The Palaeozoic and Mesozoic strata of northwestern Mexico are exposed in small, scattered areas in Sonora and Baja California because of the Tertiary tectonic events in normal faulting, the pervasive effects of Tertiary intrusives, and the widespread volcanic cover of the Sierra Madre Occidental (Figure 3.1). However, the early investigations, followed by more recent geologic work in that region, allow us to unravel the late Palaeozoic and early Mesozoic sedimentologic and tectonic history of that part of Mexico.

In this chapter we examine the sedimentologic and tectonic history of the Permian and Triassic geologic events in northwestern Mexico, with special reference to the state of Sonora. The younger Palaeozoic rocks of Sonora are of early and middle Permian age. These rocks are parts of thicker Palaeozoic successions that are of cratonic-platform character in the northeastern part of the state, of shallow-water miogeoclinal character in the northwestern and central parts of Sonora, and of deep-water eugeoclinal character in the central part of the state (Figure 3.2). Permian rocks of eugeoclinal character have been reported from one locality in Baja California. The Triassic rocks of Sonora, which are of Carnian and Norian age, can be grouped into two lithotectonic assemblages. In the central part of the state, these rocks are recognized as part of the Barranca Group (Alencaster, 1961a), whereas in northwestern Sonora they are included in the Antimonio Formation (González-León, 1980).