Provenance of Paleocene-Eocene Wilcox Group Sediments in Texas: The Evidence from Detrital Zircons

Andrew P. Hutto, Thomas E. Yancey, and Brent V. Miller

Department of Geology and Geophysics, Texas A&M University,

MS 3115, College Station, Texas 77843-3115

EXTENDED ABSTRACT

The late Paleocene and early Eocene (58 Ma – 52 Ma) Wilcox Group, a major stratigraphic unit of the Paleogene in the northwestern Gulf of Mexico geologic province, was deposited in response to tectonic uplift and erosion in the continental interior. Sediment was transported to the continental margin and deposited in a large clastic wedge in central and eastern Texas called the Rockdale delta system that was fed by the Mt. Pleasant fluvial channel system (Fisher and McGowen, 1968). The Wilcox Group (and overlying Carrizo Formation, a unit often included within the Wilcox Group) is a major producer of oil and gas in the subsurface of the Gulf of Mexico and contains large lignite coal reserves in the upper coastal plain area (Kaiser et al., 1986). Recent discovery of large amounts of oil and gas contained in Paleocene-Eocene sands in deep waters of the northern Gulf of Mexico (Meyer et al., 2005) has stimulated interest in further documentation of the depositional history of Wilcox Group strata as a means of determining the source and pathway of sediment transport into deep water environments. The possibility of major drawdown of water within the Gulf of Mexico due to basin isolation (Berman and Rosenfeld, 2007; Rosenfeld and Pindell, 2003) and the presence of Wilcox aged large submarine canyons (Galloway, 2007; McDonnell et al., 2008) adds additional complexity to the reconstruction of depositional controls and patterns for Wilcox Group deposits. Sediments of the Wilcox Group were also deposited during an interval of time spanning the PETM (Paleocene-Eocene Thermal Maximum), a time of rapid global temperature rise. Temperatures abruptly increased as much as six degrees Celsius (11 degrees Fahrenheit), as determined by stable isotope determinations of organic remains and soil precipitates (John et al., 2008; Storey et al., 2007). The PETM is recognized primarily by an associated carbon and oxygen isotope excursion and is dated as lasting 105,000 years with a recovery phase lasting 126,000 years (Giusberti et al., 2007). In the Gulf Coast region this interval lies within the Platycarya abundance zone (Elsik and Crabaugh, 2001) and corresponds with the Apectodinium flood zone (Crouch et al., 2001).

There are longstanding unanswered questions about the source areas, or provenance, of the sediments that compose the Wilcox Group. Traditional interpretations cited western Laramide aged uplifts as a major source of Wilcox sediments, a view still held by many workers. Doubt about the importance of Laramide uplifts in providing a major source of sediment to the Wilcox Group in the Texas sector of the Gulf province was raised in publications describing the petrography of Paleogene sands (Todd and Folk, 1957; Harris, 1962; McCarley, 1981). These publications reported a dominance of minerals (both heavy and light fractions) and clasts indicative of metamorphic terranes and only small amounts of materials indicative of volcanic sources. Based on these data, the early Paleogene sediments were interpreted to be derived from sources dominated by metamorphic rocks, indicating an origin in the Ouachita-Appalachian uplift trend and drainage systems headed in the northern sector. A study of the morphology of zircons in sediments (Callender and Folk, 1958) presented evidence of a change in major source area to a western source terrane occurring during the middle Eocene, when large scale and voluminous silicic volcanism began along the western margin of the continent, extending from central Mexico northward to the central Rocky Mountains (Oldow et al., 1989). Methods of determination of provenance of detrital sediments have expanded with the utilization of U-Pb age dating of populations of detrital zircons, a procedure that provides a spectrum of ages reflecting times of igneous activity that led to crystallization of zircons. Using laser ablation methods for obtaining for analysis and dating decay products from the growth center of the crystal that record the time of origination of the crystal, the age spectrum of detrital zircons can be compared with the ages of source rocks within a geographic area (Dickinson and Gehrels, 2009). A similarity in ages of detrital zircons to that of the ages of source rocks can be used as a strong positive indicator of source terrane for the sediments.

