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Younger Precambrian Geology in Southern Arizona

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Younger Precambrian Geology in Southern Arizona Younger Precambrian Geology in Southern Arizona GEOLOGICAL SURVEY PROFESSIONAL PAPER 566 Younger Precambrian Geology in Southern Arizona By ANDREW F. SHRIDE GEOLOGICAL SURVEY PROFESSIONAL PAPER 566 Stratigraphic^ lithologic^ and structural features of younger Precambrian rocks of southern Arizona as a basis for understanding their paleogeography and establishing their correlation UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress catalog-card No. OS 67-238 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 65 cents (paper cover) CONTENTS Page Page Abstract.__________________________________________ 1 Apache Group Continued Introduction. ______________________________________ 2 Basalt flows.___________________________________ 42 Older Precambrian basement----_----___--___________ 4 General features and distribution.____________ 42 Pre-Apache unconformity._----_-_-_-________________ 6 Petrology.___--_______-_____-_--__________. 43 Thickness and distribution of younger Precambrian rocks. 7 Criteria for distinguishing basalt from diabase.. 43 Apache Group____________________________________ 11 Pre-Troy unconformity.______---_-___________-.____. 44 Pioneer Shale__________________________________ 11 Troy Quartzite.____________________________________ 44 Thickness and general character-_____________ 11 Definition and subdivision.______________________ 44 Lithology and stratigraphic relations-_________ 11 Arkose member_________________________________ 46 Scanlan Conglomerate Member...________ 11 Distribution and thickness.__________________ 46 Tuffaceous mudstone..__________________ 14 Stratigraphy and lithology___________________ 46 Arkose._______________________________ 15 Chediski Sandstone Member.____________________ 47 Lateral variations.._________________________ 16 Thickness__ ______________________________ 47 Dripping Spring Quartzite___-_--___--_________ 17 Lithology and stratigraphy_._________________ 47 Barnes Conglomerate Member________________ 19 Lateral variations__._________--__-___-_-_. 50 Arkose member___________________________ 20 Quartzite member______________________________ 51 Siltstone member.__________________________ 21 Stratigraphy. _________________-_____---_-_. 52 Lithology._____________________________ 21 Lithology--___-______--_____-------___--__. 52 Weathering modifications._______________ 21 Distribution and topographic expression.______ 52 Bedding and secondary structures... ______ 22 Distinctive features of members____________----_. 52 Features that distinguish principal units. _ _ 24 Diabase and related rocks..__________________________ 53 Pre-Mescal unconformity-_______________________ 24 Form and dimensions of intrusions._______________ 53 Mescal Limestone.._____________________________ 25 Stratigraphic distribution._______________________ 55 Lower member.____________________________ 27 Pe trology_ _____________________________________ 56 Lithology and stratigraphy_______________ 27 Diabase.___.___-____________----_--------. 57 Structural and textural features._________ 29 Feldspathic differentiates..____________--_-.. 58 Halite molds in chert______________ 29 Diabase pegmatite._______________----.. 58 Sheaflike impressions._______________ 29 Granophyre.._ _____________-______---_. 58 Minor slump features._________.-__. 30 Aplite. __________________--__---_-----. 59 Intraf ormational conglomerates__ _ _ _ 30 Mode of formation..______________----_. 59 Founder breccia.___________________ 31 Metamorphism associated with diabase__________----_. 61 Algal member._____________________________ 32 Late Precambrian deformation._____________-__-----. 64 Argillite member..__________________________ 33 Prediabase deformation.-.._________--_____-----. 64 Distribution and stratigraphic relations. 33 Deformation associated with diabase intrusion __. 67 Lithology-_____________________________ 34 Relations of Paleozoic formations to Precambrian rocks.. 69 Precambrian weathering and silicification______ 35 Rocks adjacent to the pre-Bolsa unconformity _. 69 Solution features_______-___-___.._____. 35 Pre-Middle Devonian unconformity.________-_--_. 73 Late-formed cherts____________________ 36 Age of diabase and associated rocks._____--_---------- 76 Karst and related breccias.______________ 37 Hematite concentrations._-____-_--_-____ 38 Geologic history.________--_-_------_--------------- 77 Metamorphic limestones.-_--__--_-__-_-_____ 39 Interregion correlations.-__-__--_----__-------------- 80 Variations in thickness._____________________ 41 References cited._________-___-_-_--_--------------- 84 Stratigraphic nomenclature._________________ 42 Index.