Geologic Map of Empire Ranch 7.5' Quadrangle, Pima County, Arizona

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Author
Ferguson, Charles A.
Cook, Joseph P.
Affiliation
Arizona Geological Survey
Issue Date
2019-09-01

Metadata
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Citation
Cook, J.P., Ferguson, C.A., 2019, Geologic Map of Empire Ranch 7.5' Quadrangle, Pima County, Arizona. Arizona Geological Survey Digital Geologic Maps, DGM 143, 1 map sheet, map scale 1:24,000.
Publisher
Arizona Geological Survey (Tucson, AZ)
Description
The Empire Ranch map area is bisected by the Davidson Canyon fault, a west-side-down normal fault zone up to 3km wide that post-dates deposition of Oligocene ignimbrites within moderately (30-45°) east-tilted conglomerates (Pantano Formation) that overlies, with little apparent angularity, Upper Cretaceous volcanic rocks in the hangingwall of the western strand of the fault. In the hangingwall of the western strand, the ~73Ma Mt Fagan ignimbrite is at least several hundred, if not a thousand meters thick, and contains abundant lithic megabreccias. The caldera margin is exposed, in its most shallowly preserved position, in the north-central part of the map area where is not a fault, but a north-dipping buttress unconformity overlain by welded ignimbrite that overlaps folded, and typically steeply (mostly easterly) tilted lower Bisbee Group argillaceous siliciclastic rocks. The buttress can be traced northwest where it suddenly becomes a major, north-side-down, “ring-fracture” fault juxtaposing the base of the caldera-filling ignimbrite on the north with the lowermost Bisbee Group on the south. The “ring-fracture” fault can be traced further to the west into adjacent map areas, and deeper into the section, where Paleozoic rocks on the south are juxtaposed with lowermost Bisbee Group rocks to the north, and the fault is intruded by post-caldera plutonic rocks. The base of the ignimbrite inside the caldera conformably overlies a thick pile of subaqueous andesite lava that in turn overlies, with angular unconformity, the uppermost part of the Bisbee Group. Megabreccias are composed chiefly of Bisbee Group, commonly present as several-hundred meter long slabs of semi-intact stratigraphy including thick layers of the subaqueous andesite, and a third lithology, the pre-volcanic Fort Crittenden conglomerate. Rocks in the footwall to the eastern strand of the Davidson Canyon fault are Paleozoic quartzose, carbonate-rich strata, locally with wedges of Triassic-Jurassic red-bed siliciclastics, overlain by the Bisbee Group and locally intruded by Cenozoic hypabyssal igneous rocks. The lower third of the Bisbee Group is repeated by a major, northwest-side-down normal fault that is truncated by the easternmost strand of the Davidson Canyon fault, which carries in its hangingwall a complex Upper Cretaceous volcanic sequence of chaotic breccias similar to the Mt Fagan rhyolite megabreccias farther west. Ignimbrite matrix to megabreccia between the strands of the Davidson Canyon fault has been dated as slightly older (75Ma) than the Mt Fagan Rhyolite, and has a biotite-rich, mega quartz phenocryst assemblage more akin to the older, Confidence Peak Rhyolite ignimbrite. Extensive surficial deposits in the Empire Ranch quadrangle are limited to the southern half of the mapping area and are consists of deeply dissected basin fill alluvium, fanglomerate deposits, and relatively narrow but elongate alluvial terraces emplaced along incised channels. High-standing, very well-rounded hills and ridges in the southern half of the mapping area are composed of Gila Conglomerate. Thin, discontinuous relict alluvial fan remnants atop high-standing ridges record the highest point of aggradation prior to transition to incision of many small channels to meet downcutting along Cienega Creek to the east. Conglomerate and basin fill deposits are intricately dissected and well-rounded throughout the mapping area as a result of long periods of incision. Larger active channels are flanked by a suite of nested alluvial terraces. Quaternary deposits in the northern half of the mapping area are almost entirely confined to near-channel deposits and abandoned strath terraces along active channels within bedrock-dominated terrain.
URI
http://hdl.handle.net/10150/634890
Additional Links
https://library.azgs.arizona.edu/item/ADGM-1572451811712-105
Language
en
Series/Report no.
DGM-143
Rights
Arizona Geological Survey. All rights reserved.
Collection Information
Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
North Bounding Coordinate
31.878
South Bounding Coordinate
31.7531
West Bounding Coordinate
-110.752
East Bounding Coordinate
-110.616
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