New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/31 Precambrian geology of the Van Horn area, Texas Donald M. Davidson Jr., 1980, pp. 151-154 in: Trans Pecos Region (West Texas), Dickerson, P. W.; Hoffer, J. M.; Callender, J. F.; [eds.], New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, 308 p. This is one of many related papers that were included in the 1980 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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Further detailed structural analysis appears warranted. result of block faulting of a number of adjacent, but spatially Pegmatites and various quartz veins appear to have intruded disconnected mountain ranges (fig. 1). The area has been referred these units both before and after regional metamorphism (King to as the Van Horn Dome (King and Flawn, 1953) but can be con- and Flawn, 1953). sidered domical only in the sense that it has tended to be a positive structural element during Phanerozoic time. Allamoore Formation This domical physiographic structure consists of two distinctive The oldest unit exposed in the northern terrain is the Allamoore geological terrains of late Precambrian (Proterozoic) age. The Formation which consists of 800 m of volcanics, talcose phyllite southern third of the area is composed of a thick (6000-7000 m) se- and carbonate rocks. These units have been tilted and locally com- quence of older, low- to medium-grade metamorphic rocks, while plexly folded and faulted. Determination of the stratigraphic se- the northern two-thirds contains a thinner (3000 m) assemblage of quence is therefore difficult, and an estimate of total thickness is younger, relatively unmetamorphosed sedimentary and volcanic nearly impossible. units. Volcanic rocks constitute from one-quarter to one-half of the A major structural discontinuity, the Streeruwitz overthrust, total volume of the Allamoore Formation and are usually interbed- forms the boundary between these two terrains, and movement ded with limestone. These massive, mafic volcanic rocks appar- along this surface presumably placed the older, southern meta- ently formed as subaerial or subaqueous lava flows while the morphic rocks over portions of the younger lithologies, locally thicker bodies, which occur less commonly, may represent folding them, at 950 to 1000 m.y. (Denison and Hetherington, hypabyssal intrusives. Additional volcanic units are of either 1969). pyroclastic or sedimentary origin and may include pebbly sand- Erosion and incisement of the area occurred prior to the deposi- stones and interbedded volcanic conglomerates (King, 1965). tion of a late Precambrian(?) clastic unit some 300 m thick, the Van Phyllite occurs interbedded with or spatially associated with Horn Sandstone. The region underwent tilting prior to deposition volcanic rocks; this suggests that the phyllite may have a volcanic of younger Paleozoic and Mesozoic sedimentary units which, origin. Bedding, as indicated by alternation of gray, black (appar- together with the older Precambrian rocks, have been offset by ently graphitic) and calcareous varieties, is extremely contorted faults of Tertiary age. and is crossed by well-developed slaty cleavage which parallels the axial planes of folds. STRATIGRAPHY Since 1952 talc has been mined from this phyllite unit; talc con- Carrizo Mountain Group tent within the unit varies locally from 10 to 80 percent (see Ed- The Carrizo Mountain Group constitutes the oldest assemblage wards, this guidebook). The mineral may have formed as a result of Precambrian rocks within the study area (fig. 2). These Pro- of chemical alteration of an original fine-grained, magnesium- terozoic units occur in the southern part of the area under in- bearing tuff, as adjacent carbonate units contain little magnesium vestigation and have been dated at 890 to 1090 m.y. (Wasserberg (King and Flawn, 1953). and others, 1962). The upper limestone beds are characterized by interbedded The sequence consists of meta-arkose, metaquartzite, schist, seams of chert, which parallel bedding and are spaced over inter- phyllite and marble units which have been intruded by rhyolite vals of a few centimeters. The chert is either primary or diagenetic and diorite. These latter intrusive rocks have subsequently been and antedates folding and faulting, as chert seams are sliced and metamorphosed to metarhyolite and amphibolite, respectively. broken where the rocks are strongly deformed. The limestone is The sequence is at least 6300 m thick and displays moderate thin-bedded, compact, evenly laminated, generally dolomitic and homogeneity within lithologic units, yet exhibits variable intensity, commonly contains a wavy structure similar to that of stroma- as well as type, of metamorphism. Regional metamorphic facies toporoids. vary northward from amphibolite to greenschist. Dynamic (cata- clastic) metamorphism appears superimposed upon both regional Hazel Formation and thermal metamorphic events, producing retrograde and The Hazel Formation unconformably overlies the Allamoore For- cataclastic assemblages in the vicinity of the Streeruwitz overthrust mation and consists of more than 1700 m of interbedded con- zone (King and Flawn, 1953; Denison and Hetherington, 1969). glomerate, arkosic sandstone and limestone. The basal portion Foliation is well developed in these units and trends contains a thick, coarse conglomerate composed almost wholly of predominantly N40-50°E with a southeastward dip. Folded folia- angular rock fragments derived from the Allamoore. Near its top tion patterns have been noted in the high-grade rocks to the south the conglomerate is interbedded with and eventually succeeded (King and Flawn, 1953), and interference fold patterns were by a thick section of fine-grained, silty, red sandstone interbedded observed in the lower grade units. According to King and Flawn with well laminated, algae-bearing carbonate rocks (Reid, 1974). (1953), lineations are developed only in metarhyolite units from Reid (1974) stated that sediment of the Hazel Formation was the northwest part of the area where, they plunge southward at 30 deposited within an alluvial fan system, the sediment for which to 60 °; however, recent field investigations indicate that lineation was derived from a granitic highland source area to the south. CJ al_ 0 0 0 - t 0 0 0 C\0 s• - 0 (1, II 0 ‘ s q.‘ )-• _ss0. r l r o .4 • 5, -,„ ,.,01,,„-.7 r xl CD c-) -1 71 ,2 .A t z > cm cn cn C., > > 0 0 Ne 7 < --r- z 0 -4) " s CD (1) 0 n Fii > , • . • .-6 ;, () - 3> 7:, 17.1 a- CI 0 ,. 0: • ,\-- - .6, X K 4, ,- ,,, P 7 ..t" ("lilt .- ..."-.. • -< I)) . • 0 , f,) • " r)" 0 .... .. , 0 ■ 0) p 0....4 .4-6 ,s1 ,--- /--, 4 -g1 , Nso ‘,ss 4 c ..- .4 -, . m - S-1•--- >, : .--y _-= ^.., 0 -. K . = N - 4, li z , „ t I r ). I . .:- 7 , - o ^ SIERRA DIABLO 0 LJ- 0 PRECAMBRIAN GEOLOGY OF THE VAN HORN AREA 153 BLISS SANDSTONE brown thin-bedded noncalcareous sandstone, with Scofithus tubes; overlain by calcareous sandstone; 105-125 f t. t hick UNCONFORMITY VAN HORN SANDSTONE red arkosic sandstone and conglomerate; crossbedded and unfossiliferous; 1200 + f t. thick UNCONFORMITY HAZEL FORMATION : red fine- grained sandstone overlying and interbedded with conglomerate principally formed of fragments