Saturday, 15 July 2006

Gypsic, Natric, and Barite Paleosols in the Carroza Formation, La Popa Basin, Mexico, and Potential Applications of Pedogenesis to Salt Tectonics and Petroleum Exploration.

Brenda J. Buck, Univ of Nevada Las Vegas, Geoscience Dept, 4505 Maryland Pkwy MS4010, Las Vegas, NV 89154, Amy Brock, Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Parkway Box 4010, Las Vegas, NV 89154, and Timothy F. Lawton, New Mexico State Univ, P.O. Box 30001, Las Cruces, NM 88003.

Passive diapirism, whereby a salt body grows in response to the load of the surrounding sedimentary section on the parent salt bed and remains at or near the surface because diapiric rise rates roughly equal sediment-aggradation rates, has long been modeled to interpret geophysical data from salt-related petroleum provinces. Currently, direct evidence in support of passive diapirism only includes material eroded directly from the diapir and deposited in surrounding strata. The La Popa basin, 50 km northwest of Monterrey, Nuevo Leon, Mexico, contains well-exposed salt diapers and a 25 km long salt wall that is welded (the salt is completely evacuated) along half its length. This weld/salt wall is termed the La Popa salt weld. The youngest formation in the La Popa basin is the Paleogene Carroza Formation, which constitutes continental strata formed under arid conditions in an asymmetrical, synclinal salt-withdrawal basin on the down-thrown block of the La Popa salt weld. Seven sections in the Carroza Formation were measured and petrographic and SEM/EDS analyses of 72 thin sections were performed. The Carroza Formation is composed of interbedded red (less commonly green), shale (47%), siltstone (4%), and sandstone (48%) beds that are interpreted to represent an arid, low-gradient, ephemeral fluvial system (channel, crevasse splay, levee) with associated overbank deposits (floodplain, pond). Paleosols in the Carroza Formation are ubiquitous. Paleosols are most commonly compound (separated by C horizons), but cumulative paleosols (overthickened paleosols resulting from pedogenesis matching the rate of deposition; e.g., Kraus and Aslan, 1999) are also present. Paleosols vary between 40-190 cm thick, and can be grouped into four major paleosol types and four grades of maturity. The five major paleosol types are defined based upon the occurrence of: (1) natric (+/- salic) horizons, (2) gypsic horizons, (3) barite horizons, or (4) burrowed horizons lacking other soil development indicators. Often paleosols have a combination of one or more of these four types of horizons. The four grades of paleosol maturity are based on soil structure and the morphology of the salt minerals present (based on Gile et al., 1966; Reheis 1987; and Buck and Van Hoesen, 2002). Natric horizons are identified on the basis of strong columnar structure development. Columns range from fine to coarse (1 to 10 cm diameter) with domed tops. Commonly, these horizons contain halite casts (indicating salic horizon development) and some horizons also contain stage I gypsum snowballs (e.g., Buck and Van Hoesen, 2002). This study documents the second occurrence of pre-Pleistocene paleosols containing natric horizons; the other occurrence is in Permian paleosols of Kansas (McCahon and Miller 1997). Gypsic horizons are the most common feature of all paleosols in the Carroza Formation. Gypsic horizons are identified on the basis of stage I snowballs and stage II gypsum nodules (e.g., Buck and Van Hoesen, 2002). The snowballs range in diameter from 0.5-3.0 mm and locally coalesce to form composite snowballs with diameters of up to 8 mm. Stage II nodules are found up to 3 cm in diameter. The structure in gypsic horizons varies from moderate to strong, fine to coarse, subangular blocky, and when a natric horizon is also present, the structure is moderate to strong, medium to coarsely columnar. Gypsic horizons commonly underlie natric horizons, but also occur singly or in combination with natric, salic, or barite horizons. This study is the first to document the gypsum snowball morphology in pre-Pleistocene paleosols. Paleosols that contain horizons with barite demonstrate both stage I snowballs (1-3 mm) and stage II nodules (0.5-7 cm) of bladed crystals arranged in rosettes. This is the first study to describe barite snowballs in pre-Pleistocene palesols. The source of the halite, gypsum, and barite in Carroza paleosols was Jurassic evaporite exposed in the La Popa diapir during Carroza deposition. Paleosols located near the diapir have a greater degree of maturity and suggest either increased rates of salt input through fluvial or eolian processes or represent older, more stable geomorphic surfaces adjacent to the topographically higher diapir. Because these paleosols occur throughout the Carroza Formation, they indicate that the diapir was continuously exposed during Carroza deposition, thus confirming passive diapirism, perhaps enhanced by shortening during the Laramide orogeny (active diapirism). This study demonstrates a new method of using paleosols as direct evidence for passive diapirism in the interpretation of salt tectonics and offers potential for utilizing paleosols in petroleum exploration and production. References: Buck, Brenda J. and Van Hoesen, John, 2002, Snowball Morphology and SEM Analysis of Pedogenic Gypsum, southern New Mexico, USA, Journal of Arid Environments, v. 51, p. 469-487 Gile, L.H., F.F. Peterson, and R.B. Grossman. 1966. Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Sci. 101:347-360.

Kraus, M., and Aslan, A., 1999, Palaeosol sequences in floodplain environments: a hierarchical approach, in Thiry, M. and Simon-Coincon, R., eds., Palaeoweathering, palaeosurfaces and related continental deposits Special Publication of the International Association of Sedimentologists, vol.27, p. 303-321.

McCahon, T.J., and Miller, K.B., 1997, Climatic significance of natric horizons in Permian (Asselian) palaeosols of north-central Kansas, USA: Sedimentology, v. 44, p. 113-125.

Reheis, M.C. 1987. Gypsic soils on the Kane alluvial fans, Big Horn Country, Wyoming. USGS Bull. Report B-1590-C:C1-C39.

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