Abstract

The Upper Cambrian Lamotte Sandstone of Missouri is a time-transgressive marine orthoquartzite sandstone containing arkose sandstones and conglomerates in its lower part. In the outcrop area in southeastern Missouri, minor feldspathic quartzose sandstones occur throughout the formation and dolomitic and glauconitic sandstones occur at and near the top. The Lamotte Sandstone and its equivalents were deposited on a peneplaned Precambrian surface which sloped to the east-northeast in Missouri and to the south in southern Wisconsin and Minnesota. The sandstone thickens from less than 100 ft in western Missouri to 400-500 ft in eastern Missouri. Cross-bedding data obtained on the outcrop area show that the currents which deposited the orthoquartzites flowed to the south, whereas those which deposited most of the arkoses flowed to the north. These arkoses were derived from the tourmaline-poor igneous rocks of the adjacent St. Francois Mountains. Feldspathic quartzose sandstones were formed by reworking of arkosic detritus and mixing with well-rounded quartz sand. The orthoquartzite sandstones are a mixture of minor first cycle and dominant "second cycle" components. The non-opaque heavy mineral suite of the orthoquartzites is composed of well-rounded zircon and tourmaline, the latter exhibiting some abraded overgrowths, and only minor amount of angular heavy minerals. Well-rounded quartz is abundant and a few grains have abraded overgrowths. Tourmaline-bearing Precambrian sandstones in the Lake Superior region were probably the principal sources. Other Precambrian rocks in that area and in Missouri, Iowa, Oklahoma, Kansas, Nebraska, and South Dakota dakota may have supplied additional detritus.

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