PRELIMINARY GEOLOGIC MAP OF THE COTTONWOOD MOUNTAIN QUADRANGLE, IRON COUNTY, UTAH By Florian Maldonado, John J. Anderson, and Robert F. Biek 2011 U.S. Geological Survey Open-File Report 2010–1334 ABSTRACT The Cottonwood Mountain quadrangle is a structurally complex terrane composed of several features, from oldest to youngest:(1) low-angle faults, (2) megabreccia deposits, and (3) horsts and grabens. Low-angle faults are of two ages: an older age group referred to as the Red Hills low-angle shear zone, and younger faults related to later uplift mapped in the Parowan quadrangle. The Red Hills shear zone in the Red Hills, approximately 8 km west of quadrangle, is interpreted to be a low-angle Miocene structure that extends into the Markagunt Plateau and is of regional extent. This shear zone has detached part of the Brian Head Formation and overlying rocks from underlying rocks, possibly along the contact between the Brian Head Formation and the base of Tertiary volcanic rocks, or in some areas, within the upper part of the Brian Head Formation. On the geologic map, the shear zone is approximately placed at the contact between the Brian Head Formation and the base of Tertiary volcanic rocks even though it may occur below that contact. The shear zone is variously characterized by pulverized rock and discordance in bed attitudes and by thinning or omission of rock units overlying the shear zone. The structurally incompetent rocks of the Brian Head Formation allowed slip to be localized within this unit. The shear zone and overlying detached plate are offset by later high-angle faults and are preserved in the grabens and eroded from horsts. The age of the Red Hills low-angle fault is older than about 20 Ma since the dated northwest trending mafic dikes intrude both the lower and upper plate of the Red Hills shear zone. Other low-angle faults are found in the adjacent Parowan quadrangle to the southwest of map area but have not been found in the map area. These faults are younger than the Red Hills shear zone, possibly related to later uplift of the Markagunt Plateau or adjacent Red Hills. The faults merge locally with the Red Hills shear zone, and it is impossible to differentiate the two in some areas. Locally, the faults become steeper along part of their segment suggesting rotation of blocks containing these faults. The purpose of this report is to map the geology of the quadrangle at 1:24,000 to be used to compile the Panquitch 1:100,000 scale sheet. DISCLAIMERS This database, identified as OFR 2010–1334, has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review.Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty, expressed or implied is made regarding the display or utility of the data on any other system, or for general or scientific purposes, nor shall the act of distribution constitute such warranty. The U.S.Geological Survey shall not be held liable for improper or incorrect use of the data described and/or contained herein. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. Although this information product, for the most part, is in the public domain, it also contains copyrighted material as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. DATA FILES List of files: 00ReadMe.txt OF2010-1334.pdf OF2010-1334.met Cottonwood.gdb Cottonwood_shapefiles.zip Geodatabase feature classes (Cottonwood.gdb): cottonwood_geo---geologic unit polygons cottonwood_geo_label---geologic unit labels cottonwood_line---contacts and faults cottonwood_dike---dikes cottonwood_pnt---geologic point features: bedding, folds, joints, azimuth and dip of fault locations cottonwood_xsection---cross section lines shapefiles directory: cottonwood_geo.shp (.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) cottonwood_geo_label.shp (.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) cottonwood_line.shp (.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) cottonwood_dike.shp (.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) cottonwood_pnt.shp (.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) cottonwood_xsection (.dbf/.prj/.sbn/.sbx/.shp.xml/.shx) HOW TO OBTAIN THE DIGITAL FILES The digital files constituting the geologic map database of this report can be obtained via the Internet from the U.S.Geological Survey publications website. Go to the web page at http://pubs.usgs.gov/of/2010/1334 and follow the directions to download the files. ArcView shapefiles may be extracted from a zip-compressed file (Cottonwood_shapefiles.zip). In addition, an Adobe Portable Document Format (PDF) file (OF2010-1334.pdf) can be downloaded, from which paper copies may be printed. The main product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com.