______________---_---_---------------------- 87 ILLUSTRATIONS Page FIGURES 1-4. Maps: 1. Outcrops of younger Precambrian rocks in Arizona._______________-____--_------------------- 2. Outcrops of older Precambrian terrane, showing types of rocks that were sources of elastics during late Precambrian time._____________________________-___-------_---------------------- 5 3. Outcrops of younger Precambrian strata and coextensive diabase intrusions in southeastern Arizona. 12 4. Some paleogeographic features of younger Precambrian formations in southeastern Arizona .____ 13 m IV CONTENTS Page FIGURE 5. Photograph showing delicate cross-lamination in tuffaceous siltstone of the Pioneer Shale._______________ 15 6. Photomicrograph of tuffaceous siltstone of the Pioneer Shale.________________________________________ 15 7-9. Photographs: 7. Typical lamination and irregular bedding in siltstone member of Dripping Spring Quartzite______ 22 8. Scour-and-fill and compaction features in the siltstone member of the Dripping Spring Quartzite-__ 23 9. Structures typical of the stromatolite beds of the algal member of the Mescal Limestone.---______ 33 10-14. Diagrams: 10. Schematic sections showing relations of multiple injections of diabase to plates of host rock isolated between intrusions._________________________________________________________________ 55 11. North-south geologic section after the Troy Quartzite was deposited,__________________________ 64 12. North-south geologic section at the end of Devonian time._---__-______-__---_________-______ 64 13. Effects of sill intrusion on displacement along a preexisting fault-__-___--__-_-____-____-_-____ 67 14. Idealized geologic section showing relations of faults to diabase intrusions and to pre-Paleozoic unconformities_ _ __________________________________________________________________ 67 TABLES Page TABLE 1. Pre-Devonian formations of southeastern Arizona.-_________________________________________________ 8 2. Criteria for distinguishing clastic strata of the Bolsa Quartzite and Abrigo Formation from those of the Troy Quartzite-_________________________________________________________________________________ 74 YOUNGER PRECAMBR1AN GEOLOGY IN SOUTHERN ARIZONA By ANDREW F. SHEIDE ABSTRACT changes where it laps onto older terrane. The recognition of these aspects of the Apache and Troy sequence provides new Younger Precambrian rocks of southeastern Arizona con­ bases for speculation on interregional correlations of younger sist of the Apache Group, the overlying Troy Quartzite, and Precambrian rocks. large diabase bodies intrusive into and coextensive with the At least two major marine transgressions, sharply marked sedimentary formations. These rocks are widely exposed by the unconformities at the bases of the Pioneer and the and little deformed in northern Gila County and in the south­ Dripping Spring, occurred during Apache time; the Chediski west corner of Navajo County areas that are structurally Sandstone Member of the Troy was deposited during another part of the Colorado Plateau. In the Basin and Range prov­ transgression. The erosional unconformity between the Mes­ ince farther south, outcrops are scattered and less extensive, cal and the Dripping Spring and the unconformity at the and structural relations with younger formations are com­ top of the algal member of the Mescal could also represent monly obscure. appreciable time intervals. The unconformities that separate The Apache Group ranges in thickness from 1,250 to 1,600 the argillite member from the overlying and underlying basalt feet, and includes, in ascending order, the Pioneer Shale, the are apparently effects of brief subaerial exposures during re­ Dripping Spring Quartzite, the Mescal Limestone, and un­ current broad warping which preceded uplift that terminated named basalt flows. The Scanlan and Barnes Conglomerate Apache sedimentation. Apache strata were variously eroded Members, formerly designated as separate formations, are and locally stripped completely away before the Troy was considered basal conglomerate members of the Pioneer Shale deposited. Conceivably, unconformities also separate the and Dripping Spring Quartzite, respectively. The upper part members of the Troy. of the Dripping Spring is divided into a massive arkose mem­ Before and during the pre-Troy erosion, solution caverns ber and an upper thin-bedded member of siltstone and arkose. were formed in the dolomite of the Mescal, and in a few The Mescal Limestone comprises three members: a lower areas a karst topography was formed. During this period thin- to medium-bedded member of cherty dolomite or of cal- the dolomite was extensively silicified and locally was con­ citic limestone that is
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