Weathering and tectonic settings of the siliciclastic rocks from the Morita Formation, northeastern Sonora, Mexico

Abstract

Geochemistry of major oxides was carried out on the siliciclastic rocks collected from the Sierra Anibacachi and Rancho Búfalo sections of the Morita Formation (Early Cretaceous) exposed in the northeast of Sonora, to interpret the paleoweathering conditions and tectonic settings of the source rocks. Geochemically, the siliciclastic rocks were classified as sub-litharenite, litharenite, subarkose and Fe-sand. The Morita Formation consists mainly of siliciclastic rocks that were deposited under fluvial conditions and the total thickness vary between 560 and 855 m in Sierra Anibacahi and Rancho Búfalo, respectively. The CIA and PIA values and A-CN-K diagram of sandstones from Sierra Anibacachi section indicate a low to intense chemical weathering of the source area, whereas CIA values for siltstone and shale show moderate intensity of chemical weathering in the source region. Similarly, the CIA and PIA values and the A-CN-K plot suggest that the sandstones from Rancho Búfalo section were derived from low to intensely weathered source region, whereas siltstones were received sediments from source rocks affected by low to moderate degree of chemical weathering. The siliciclastic rocks from Sierra Anibacachi and Rancho Búfalo sections show large variations in ICV values, indicating that these samples are compositionally immature to mature. The tectonic discriminant-function-based multidimensional diagram shows arc, rift and collision fields for the sandstones from Sierra Anibacachi section, rift and collision for siltstone, while collision setting for shales. However, most of the sandstones and siltstones from Rancho Búfalo section fall in the arc field, whereas some sandstones fall in the collision field. The siliciclastic rocks from Sierra Anibacachi and Rancho Búfalo section show large variations in the tectonic settings of the source rocks. However, the results obtained from the discriminant function diagrams are consistent with a regional tectonic evolution of the study area.