Detrital zircon dating was performed on sediment samples from two geographic areas of Wilcox Group fluvial deposits. Two samples were taken from one large fluvial channel fill deposit within the lower part of the Calvert Bluff Formation, exposed in the Big Brown lignite mine near Fairfield, Texas (31°50.2’N; 96°03.5’W). One sample was obtained from a small fluvial channel fill deposit present at the base of the Carrizo Formation near Bastrop, Texas (30°04.4’N; 97°16.9’W). The lower part of the Calvert Bluff contains the major lignite deposits of Texas and is late Paleocene in age (O’Keefe et al., 2005), whereas the Carrizo Formation is early Eocene in age (Elsik and Crabaugh, 2001), although it is not known if the channel fill interval lies within the earliest part of the Eocene or at a higher level. The fluvial sediments at Bastrop are deposits of a channel cut during subaerial exposure and occur above a major sequence boundary (Ayers and Lewis, 1985) that might be associated with the hypothesized drawdown of water in the Gulf of Mexico (Rosenfeld and Pindell, 2003).

The age distribution of zircon ages in the sampled sediments is shown in the form of age histogram plots (Fig. 1) and the accuracy of individual analyses is indicated in the concordia plots of the data points (Fig. 2). Superimposed on the histogams of calculated age dates are probability density plots that emphasize concentrations of age determinations with low margins of error and greater accuracy. The data in the three samples is similar, suggesting that sediments deposited in both areas were derived from the same or similar sources. All three samples have a large dominant concentration of age determinations in the interval of 50-200 Ma, containing 40-50% of the age determinations for each sample.. This interval has dual peaks, with the dominant peak in the narrow age interval of 50-100 Ma and a smaller mode in the 150-200 Ma age interval. The 50-100 Ma peak contains 20-30% of the total age determinations in each sample and is by far the dominant peak on the probability density plot. The 150-200 Ma peak shows as the secondary peak on probability density plots, although not on the histogram plots. There is a strong secondary mode of age determinations spanning the interval from 1300-1700 Ma that contains 30-40% of the data points. This concentration has weakly defined peaks at 1300-1400 Ma and 1500-1700 Ma, with the larger peak occurring at 1500-1700 Ma. The remainder of the data is scattered, with small modes in the intervals 500-600 Ma, 1000-1200 Ma, and 2400-2700 Ma. There also are notable gaps in distribution, with an absence or very few ages in the 600-900 Ma interval and the 2000-2400 Ma interval.

Except for the 500-600 Ma age grouping, the major and minor modes of age determinations correspond with well known tectonic events in North America (Oldow et al., 1989). The 50-200 Ma interval corresponds with subduction and associated igneous activity and deformation of the western margin of North America, including the late Mesozoic deformations known as the Laramide event (Oldow et al., 1989). The secondary mode at 1500-1700 Ma is coincident with igneous activity in the south and southwestern parts of North America that produced the Yavapai-Matzatzal geologic province (Hoffman, 1989). The oldest mode at 2400-2700 Ma is coincident with Trans-Hudson and related tectonism that created the stable interior craton of North America. What is conspicuously absent is a mode of age determinations from rocks of Grenville age, ranging from 900-1200 Ma (Hoffman, 1989; Mosher et al, 2008). There is no obvious source for the small mode of age determinations at 500-600 Ma.

