Late Cambrian

In the Late Cambrian, Laurentia and Siberia continued to drift toward the equator (figure 3.4). The Iapetus Ocean increased in area and had its maximum width at this time (Harris and Fettes 1988; Khain and Seslavinsky 1995). Two additional oceans, the Panthalassa and the Paleoasian, also existed. The imbrication of oceanic crust on the periphery of Siberia and the central Kazakhstan terranes continued during the Late Cambrian. It was probably related to marked spreading of the ocean floor between the North China and Bureya-Khanka, South Gobi, and Central Mongolia ter-ranes, where new oceanic crust continued to grow and where accumulation of silici-clastics, often ore-bearing formations, and, on local uplifts, reef limestones occurred. During this time, the Bureya-Khanka terrane appears to have amalgamated into the single large Amur Massif (Amuria), where coarse molasse developed and orogenic acid volcanism occurred (Kheraskova 1995).

Widespread transgression on Laurentia was an important Late Cambrian event, and shelf basins covered vast areas of the midcontinent from the Cordillera to the Appalachians (Link 1995; Long 1995). Marine conditions were reestablished over the whole of Siberia (Budnikov et al. 1995), but the proportions of siliciclastic sediments increased in marginal sedimentary basins (Markov 1979). Scarcity of marine Late Cambrian deposits, and the onset of bimodal subaerial volcanism, are indicative of uplift of the peri-African shelf of Gondwana (Linan and Gamez-Vintaned 1993; Geyer and Landing 1995; Buschmann and Linnemann 1996). A general restriction of marine basins occurred in Australia and in North and South China (Atlas 1985; Cook 1988).

Figure 3.5 Early Ordovician paleogeography (490 Ma). See legend on page 50.

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