Mineralogy and provenance of Pleistocene outwash-plain and modern beach sands of outer Cape Cod, Massachusetts, USA

Abstract

Sand and pebbles of the beaches of outer Cape Cod are derived from eroding coastal cliffs of lacustrine-fluvial outwash plains of Wisconsin age. The beach sands and the sands of the older and younger Wellfleet, Highland and Truro plains have similar mineralogically mature, quartzose compositions. These sands are quartz-arenites to subarkoses with small amounts of glauconite pellets. The complete samples of the combined outwash and beach groups average 89% quartz and 4% feldspar; the medium-sand fractions average 91% quartz and 5% feldspar. The non-opaque, non-micaceous heavy minerals in the medium-sand fraction of the beaches average 42% tourmaline, 16% staurolite, 14% garnet, 13% sillimanite, 5% amphibole, 5% andalusite, and 5% others. Although the proportions of heavy minerals in the outwash sands vary among samples, the average composition is similar to the beach sands: 40% tourmaline, 16% staurolite, 16% sillimanite, 12% garnet, 6% andalusite, 4% amphibole, and 6% others. A common provenance for outwash and beach sands is also reflected in the nearly uniform ratios of color varieties of tourmaline in all samples. Most of the quartzose sand of the beaches and outwash has a long history of recycling from Atlantic coastal-plain strata of Cretaceous to Tertiary age. These sands were entrained by Illinoian (?) glacial ice, which deposited gravel and sand in till and outwash over the Gulf of Maine. Sangamon marine reworking of the surface of these deposits mantled them with glauconitic, quartzose sand and gravel. Wisconsin ice moved southeastward, stripping the floor of the Gulf of Maine down to crystalline rocks and remnants of Cretaceous to Tertiary strata. Melt-water streams flowed southwestward from the South Channel Lobe into lakes, building the outwash plains with quartzose sand and lithic pebbles. The shapes and surfaces of many round, subspherical quartz grains were modified by grinding during transport in the ice, producing conchoidal fractures, flat-edged plate-like fragments, sharp edges, sliver-like chips, and grooves. Subsequent longshore transport of the quartzose sand and lithic pebbles north and south from a nodal zone has somewhat rounded the grains, but has not made the medium-sand fraction more quartzose. The nodal zone is northeast of Wellfleet, as evidenced by an increase in hydraulic equivalent ratios for tourmaline, sillimanite, staurolite, garnet and hornblende north and south along the coast from this location.