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Middle Paleozoic strata of the Sierra Las Pintas, northeastern Baja California Norte, Mexico

By
Paula Leier-Engelhardt
Paula Leier-Engelhardt
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Published:
January 01, 1993

The bedrock strata of the northern Sierra Las Pintas are approximately 520 m thick and are here divided into six informal units of formational rank: unit SP1, calcareous sandy siltstone and calcareous siltstone; unit SP2, siltstone interbedded with calcareous siltstones, crinoidal grainstone, and granule conglomerates; unit SP3, graded crinoidal grainstone; unit SP4, massive and normally graded beds of coarse sandstone and granule to cobble conglomerate; unit SP5, basalt flows with pillow breccia and hyalotuffs; and unit SP6, bedded chert and calcareous argillite in thrust contact with underlying units. Lophyophyllid corals and brachiopods from unit SP3 suggest a Carboniferous age.

The bedrock strata of the southern Sierra Las Pintas are approximately 720 m thick and divided into four informal units of formational rank: unit AG1, bedded chert and argillite; unit AG2, sandstone, siltstone, and debris flow; unit AG3, limestone and shale; and unit AG4, pillow basalt and basalt flows. Based on conodonts recovered from unit AG1 and conodont fragments recovered from unit AG3, the section is bracketed between Early Devonian and Early Mississippian.

Thin-section analysis suggests that the terrigenous rocks from the two areas were derived from similar terranes with cratonal source areas. Minor- and trace-element analyses of basalt indicates eruption during a rifting event, perhaps near a continental margin. The basement exposures of the northern and southern Sierra Las Pintas are separated by a cover of Miocene volcanic strata and can not be correlated on a unit-to-unit basis, but could have been deposited in the same basin. Rocks of similar provenance, age, and lithologic associations are found elsewhere in Baja California Norte, Sonora, Sinaloa, and the Havallah and Schoonover sequences of Nevada.

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Contents

GSA Special Papers

The Prebatholithic Stratigraphy of Peninsular California

R. Gordon Gastil
R. Gordon Gastil
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R. H. Miller
R. H. Miller
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Geological Society of America
Volume
279
ISBN print:
9780813722795
Publication date:
January 01, 1993

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