tide on each side of the lagoon were Hyotissa oyster reefs, and marginal to them, living intertidally, were clusters of corkscrew-shaped Vermicularia gastropods. These two very shallow-water elements are positioned both above and below the central lagoonal unit to form a symmetrical sandwich-like package, illustrating Walther’s Law. Above them, showing that this pattern represents a transgression, are units associated with the marginal barrier island or spit: blackened sands that accumulated in swamps and ponds on the back of the barrier and contain brackish-water mollusks, a variety of mammals, meter-scale tidal channels, catastrophic inlet deposits produced by hurricanes, and a washover fan. Unlike what Al Fischer showed to be happening today along the New Jersey coast, where there is a weak supply of sediment, this barrier-island-lagoon complex was preserved during a transgression because the high rate of sedimentation in the ancient Gulf of Mexico produced a gentle shore face that left the transgressed lagoon deposits preserved below the seafloor seaward of the barrier.