IRONSTONE CONCRETIONS WITHIN THE CRETACEOUS FORMATIONS ON STATEN ISLAND, NY

The Atlantic Coastal Plain sediments deposited along southeastern Staten Island, NY generally consist of unconsolidated gravel, sand, and clay that may contain minor pyrite, lignite and amber. These Late Cretaceous materials were deposited in a variety of terrestrial, transitional, and marine environments caused by fluctuating sea level. The two formations usually recognized on SI are the Raritan and Magothy which occur in small outcrops in Kreischerville and at Mt. Loretto bluff adjacent to Raritan Bay. Due to later Pleistocene glacial activity, large portions of these formations have been destroyed, reworked or transported, and in several localities deformed into recumbent folds, making their individual identities uncertain. Within these deposits are variously-sized amorphous and yellow-brown ironstone concretions. Petrographic analyses show the presence of roughly concentric layers of an outer shell of arenitic rock with much angular and poorly-sorted clear quartz with rich hematite and limonite cement that surpasses 15% by volume. The mineralogical makeup of the interior varies from nodule to nodule. Some contain a fine-grained mixture of clear angular quartz and marcasite. Others contain overall grey to white to buff fine-grained goethite, hematite, limonite, siderite, magnetite, chamosite, quartz, and colloform opal. These minerals surround millimeter-size hematite ooids and chamosite-containing pisolites that lack concentric/radial structure, which may have been destroyed by changing redox conditions. To date, no micro/macro fossils or glauconite have been found in these concretions. The Cretaceous shoreline on S.I. changed from submergence to emergence and back again several times, which left many interior lagoons, tidal pools, and deltas subject to flooding with iron-rich fresh water from chemical weathering of exposed Triassic rocks and later sea water, causing wide variations in pH and redox. The now interior-located water bodies became stagnated under reducing conditions which led to formation of marcasite and pyrite. Further exposure and drainage led to more oxidizing conditions producing the limonite, hematite, and goethite.