299-5 From Rockets to Rocks: Anthropogenic Ironstones of Gateway National Park

Session: Geochemical Studies of Sediments (Posters)

Presenting Author:

Authors:

Abstract:

The New York Bight, home to Gateway National Recreation Area, lies between the peninsula of Sandy Hook, New Jersey and the barrier island of Riis Park, New York City. Alteration and excavation of its beaches for extensive military installations began during the Civil War and continued well into the 1950s. Several Nike missile bases that once housed dozens of rockets along the shores of the Bight now lie in decay. These beaches exhibit both the lithification of unusual iron-cemented beachrock as well as iron-rich concretions that wash ashore. Iron-oxide cementation of sand grains against anthropogenic iron sources is associated with electrolytic reactions. High pH (>10) as a result of these reactions resulted in dissolution and embayment of quartz. Mineral assemblages share common morphology, though tabular concretions precipitate with a more defined chemical zonation in cement as distance increases from adjoining infrastructure.The origin of areally extensive tabular ironstones (beachrock) is more enigmatic. Although they also exhibit embayed quartz, they are distinct texturally (finer grained, less porous), mineralogically (presence of aluminous cements: clays, adularia, diaspore), and chemically (enriched in P and depleted in S and Cl). Thus, they likely formed by a process other than electrolytic oxidation of adjacent infrastructure. The presence of embayed quartz may indicate a link to bog iron, in which organic acids can result in quartz dissolution. The presence of bog-related microfossils further support this conclusion. However, it is unclear whether the iron present in the beachrock’s cement was derived from large-scale infrastructure or underlying glauconitic sands. The presence of anthropogenic fibers in the cement further complicates this story. Varying forms of beachrock are present on the shores of both Sandy Hook and Riis Park, some having clear anthropogenic influence while others may have formed without human intervention.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.