Abstract

Thick Upper Paleozoic offshore limestones and Cretaceous near-shore deposits in northeastern Sonora indicate the extent of the former seaways. Near El Tigre the Upper Paleozoic limestones attain a thickness of about 5500 feet, are like those in southeastern Arizona, and show the existence of a fairly broad persistent sea. Marked shoaling is indicated by the shaly character of the Upper Mississippian deposits.

Near El Tigre the Paleozoic limestones are overlain by several thousand feet of near-shore deposits representing the Trinity division of the Cretaceous. Significant fossils include Parahoplites, Acanthoplites, Cheloniceras, Beudanticeras, and Exogyra aff. latissima var. aquila (Brongniart). Southward from the Bisbee district the Glance conglomerate and Morita formation increase markedly in coarseness and thickness, attaining over 8000 feet along the northern part of the Sierra de los Ajos. Similar coarse clastic deposits, extending from Cabullona to Santa Cruz, show deposition in a geosyncline adjacent to a rising and probably fairly high landmass which lay south of the Bisbee district and west of the El Tigre district. The transgressive character of the Trinity sediments on the Upper Paleozoic limestones suggests that the Jurassic and Lower Cretaceous seas older than Trinity time never extended as far west as Sonora.

During the Laramide revolution the sedimentary rocks of the geosyncline were intensely folded, uplifted, and eroded to a surface of high relief which subsequently was buried by thousands of feet of Tertiary lavas. Extensive steep-angled faulting along north-northwest lines began in middle or late Tertiary time and at present controls most of the topographic features. . . .

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