New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/54 Pedogenic features of the Chinle Group, Four Corners region: Evidence of Late Triassic aridification Lawrence H. Tanner, 2003, pp. 269-280 in: Geology of the Zuni Plateau, Lucas, Spencer G.; Semken, Steven C.; Berglof, William; Ulmer-Scholle, Dana; [eds.], New Mexico Geological Society 54th Annual Fall Field Conference Guidebook, 425 p. This is one of many related papers that were included in the 2003 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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No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geological Society Guidebook, 54th Field Conference, Geology of the Zuni Plateau, 2003, p. 269-280. 269 PEDOGENIC FEATURES OF THE CHINLE GROUP, FOUR CORNERS REGION: EVIDENCE OF LATE TRIASSIC ARIDIFICATION LAWRENCE H. TANNER Department of Geography and Geosciences, Bloomsburg University, Bloomsburg, PA 17815 ABSTRACT.—Paleosols and pedogenic features in formations of the Upper Triassic Chinle Group preserve a record of changing paleoclimate in the Four Corners region. The Carnian mottled strata and Shinarump formations contain strongly gleyed and bioturbated paleosols with a prismatic fabric that formed under the infl uence of a subhumid but (likely) strongly seasonal climate. Thick argillic horizons with epipedons in the overlying Cameron Formation and the Blue Mesa Member of the Petrifi ed Forest Formation, and the prominence of vertic features and calcrete horizons in the Blue Mesa and Painted Desert members suggest that the climate became less humid, but remained strongly seasonal, from late Carnian through early Norian time. Mature calcrete profi les in the middle Norian Owl Rock Formation document continued aridifi ca- tion leading to semiarid conditions. Strata of the Rhaetian Rock Point Formation contain few paleosols other than immature calcretes, but the abundance of eolian and playa facies in this formation suggest increasing aridity through the end of the Triassic. Interformational variations in the carbon-isotope composition of pedogenic carbonate are consistent with this trend of aridifi cation at a time of elevated atmospheric CO2. Although not world-wide, aridifi cation was widespread across Pangea, and possibly resulted from the increasing strength of monsoonal fl ow and the weakening of zonal circulation, particularly at lower latitudes, as the continent drifted northward. INTRODUCTION source areas for these sediments included the Mogollon high- lands, located approximately 500 km to the south and southwest, The spatial and temporal resolution of paleoclimate models the Uncompahgre highlands located 200 to 300 km to the east has improved dramatically since the interpretations of Robinson and northeast (Blakey and Gubitosa, 1983; Marzolf, 1994), and (1973), in part through the increasing use of paleosols and pedo- more distant upland areas in Texas (McGowan et al., 1983; Riggs genic features as paleoclimate archives. Indeed, the morphology et al., 1996). Across most of the Colorado Plateau, lowermost and composition of paleosols have long been used to ascertain past Chinle strata were deposited unconformably on Moenkopi Group climate conditions (see Kraus, 1999 for a review). Although com- and older strata (the Tr-3 unconformity) following an interval of paction, erosion, and diagenetic alteration of paleosols may render lowered base-level and incision. The Shinarump and Cameron precise interpretation of the conditions of paleosol formation formations, and Blue Mesa Member of the Petrifi ed Forest For- problematic, many climate-dependent pedogenic features, such as mation, and lateral equivalents, were deposited during late Car- calcrete horizons, gleying, and translocation of clay and sesquiox- nian on this incised surface (Lucas, 1993; Lucas et al., 1997). The ides, have high preservation potential, rendering paleosols a valu- Tr-4 unconformity separates the Sonsela Member of the Petrifi ed able record of paleoclimate. The geochemistry of paleosols, in Forest Formation and the laterally equivalent Moss Back Forma- particular, the stable isotope composition of calcareous nodules in tion from the underlying Blue Mesa strata (Heckert and Lucas, calcretes, has been shown to be useful for the analysis of paleocli- 1996, 2002; Lucas et al., 1997). The Owl Rock Formation was mate and atmospheric composition (Cerling, 1991, 1999; Driese deposited conformably to locally unconformably on the Painted et al., 1992; Driese and Mora, 1993; Caudill et al., 1996; Liu et al., Desert Member of the Petrifi ed Forest Formation during the δ13 1996). The estimation of atmospheric paleo-p(CO2) using C of middle Norian time. The Rhaetian Rock Point Formation uncon- pedogenic carbonate and the diffusion-reaction model developed formably overlies the Owl Rock Formation (Tr-5 unconformity) by Cerling (1991) is now widely considered reliable because and grades vertically to the mainly Hettangian Wingate Forma- values of δ13C do not appear susceptible to diagenetic modifi ca- tion. Previous studies have established the details of the stratigra- tion (Cerling, 1991; Driese and Mora, 1993; Mora et al., 1996), phy and architectural elements of these formations (Stewart et al., and calculated paleo-p(CO2) levels compare favorably with values 1972; Blakey and Gubitosa, 1983; Kraus and Middleton, 1987; from geochemical modeling (Berner, 1993). Dubiel, 1987; Lucas, 1993; Lucas et al., 1997; Tanner, 2000). Geologic Setting and Stratigraphy PEDOGENIC FEATURES OF CHINLE GROUP STRATA The Four Corners region was located at near-equatorial lati- Paleosols have long been recognized as prominent components tudes (less than 10o N) during Late Triassic time (Scotese, 1994; of the Chinle Group formations, and general features of these Molina-Garza et al., 1995; Kent and Olsen, 1997). Late Carnian paleosols have been cited in previous interpretations of Triassic to Rhaetian strata of the Chinle Group, now exposed across much paleoclimate (Dubiel, 1987; Dubiel et al., 1991; Parrish, 1993). of the Colorado Plateau (Fig. 1), were deposited in a continental Yet few studies have examined the pedogenic features and asso- back-arc basin that extended from southwestern Texas to north- ciated climatic implications in substantial detail. Most existing ern Wyoming (Lucas, 1993; Lucas et al., 1997; Lucas, 1999). descriptions have focused on individual formations (Blodgett, Deposition within the Chinle basin was controlled primarily 1988; Lucas and Anderson, 1993; Tanner, 2000; Therrien and by streams fl owing northwest across a broad alluvial plain. The Fastovsky, 2000), or are cursory summaries of the features of the 270 TANNER Field Example: Mottled Strata Near Fort Wingate, New Mexico, over 20 m of strongly mottled mudstone and sandstone beds rest unconformably on strata of the Moenkopi Group (see Stop 1, Day 2 in Roadlogs, this volume; also Lucas and Hayden, 1989). The stratigraphic position of these strata below the coarse sandstones and conglomerates of the Shinarump Formation indicates that they belong to the mottled strata. Pedogenic modifi cation, consisting of some combination of gleying (color mottling), desiccation, or root traces or casts, is evident in all of the strata in the sequence. Mudstone beds are up to 1.5 thick and typically display a crudely prismatic fabric and medium bluish gray (5B 5/1) to light yellowish gray (5Y 8/1) gley mottling in a host that varies from dark reddish brown (10R 3/4) to dark yellow orange (10YR 6/6) (Fig. 2A). Locally, the gley mottling follows a coarse reticulate pattern. Through most of the sequence, the beds are penetrated by near-vertical sandstone cylinders, some up to 1.5 m long. These have a nearly straight