Revista Mexicana de Ciencias Geol6gicas, volumen 14, numem 2,

115

1997, p. 115-135.

Univcrsidad Nacional Autdnoma de Msxico, Instituto de Geologia,
Mdxtco, D.P.

STRATIGRAPHY AND REGIONAL SIGNIFICANCE

OF MISSISSIPPIAN

TO JURASSIC ROCKS IN SIERRA SANTA TERESA, SONORA,

MEXICO

John H. Stewartl, Ricardo Amaya-Martinez2,
Robert 6. Stamm3, Bruce R. 8'ardlaw3,
George D. Stanley, Jr.4, and Calvin H. Stevens5

ABSTRACT

In Sierra Santa Teresa, about 20 km southeast of Hermosillo, Sonora, Mexico, Paleozoic strata are

exposed in the upper plate, and Mesozoic rocks in the lower plate, of a post-Early-Jurassic thrust fault.

Paleozoic rocks in the upper plate are divided into seven units. Units I through 5 are about 1,500-m thick,

composed of shallow-water lime mudstone and minor wackestone, packstone, and bioclastic packstone, and

range in age from Late Mississippian to Middle Pennsylvanian. Unit 6 is about 110- to 120-m thick and con- sists of a distinctive shallow-water packstone with abundant fusulinids and pelmatozoan detritus. Unit 7

appears to lie depositionally on unit 6 and is at least 610-m thick. It consists of relatively deep-water silty

limestone, sandy limestone, limy siltstone, and limy very fine-grained sandstone, and interbedded 0.5 to 2.5-

m-thick layers of conglomerate and calcarenite, commonly containing detrital fusulinids. The detrital fusulin- ids are late Early or Middle Permian in age and considered to indicate the approximate age of deposition of

unit 7.

The lower plate rocks consist of at least 544 m of Upper Triassic and Lower Jurassic strata overlain

by about 600 m of Mesozoic andesitic rocks. Upper Triassic and the Lower Jurassic strata consist of a variety

of rock types including argillite, siltstone, quartzitic siltstone, limy siltstone, sandstone, limy sandstone,

quartzite, conglomerate, conglomeratic sandstone, sandy calcarenite, sandy limestone, calcarenite, lime mud- stone, wackestone, bioclastic packstone, boundstone, and recrystallized limestone. Corals and sponges in

limestone in the lower part of the section indicate a Late Triassic age, and ammonoids in the more silty upper

part of the section are considered to be Early Jurassic in age.

The relatively deep-water depositional environment of the Lower or Middle Permian siliciclastic stra- ta of unit 7 marks a distinct change from the underlying shallow-water depositional environment of the

Mississippian to Lower Permian strata of units I to 6. Tectonically this change can be interpreted either as (I)

the development of a foreland basin in response to a subduction system to the south; or (2) development of a

basin during fragmentation of the continental margin by strike-slip and transtensional faulting in an unstable

continental borderland.

Upper Triassic rocks in Sierra Santa Teresa have been interpreted either as (1) an allochthonous ter- rane unrelated to largely nonmarine strata of the Upper Triassic and Lower Jurassic Barranca Group to the

east in east-central Sonora; or (2) a facies of Upper Triassic rocks intermediate between the largely nonma- rine Barranca Group to the east and marine strata of the Antimonio Formation to the northwest in northern

Sonora.

Keywords: Stratigraphy, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Sierra Santa Teresa,

Sonora, Mexico.

RESUMEN

En la Sierra de Santa Teresa localizada a 20 km al SE de Hermosillo, Sonora, afloran estratos paleo- zoicos (placa superior) que tectonicamente yacen sobre rocas mesozoicas (placa inferior), a lo largo de una

falla de cabalgadura post jurasica temprana. Las rocas paleozoicas de la placa superior se dividen en siete

unidades. Las unidades 1 a la 5 tienen un espesor de aproximadamente 1,500 m y estan compuestas por

lodolitas calcareas, wackestone, packstone y packstone bioclastica que indican ambientes de aguas marinas

someras; la edad de estas unidades varia del Misisipico Tardio al Pensilvanico Medio. La unidad 6 varla en

espesor de 110 a 120 m y consiste en un packstone con abundantes fusulinidos y fragmentos de briozoarios.

La unidad 7 yacc sobre la unidad 6 en aparente contacto sedimentario y es de al menos 610 m de espesor.

Consiste en caliza arenosa y limolitica, limolita calcarea, arenisca calcarea de grano muy fino e intercala- ciones de conglomerado y calcarenita en capas de 0. 5 a 2.5 m de espesor con detritos de fusulinidos.

iU.S. Geological Survey, 345 Middlefield Road, MS-901, Menlo Park,

California, 94025, USA. (E-mail address: stewart®mojavewrusgsgov).

2Departamento de Geologia, Universidad de Sonora, Hermosillo, Sonora,

Mexico.

3U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, Virginia,

22092, U.S.A.

4Department of Geology, The University of Montana, Missoula, Montana,

591312, U.S.A.

sGeology Department, San Jose State University, San Jose, California, 95192,

U.S.A.

STEWART, AMAYA-MARTINEZ, STAMM, WARDLAW, STANLEY, AND STEVENS