MESOZOIC THROUGH EARLY TERTIARY SEDIMENTATIONAL AND TECTONIC PATTERNS OF NORTHEAST SONORA AND SOUTHEAST ARIZONA * led by: 2 3 Stanley B. Keith!, Francisco Longoria , and William L. Bilodeau Arizona Geological Survey Open-File Report 81-3 1981, Arizona Geological Survey 416 W. Congress, Suite #100, Tucson, Arizona 85701 I. Arizona Bureau of Geology and Mineral Technology, 845 N. Park Avenue Tucson, Arizona 85719 2. The University of Texas at Dallas, Box 688, Richardson, Texas 75080 3. Department of Geology University of Colorado at Denver, 1100 14th Street Denver, Colorado 80202 * Field Trip No. 12 in conjunction with the 1981 Cordilleran Geological Society of America c:onference held ill Hermosillo, Sonora, Mexico (March 26-28, 1981) This report is preliminary and has not been edited or reviewed for conformity with Arizona Geological Survey standards NORTHEAST SONORA LEG Leader: Francisco Longoria ......... AGUA PRIETA """ <tt", o '?- ~ ('~ '\" -vi J I I \~~, \ 'II r I - \ I E XPL ICA CION DEPOSITOS DE \ALLE -----l~ TERCIARIO SlIPERIOR--[ BASALTOS----- ---[]I] TE~IARIO MEOIO-----{ IGNWBRITAS Y RIOLITAS----IT;7jl RlCAS \Cll.£:AHICAS Y f\~ PI.JJT1:NCAS ca.t SU --- ~ CRETACICO SlIPERIOR_ r FRANJA BASAMENTO VCLCAHO INTRUSIONES----- ~ T£HCIARI: INFERIO..?1 PLUTONICA{ AtaSITAS Y RIOLITAS - ~ CRETACICO SlIPCRIOR-{ MQLASAS -------F----=---1 @zLOCALlZACION o OE PARAOAS ME.lV.!Ota7-lF~l4.W------{ INTRUSIVOS INOIFERENClAOQS-- - --- ~t +71 [SCALA GRAflCA CRETACCO INFERKJR-{ ZONA EXTERNA ORIENTAL_ r: "~ (SEDIMENTOS) ~ .:...!.:...d I o 10 20 }OK", ZCflA INTERNA OCCIDENTAL -" If! ,~! 1 JlIRASICO-------t (SEDIMENTOS VOLCANOCLASTICOS - G:~ ... [ (;Of VQLCANICAS) -" J PALEOZOICO [ CAUIA DE TJPO PLATAfORMA---- kg ESQUEMA GEOLOGICO DEL MJRTE DEL ESJ;1[)() DE SONORA (lJ) PNECAMBRICO ANTER.QR--{ ROCAS METAMORfICAS--------- - L~ ITINERARY FOR NORTHEAST SONORA LEG Leader: Francisco Longoria GEOLOGY OF THE AGUA PRIETA REGION, NE SONORA. MEXICO. Objectives: 1) To examine the lithostratigraphy of Paleozoic and Mesozoic rocks of the Agua Prieta Region. 2) To study the structural relationship between Paleozoic rocks and Mesozoic rocks (lower Cretaceous Bisbee and upper Cretaceous Cabullona groups). 3) To visit two localities in the Agua Prieta Region whereby southwest directed thrusting in the Cabullona Basin~been proposed. h:L ITINERARY Saturday, March 28,1981. 07:30 Meet at main entrance of Motel Valle Grande. 08:00 Leave Hermosillo from Motel Valle Grande. 14:00 Arrive in the Agua Prieta Region. 14:30 Sierra de Anibacachi. On the SSW flank at the Sierra de Anibacachi we will examine the Bisbee Group. At this locality fossiliferous Morita and Mural are well exposed. 16:30 Cerro la Morita (Mina la Negrita). Good outcrops of Paleozoic rocks a~signed to the Mississippian Escabrosa through Pennsylvanian permian Naco limestone sequence are exposed on Cerra la Morita. Here we will focus on the structural relationship between Paleozoic and Lower Cretaceous rocks. 17:30 - Drive to the Hotel el Greco in Agua Prieta. Have dinner. 18:00 Spend the night and be ready for next morning ~t 07:00 hrs. 2 Sunday, March 29, 1981. 07:00 Leave from Hotel el Greco in Agua Prieta and drive to Sierra de Cabullona. Be certain you are carrying all of your luggage because we will not be returning to Hotel el Greco. 8:00 Sierra de Cabullona. Here, we will have a look at the stratigraphic sequence exposed on the SSW flank of Sierra de Cabullona. Paleozoic rocks are exposed in the lower and middle parts of the range. The Paleozoic/Upper Cretaceous contact at this locality is thought to be a thrust fault. 11 :00 Leaver Sierra de Cabullona. 12:00 Cross the USA/Mexican Border at Douglas, Arizona. 3 GEOLOGY OF THE AGUA PRIETA REGION by Francisco Longoria INTRODUCTION The region under consideration (Figure 1) lies within the Cabullona Basin of northeast Sonora. It is located south of the towns of Agua Prieta and Naco. The region covers an area of approximately 1850 square kilometers. Physiographically, the Agua Prieta region lies within the Basin and Range Province of northeast Sonora. The rocks exposed in the area are predominantly sedimentary rocks, although volcanic and plutonic igneous rocks and meta­ morphic rocks, are locally abundant (Figure 2). The Agua Prieta Area plays an important role in the paleotectonic interpreta­ tion of SE Arizona and NE Sonora. In general, the structural evolution of the region is poorly known and controversial. Pioneer field studies in the Agua Prieta region by Taliaferro (1933), Imlay (1939), and King (1934); as well as in adjacent areas by Ransome (1904) and Willson (1934) demonstrated that this region displays several structural and stratigraphic features that deserve more investigation. Recent studies by Rangin (1977) into late Cretaceous - early Tertiary tectonics of the Northern part of Sonora (Figure 2) suggested superimposed deformations and southwest directed thrusting in the Cabullona Basin. Paleontologic studies by Gamper and Longoria (1980) have shown that