Description
The Sierra las Pinta is located 122 km south of Mexicali, Baja California Norte, Mexico. Paleozoic rocks are on the extreme north and south ends of the range. The Paleozoic rocks were studied to describe the stratigraphy, determine if correlations could be made between units in each area and interpret the regional tectonic implications. The section in the northern area (Area 1) is approximately 520 m thick and is divided into six units: Unit 1, calcareous siltstone and calcareous sandy siltstone; Unit 2, siltstone interbedded with calcareous rare crinoidal grainstone and rare granule conglomerates; Unit 3, graded sandy crinoidal grainstone; Unit 4 , massive and normally graded beds of coarse sandstone and granule to cobble conglomerate; Unit 5, basalt flows with rare basalt conglomerate and hyalotuffs; and Unit 6 , bedded chert and calcareous argillite in thrust contact with underlying units. Lophyophyllid corals and brachiopods from Unit 3 suggest a possible Carboniferous age. The section in the southern area (Area 2) is approximately 720 m thick and is divided into four units: Unit A, bedded chert and argillite; Unit B, fine sandstone , coarse siltstone and granule conglomerate debris flows , probably formed in a submarine fan environment ; Unit C, a deformed limestone and shale sequence ; and Unit D, pillow basalts and basalt flows. Based on conodonts recovered from Unit A and conodont fragments recovered from Unit C, the section is Early Devonian to Early Mississippian in age. Detailed thin section analysis suggests the terrigenous rocks from each area were derived from similar terranes and had a definite cratonal source. The lithic grain population includes quartz arenite and chert grains, no metamorphic lithic grains lithic basalts and a small but persistent percentage of volcanic grains. Minor and trace element analyses of from each area indicate the basalts were erupted during a rifting event, perhaps in a back-arc basin near a continental margin. The rocks from Area 1 and Area 2 cannot be correlated on a one-to-one basis, but could well have been deposited in the same basin. Rocks of similar provenance, age and lithologic associations are found in Baja California Norte, Sonora, and Sinaloa, and all these sections may relate to events which formed the Havallah and Schoonover sequences in Nevada.
