LAURENTIA

Laurentia mainly consists of the North American craton and most of the marginal Cordilleran, Inuitian, Appalachian and Ouachita foldbelts, excluding possibly parts of western Alaska and the Avalonian regions of northeastern U.S. and eastern Canada. In addition, Svalbard, northwestern Scotland, and the Argentine Precordillera were originally parts of Cambrian Laurentia.

Cambrian sequences in the craton are exposed around basement uplifts in the U.S. Midcontinent (e.g., Indiana); in the Rocky Mountains of U.S. and southern Canada east of the belt of late Cretaceous and early Tertiary thrusting; and in parts of northern Canada, the Canadian Arctic Islands and northern Greenland. The Cambrian of northwestern Scotland and of the eastern part of the Argentine Precordillera is probably also cratonic.

Middle Cambrian rocks (calcareous formations in the upper part and highly fossiliferous Wheeler Shale forming lower slope) at Marjum Pass, House Range, Utah

The Greast Basin region of the U.S., and the Rocky Mountains of the U.S. and Canada present spectacular sections which are often abundantly fossiliferous. Particularly famous are the Middle Cambrian localities at Burgess Pass in British Columbia and the House Range in western Utah, and the Lower Cambrian Sirius Passet fauna of northern Greenland, which are Cambrian lagerstatten (see fossil archives). Other well known areas with Cambrian outcrops include Lower Cambrian sections in the White-Inyo Mountains of eastern California and comparable sections in adjacent western Nevada, Lower Cambrian in the Mackenzie and Wernecke Mountains of the Canadian Northwestern Territory, Middle and Upper Cambrian in the Rocky Mountain ranges of the northwestern U.S., British Columbia and Alberta. Almost all of the lithostratigraphic units that form these sections are under comprehensive investigation (e.g., the Poleta Fm.).

Section at Eagle Mountain, eastern California,

mainly consisting of rocks of the

Lower-Middle Cambrian Carrara Formation

Cambrian depositional environments reflect a concentric development around the core of the Laurentian platform. These range outward from littoral sands via inter- and infratidal sands and muds lagoonal muds, and algal shoal carbonates across often broad carbonate platforms to shales of the outer shelf, and broadly characterize three facies belts: an inner detrital belt, a carbonate belt, and an outer detrital belt, that are well developed from early Middle Cambrian time onward. Abrupt regional lateral shifts of the facies belts define the boundaries of Grand Cycles.

A transgression in the Early Cambrian flooded the marginal areas of Laurentia, but huge areas remained land until the time of maximum flooding in the Late Cambrian. Accordingly, regional differences in facies and thickness are conspicuous. Thicknesses vary from about 200 m in the Upper Mississippi Valley to about 5,000 m in parts of British Columbia. Laurentian trilobites and other fossils give evidence for strong ecological and geographic separation of Laurentia from the other continents during most of Cambrian time.

A. R. Palmer

Middle Cambrian Bright Angel Shale, an inner detrital unit,

overlain by the Muav Limestone, which suggests carbonate shoal deposition.

Seen from Bright Angel Trail, Grand Canyon, Arizona.

Huge stromatolitic build-ups formed by Cryptozoon proliferum.

Upper Cambrian. Petrified Gardens, Saratoga, N.Y.