Comparison of the age distributions of detrital zircon grains with tectonic events indicates a large majority of the grains are derived from a western source. The concentrations of ages, containing 70-80% of the data, correspond with granite-rhyolite igneous activity of the Yavapai-Matzatzal geologic province (Hoffman, 1989) and the Laramide tectonism of the central and eastern Cordillera (English and Johnston, 2004). This is supported by a scarcity of dates derived from Grenville age igneous activity or from Ouachita-Alleghenian igneous activity, which are the major tectonic events producing the basement rocks of the eastern part of North America underlying the Ouachita and Appalachian mountain trends. The youngest zircon age determinations in the samples are about 55 Ma, which is close to the estimated age of deposition of the sedimentary deposits sampled. This is within the age range of Laramide tectonism at 80-55 Ma (English and Johnston, 2004), suggesting that the zircons crystallized, were eroded and then transported to the site of deposition within a very short time and are likely to have been derived from extrusive volcanics. The Laramide uplifts occur in an area with Precambrian basement rocks of the 1600-1800 Ma Yavapai-Mazatzal geologic province, providing a source for volcanic recycling of the older large population of zircons in the samples as well as regular erosional sculpting of uplifted basement rock. The Laramide tectonic event produced large uplifts and highlands in the Rocky Mountains area that positioned the continental divide in the eastern Cordillera. Therefore, drainage systems carrying sediment to the Texas segment of the Gulf of Mexico had drainage basins that headed in the general area of the Rocky Mountains and the Laramide block uplifts. It also implies that the input from other geologic provinces to the north and to the east was minimal during deposition of the Wilcox Group strata.

ACKNOWLEDGMENTS

Support for this study was provided by Devon Energy Corp., as part of its program of support for students at Texas A&M University, and their support is greatly appreciated.

REFERENCES cited

Ayers, W. B., Jr., and A. H. Lewis, 1985, The Wilcox Group and Carrizo Sand (Paleogene) in east-central Texas: Depositional systems and deep-water lignite: Texas Bureau of Economic Geology, Geological and Hydrological Folio Special Report, Austin, 19 p.

Berman, A. E., and J. H. Rosenfeld, 2007, A new depositional model for the deep-water Gulf of Mexico Wilcox equivalent Whopper Sand—Changing the paradigm, in L. Kennan, J. Pindell, and N. C. Rosen, eds., The Paleogene of the Gulf of Mexico and Caribbean basins: Proceedings of the 27th Annual Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists Foundation Bob. F. Perkins Research Conference, p. 284-297.

Callender, D. L., and R. L. Folk, 1958, Idiomorphic zircon, key to volcanism in the lower Tertiary sands of central Texas: American Journal of Science, v. 256, p. 257-269.

Crouch, E. M., C. Heilmann-Clausen, H. Brinkhuis, H. E. G. Morgans, K. M. Rogers, H. Egger, and B. Schmitz, 2001, Global dinoflagellate event associated with the late Paleocene thermal maximum: Geology, v. 29, p. 315-318.

Dickinson, W. R., and G. E. Gehrels, 2009, U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment: Geological Society of America Bulletin, v. 121, p. 408-433.

Elsik, W. C., and J. P. Crabaugh, 2001, Palynostratigraphy of the upper Paleocene and lower Eocene Wilcox Group in the northwestern Gulf of Mexico Basin, in D. K. Goodman and R. T. Clarke, eds., Proceedings of the IX International Palynological Congress, Houston, Texas: American Association of Palynologists Foundation, Baton Rouge, Louisiana, p. 233-237.

English, J. M., and S. T. Johnston, 2004, The Laramide orogeny: What were the driving forces?: International Geology Review, v. 46, p. 833-838.

Fisher, W. L., and J. H. McGowen, 1968, Depositional systems in the Wilcox Group of Texas and their relationship to occurrence of oil and gas: American Association of Petroleum Geologists Bulletin, v. 53, p. 30-54.

Galloway, W. E., 2007, Wilcox submarine canyons: Distribution, attributes, origins, and relationship to basinal sands, in L. Kennan, J. Pindell, and N. C. Rosen, eds., The Paleogene of the Gulf of Mexico and Caribbean basins: Proceedings of the 27th Annual Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists Foundation Bob F. Perkins Research Conference, Houston, Texas, p. 271-272.

Giusberti, L., D. Rio, C. Agnini, J. Backman, E. Fornaciari, F. Tateo, and M. Oddone, 2007, Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pro-Alps: Geological Society of America Bulletin, v. 119, p. 391-412.

Harris, J. R., 1962, Petrology of the Eocene Sabinetown-Carrizo contact, Bastrop County, Texas: Journal of Sedimentary Petrology, v. 32, p. 263-283.

