Mineral Composition and Dispersal Patterns of Deep-Sea Sands in the Western North Atlantic Petrologic Province
The western North Atlantic petrologic province consists of terrigenous sands of glacial and postglacial age on the shelf and in the deep sea, north of a line extending east of Cape Fear, North Carolina. The province includes the northern part of the Hatteras Abyssal Plain, all of the Sohm Abyssal Plain, and an undefined area east of the Mid-ocean Canyon. Within the province a western, light-mineral subprovince extends northward from Cape Fear, and eastward through the Hatteras Abyssal Plain and the central Sohm Abyssal Plain, to the Southeastern Newfoundland Ridge; this area is characterized by numerous glauconitic quartzose sands derived by selective sorting and a brasion of arkosic glacial detritus on the shelf. An eastern light-mineral sub-province contains relatively unmodified arkosic sands transported southward around the Southeastern Newfoundland Ridge to the eastern part of the Sohm Abyssal Plain. The glacially derived, heavy-mineral assemblage of the western North Atlantic province is complex; the nonopaque, nonmicaceous minerals are characterized by amphibole (44 per cent), garnet (19 per cent), and pyroxene (5 per cent). Five heavy-mineral subprovinces are mapped by areal variations in the proportions of kyanite-sillimanite-staurolite, epidote, tourmaline, and pyroxene plus the ratio of garnet to kyanite-staurolite. None of these indices correlates with mean grain size of the sands and silts. Amphibole increases and garnet decreases in the finer sands and silts. In the more quartzose sands the light-mineral assemblages are relatively mature, whereas the heavy-mineral assemblages are immature. This is because modification of the light minerals toward an ultra-stable quartz-chert composition occurs more rapidly than does modification of the heavy minerals toward an analogous zircon-tourmaline-rutile ultrastable assemblage. The light minerals are thus a more sensitive indicator of abrasion history than are the heavy minerals, which better reflect source-rock lithology. Bottom topography largely controls sand-dispersal patterns. Submarine canyons act as sediment traps that funnel the downslope movement of sand to deep-sea fans. The sand is then transported across the abyssal plains.