the Lower Cretaceous rocks of NE Sonora are predominantly open marine deposits, locally rich in planktonic foraminifera. Nevertheless, much remains to be learned about the tectonic evolution of the whole area. The main mountain ranges in the area are: 9) Sierra de San Jose, 2) Sierra de Anibacachi, 3) Cerro de la Morita, 4) Sierra del Coloso, 5) Sierra Lorna Blanca, 6) Cerro de Cabullona, and 7) Sierra de Magallanes (See Figure 2). Paleozoic, ~lesozoic and Tertiary rocks are well exposed in several of these mountain ranges and allow us an excellent view of the stratigraphic and structural relationships. During our field excursion we will concentrate our attention on the rock and structural sequences exposed at Sierra de Anibacachi, Cerro de la Morita and Cerro de Cabullona. STRATIGRAPHY (Figure 3) Precambrian Rocks Two main lithostratigraphic units are recognized in the Precambrian of the Agua Prieta Region: 1) The Pinal Schist, and 2) Las Mesteoas Granite. Only small patches of these two units are found within the area covered by the field trip. 4 The Pinal Schist crops out in the lower part of the Sierra de Las Mestenas, as well as in the SW flank of the Sierra de los Ajos. Both localities are a few kilometers south of Sierra de Cabullona. The Pinal Schist is also exposed on the SW flank of the Cerro de la Morita. The Pinal Schist contains a wide variety of lithologies: phyllites, quartz gneiss, muscovite-biotite-gneiss, sericite-chlorite-quartz-schist. The base of the Pinal Schist is not exposed in the Agua Prieta Region. The upper boundary is a nonconformity. The term Las Mestenas Granite refers to a lithostratigraphic unit of felsic composition containing xenoliths of the Pinal Schist. The Las Mestenas Granite i§ well exposed in the Sierra de Las Mestenas. A few outcrops of the Las Mestenas Granite can be found along the tributary creeks of the Rio Cabullona. The Las Mestenas Granite generally is strongly weathered and forms the lower parts of the hills near Sierra de Las Meste~as. At several localities a polymictic conglomerate can be found. In the Cerro de la Morita area along the creek beds a conglomerate mainly composed of angular to subangular cobbles and boulders of metamorphic rocks is common. Most of the clasts found in this conglomerate are derived from the Pinal Schist. The stratigraphic age of this conglomerate is unknown. Paleozoic Rocks In the Agua Prieta Region the Paleozoic rocks can be assigned to the following lithologic units: Bolsa Quartzite, Abrigo Limestone, Martin Limestone, Escabrosa-Naco limestone. Paleozoic rocks are present in isolated localities and their stratigraphic relationship is still poorly understood. However, Paleozoic rocks are known from several localities: Cerro de la Morita, and Cerro de Cabullona. South of the Agua Prieta Region Paleozoic rocks are present at Cerro de Las Mestenas and Sierra de los Ajos. Bolsa Quartzite The oldest Paleozoic unit known in the region of Agua Prieta is the Bolsa Quartzite. This unit is mainly composed of pure quartz sandstone and some conglomeratic sandstone. Locally the sandstones of this unit are metamorphosed and become metaquartzite. The lower part of the Bolsa Quartzite is characterized by the presence of several thin layers of conglomeratic sandstone. These layers vary from 20 cm up to 1.0 m in thickness. The sandstone often displays cross-stratification. In general, the rocks of the Bolsa Formationweatnerpink to white and give the outcrops their typical pinkish appearence. The sand grains are well­ rounded to subrounded and cemented by quartz, chlorite or sericite. The quartz grains vary in diameter from 0.5 to almost 2.0 cm. Several conglomeratic sandstone layers can be observed in the middle and upper parts of the Bolsa Quartzite. These conglomeratic layers are formed of 5 subrounded quartz grains which vary from 2.0 to 4.0 cm in diameter. The conglomeratic layers display a somewhat darker pink~reddish color. The total thickness of the Bolsa formation in the area is unknown. The Bolsa Quartzite is found in the SW flank of the Sierra de Cabullona where it reaches a maximum thickness of about 18 meters. Small outcrops of this unit can also be observed near the Cerro de la Morita. The Bolsa Quartzite overlies the Las Mestenas Granite. This contact is only observed at isolated outcrops in the Sierra de Las Mestenas. At this locality a basal conglomerate can be observed. The upper limit of this unit is placed at the base of the first appearance of limestone beds assigned to the Abrigo Formation. Abrigo Formation The Abrigo Formation is composed of an intercalation of limestone and marls. In the Agua Prieta region the Abrigo Formation is frequently covered by alluvium deposits. It always forms low relief. The limestones of the Abrigo are thin­ bedded and gray in color. They are consistently interbedded with marls and calcareous shales. The lower contact, i.e., the Bolsa-Abrigo formational boundary, is difficult to see in the field because it is always covered by soil.