Hoffman, P. F., 1989, Precambrian geology and tectonic history of North America, in A. W. Bally and A. R. Palmer, eds., The geology of North America; an overview: Geological Society of America, Boulder, Colorado, p. 447-512.

John, C. M., S. M. Bohaty, J. C. Zachos, A. Sluijs, S. Gibbs, H. Brinkhuis, and T. J. Bralower, 2008, North American continental margin records of the Paleocene-Eocene thermal maximum: Implications for global carbon and hydrological cycling: Paleoceanography, v. 23, 20 p.

Kaiser, W. R., M. L. Ambrose, W. B. Ayers, Jr., P. E. Blanchard, G. F. Collins, G. E. Fogg, D. L. Gower, C. L. Ho, C. S. Holland, M. L. W. Jackson, C. M. Jones, A. H. Lewis, G. L. Macpherson, C. A. Mahan, A. H. Mullin, D. A. Prouty, S. J. Tewalt, and S. W. Tweedy, 1986, Geology and ground-water hydrology of deep-basin lignite in the Wilcox Group of east Texas: Texas Bureau of Economic Geology Special Report, Austin, 182 p.

McCarley, A. B., 1981, Metamorphic terrane favored over Rocky Mountains as source of Claiborne Group, Eocene, Texas Coastal Plain: Journal of Sedimentary Petrology, v. 51, p. 1267-1276.

McDonnell, A., R. G. Loucks, and W. E. Galloway, 2008, Paleocene to Eocene deep-water slope canyons, western Gulf of Mexico: further insights for the provenance of deep-water offshore Wilcox Group plays: American Association of Petroleum Geologists Bulletin, v. 92, p. 1169-1189.

Meyer, D., L. Zarra, D. Rains, R. Meltz, and T. Hall, 2005, Emergence of the Lower Tertiary Wilcox trend in the deepwater Gulf of Mexico: World Oil, May 2005 issue, p. 72-77.

Mosher, S., B. Davis, and S. W. Grimes, 2008, Grenville tectonics in Texas: A diachronous orogeny: Geological Society of America Abstracts with Programs, v. 40, no. 6, p. 289.

O’Keefe, J. M. K., R. H. Sancay, A. L. Raymond, and T. E. Yancey, 2005, A Comparison of late Paleocene and late Eocene lignite depositional systems using palynology, Upper Wilcox and Upper Jackson groups, east-central Texas, in P. D. Warwick, ed., Coal systems analysis: Geological Society of America Special Paper 387, Boulder, Colorado, p. 59-71

Oldow, J. S., A. W. Bally, H. G. Ave Lallemant and W. P. Leeman, 1989, Phanerozoic evolution of the North American Cordillera; United States and Canada, in A. W. Bally and A.R. Palmer, eds., The geology of North America; an overview: Geological Society of America, Boulder, Colorado, p. 139-232.

Rosenfeld, J., and J. Pindell, 2003, Early Paleogene isolation of the Gulf of Mexico from the world’s oceans? Implications for hydrocarbon exploration and eustasy, in C. Bartolini, R. T. Buffler, and J. Blickwede, eds., The Circum-Gulf of Mexico and the Caribbean: Hydrocarbon habitats, basin formation, and plate tectonics: American Association of Petroleum Geologists Memoir 79, Tulsa, Oklahoma, p. 89-103.

Storey, M., R. A. Duncan, and C. C. Swisher, 2007, Paleocene-Eocene thermal maximum and the opening of the northeast Atlantic: Science, v. 316, p. 587-589.

Todd, T. W., and R. L. Folk, 1957, Basal Claiborne of Texas, record of Appalachian tectonism during Eocene: American Association of Petroleum Geologists Bulletin, v. 41, p. 2545-2566.

Hutto, A. P., T. E. Yancey, and B. V. Miller, 2009, Provenance of Paleocene-Eocene Wilcox Group sediments in Texas: The evidence from detrital zircons: Gulf Coast Association of Geological Societies Transactions, v. 59, p. 357-362.