Abstract of my October, 2001 article in Journal of South American Earth Sciences special issue on the Geology of Northwestern Mexico and Adjacent Areas, volume 14, Number 5, pages 453-474:    (Click here for link to complete article)

Tectonic Insights From an Upper Jurassic-Lower Cretaceous

Stretched-Clast Conglomerate, Caborca-Altar Region, Sonora, Mexico

Jonathan A. Nourse

Department of Geological Sciences

California State Polytechnic University

Pomona, CA 91768

ABSTRACT

The ranges directly northeast of Caborca and Altar preserve a 2000+ m-thick folded section of the Altar Formation, composed of ductilely deformed Upper Jurassic-Lower Cretaceous polymict conglomerate, sandstone, mudstone, and andesite.  Stratigraphic relationships indicate that this section, which accumulated in a 60 km-long northwest-trending basin, is broadly correlative with the Glance Conglomerate.  Coarse (boulder-cobble) detritus in the lower part of the section was shed from prominent highlands consisting of the Middle Jurassic magmatic arc and the Neoproterozoic-Paleozoic miogeocline.  In the pebbly upper part of the section, abundant sandstone, mudstone, lacustrine(?) carbonate, and felsic tuff interstratified with andesite flows and breccias record lower-energy depositional conditions interrupted locally by volcanism.  By the time of Bisbee Sea incursion into northwestern Mexico, the Jurassic arc source had disappeared and the miogeoclinal source was topographically subdued.  The Lower Cretaceous Morita Formation, which conformably overlies the Altar Formation along a gradational contact, records marine transgression that accompanied regional subsidence.  

            Significant northeast-southwest contraction affected the Altar Formation, Bisbee Group, and Upper Cretaceous strata between 71 Ma and 51 Ma.  Characteristics of this Laramide event include asymmetric, northeast-verging folds and thrusts accompanied by penetrative southwest-dipping stretched-clast foliation or cleavage and greenschist-facies metamorphism.  Mid-Tertiary normal faults overprint the compressional structures such that deeper levels of the stratigraphy are juxtaposed against shallower levels, and axial planes of major range-scale folds are rotated about horizontal axes relative to one another.

Abstract of my 1995 article in Geological Society of America Special Paper #301--Studies of Sonora and Adjacent Areas, p. 59-78:

JURASSIC-CRETACEOUS PALEOGEOGRAPHY OF THE MAGDALENA REGION, NORTHERN SONORA, AND ITS INFLUENCE ON THE POSITIONING OF TERTIARY METAMORPHIC CORE COMPLEXES

Jonathan A. Nourse

Department of Geological Sciences

California State Polytechnic University

Pomona, California 91768

ABSTRACT

Jurassic and Cretaceous rocks in the vicinity of Magdalena de Kino, Sonora record the transition from a shallow-level magmatic arc to a northwest-trending marine embayment.  The stratigraphy may be divided into three lithologic packages: (1) Lower or Middle Jurassic rhyolite porphyry interstratified with quartz arenite and rhyolite-quartz arenite clast conglomerate and intruded by Late Jurassic granite porphyry, unconformably overlain by (2) Upper Jurassic-Lower Cretaceous Glance Conglomerate: quartz-dominant polymictic conglomerate interbedded with lithic sandstone and siltstone, which grades upward into (3) Lower Cretaceous basinal marine strata containing siliciclastic and carbonate lithologies correlative to the Bisbee Group of southeastern Arizona.  This tripartate section was later affected by Laramide compression and/or middle Tertiary magmatism, mylonitization, and detachment faulting.  Although the imprint of Tertiary extensional strain is locally intense, it has been possible to reconstruct the Mesozoic paleogeography.

The lowermost Mesozoic stratigraphic units in the Magdalena area represent a portion of the Jurassic magmatic arc in northern Sonora.  Disturbance of the arc during Late Jurassic-Early Cretaceous time is recorded by syntectonic deposition of the Glance Conglomerate.  This fluvial unit contains distinctive clasts eroded from predominantly northeasterly sources.  Outcrops of Glance Conglomerate delineate the northeast boundary of a basin that extended northwestward to the vicinity of Tubutama and Altar, and southeastward to Arizpe.  As the basin subsided, it was inundated by an Early Cretaceous sea, and Bisbee Group sediments were deposited.  The southwest margin of this basin may be marked by exposures of conglomerate that underlie Lower Cretaceous strata near Cucurpe, Estacion Llano, and Altar.  These conglomerate localities preserve clast assemblages suggestive of southwesterly source regions.  The Glance Conglomerate equivalent near Altar also contains clasts eroded from northwesterly sources.

Early Cretaceous basins in Sonora and southern Arizona record a period of marine deposition that followed Late Jurassic movements on the Mojave-Sonora megashear and an associated family of faults.  Deformation related to broadly distributed Late Jurassic-Early Cretaceous sinistral shear produced several geographically distinct regions of subsidence regionally defined by outcrops of Bisbee Group strata.  Fault bounded margins of these basins trended northwest and northeast, and may be delineated by exposures of Glance Conglomerate.  It is postulated that  Tertiary metamorphic core complexes of the region developed preferentially within areas of crust that were thinned and fractured by Late Jurassic-Early Cretaceous transtensional faulting.  Margins of the Early Cretaceous sedimentary basins first acted as mechanical discontinuities that focused Laramide contractional strain.  Many of these same areas were also affected by peraluminous magmatism and metamorphism during Tertiary time.  As the region extended orthogonal to Late Jurassic-Early Cretaceous lineaments, mylonitized mid-crustal granitic and metamorphic rocks were uplifted and exposed via normal displacement on middle Tertiary detachment faults.