Conodont-Based Thermal Maturation of Paleozoic Rocks in Arizona

The American Association of Petroleum Oeologisis Bulletin V. 68. No.'J (September 1984), P. 1101-1106. 4 Figs.

Conodont-Based Thermal Maturation of Paleozoic Rocks in Arizona^ BRUCE R. WARDLAW and ANITA G. HARRIS'

ABSTRACT

PURPOSE

Color changes in conodonts are established thermal indices that have been related to the thermal window tor hydrocarbon generation and preservation and that also have value for targeting mineralization potential. Conodont color-alteration values from 141 localities in Arizona appear to characterize the thermal maturation of the major Paleozoic outcrop areas in the state. Paleozoic rocks in the Colorado Plateau, in part of the belt of faulted plateau-like rocks bordering the plateau, and in a small area (from 31°20' to 31°44' N and from 110°00' to 110° 40'W) in southeast Arizona have thermal potential for oil. Paleozoic rocks in the same areas as well as in the Pedregosa basin (southeasternmost Arizona) have thermal potential for gas. Of these areas, southeast Arizona appears to have the best hydrocarbon potential because of the association of possible source beds and favorable thermal-maturation levels.

Generalized CAI maps for the Basin and Range province in Idaho, Utah, Nevada, and easternmost Cahfornia have been published (Harris et al, 1980). Arizona was excluded from this compilation because data were too sparse in this vast area of diverse geology. Recent oil and gas successes in the western Overthrust belt in Utah and Wyoming (Blackstone, 1980; Dixon, 1982), however, have spurred interest in oil and gas leasing along the southern extension of the Overthrust belt as far south as Arizona (Anschutz, 1980). Because of this increased interest and because Arizona has large tracts of unevaluated federal land, there was impetus to collect additional samples, which would help in interpretation of unpublished data for Paleozoic rocks, and to make this information available.

COMPILATION INTRODUCTION Conodonts are apatitic microfossils that contain trace amounts of organic matter that visibly changes color from 50° to 500°C (122° to 932°F). Conodonts are valuable biochronologic and thermal-maturation indices for marine rocks of Ordovician through Triassic age. Consequently, they can be used to produce time-based isograd maps, which can then be used to assess oil, gas, and some types of mineralization potential. Several papers have described the origin, calibration, and geologic applications of conodont color alteration (CAI), particularly as related to hydrocarbon and mineral exploration (Epstein et al, 1977; Harris et al, 1978, 1980, 1981; Harris, 1979). All these papers demonstrate that CAI is time and temperature dependent and resuhs chiefly from burial metamorphism and/or heat associated with igneous activity. Less commonly, conodont color alteration can result from contact with warm saline solutions that produce thermally anomalous high CAI values due to corrosion of conodont apatite and oxidation of organic matter.

^Manuscript received, October 11,1983; accepted, IVIarchS, 1984. ^U.S. Geological Survey, U.S. National Mjseum E-501, Washington, D.C. 20560. About 30% of our localities are from collections made by other conodont specialists in the course of their biostratigraphic studies. We thank R. H. Bruns, D. L. Clark, R. L. Ethington, William Purves, J. E. Repetski, and Dietmar Schumacher for making their collections available to us. R. T Herman, U.S. Geological Survey, helped in the collection of outcrop samples. Wallace de Witt, Jr., R. T. Ryder, and C. W. Spencer, U.S. Geological Survey, reviev^ed the manuscript initially and T. R. Carr and David Mason were reviewers for AAPG; they all made many useful suggestions.

CAI was determined on surface samples from 141 locaUties. Many localities represent several collections distributed through the Paleozoic section or within an area too small to show at the compilation scale. Thus, many of the CAI localities show a range value that represents many samples. Detailed geographic and biostratigraphic data for all localities used in this report are given in Wardlaw et al(1984). The Colorado Plateau contains most of the outcrops of Paleozoic rocks in Arizona (Figure 1). Data points in the plateau in Utah (Harris et al, 1980) and Arizona are widely scattered, but the consistently low thermal values adequately represent this relatively noncomplex geologic province. South and west of the plateau, the narrow and spotty distribution of data points reflects the distribution of Paleozoic outcrops (Figure 1). Thermal assessment of Paleozoic rocks in Arizona required new strategies because of: (1) tectonic diversity, (2) limited areas of Paleozoic outcrop beyond the Colorado Plateau, (3) widespread post-Paleozoic igneous activity, metamorphism, and mineralization, and (4) locally thick post-Paleozoic deposits in southeast Arizona. Moreover, the same restrictions used to interpret other Basin and Range CAI maps (Harris et al, 1980) also apply here: (1) CAI patterns shown across basin areas do not necessarily characterize the thermal maturity of Paleozoic rocks beneath Tertiary basin-fill deposits, and (2) any of the mapped CAI areas may have smaller areas of higher or lower CAI values within them because of geologic complexities, broad distribution of data points, and cartographic constraints.

1101

Conodont-Based Thermal Maturation, Arizona

1102

114*^

112"^

no''

Figure 1—Map of Arizona showing physiographic provinces and generalized distribution of Paleozoic outcrops (modified from WUsonetal,1969).

Initially, all localities were plotted on one map, which produced a confusing distribution of CAI values beyond the Colorado Plateau. Significantly, isopach data for Arizona (Cook and Bally, 1975, and references cited therein) show that post-Cambrian Paleozoic rocks are relatively thin throughout the state, commonly less than 5,000 ft

(1,524 m) and that thick post-Paleozoic deposits are virtually restricted to the northern and southern extremities of the state. Therefore, burial alone cannot account for CAI values greater than 3. Consequently, we removed all CAI values above 3 from the original compilation and plotted them on a separate map, together with relatively large

Bruce R. Wardlaw and Anita G. Harris

114^

112'

I

>

5.5-7

I O

-34^

__j

Cr"

}

J

110*

__i

7~

1103

,--J

I

\/

> \

\ ._-_., 3.5-4 ^ . ^ . 4-4.5 «!a /"

I

>

t 32^

EXPLANATION

0 D

Area having all CAI values >4.5

^ ^ -

Post-Paleozoic intrusive and(or) metamorphic rocks

0 I 0

25 50 '—r-" 25

75 100 KM s—' 50 Ml

Figure 2—Map of southern Arizona showing localities having CAI values greater than 3, large areas of post-Paleozoic intrusive and (or) metamorphic rocks (modified from WUson et al, 1969), and a large area having CAI values greater than 4.5.

areas of post-Paleozoic intrusive rocks and metamorphism (Figure 2). We believe that these anomalously high CAI values (Figure 2) were caused by heating events associated with the intrusions. All localities having CAI values of 3 or less were separated by geologic age into two groups (Ordovician through Mississippian, and Pennsylvanian and Permian), and each group was plotted on a separate map. We found, however, that these two CAI maps were nearly identical, because Ordovician through Mississippian rocks in Arizona are relatively thin and have virtudly the same burial metamorphic imprint as the much thicker overlying Pennsylvanian and Permian rocks. Therefore, only one map for CAI values of 3 or less is necessary to characterize thermal maturity related to burial metamorphism of the entire Paleozoic section in Arizona (Figure 3). INTERPRETATION Total overburden on even the oldest Paleozoic rocks in Arizona is inadequate to account for thermal levels above a CAI of 2 in most of the state or 3 in southeasternmost Arizona. Thus, CAI values of 3.5 or greater are probably related to high heat flow associated with igneous activity. Figure 2 shows most localities that have anomalously high CAI values with respect to regional burial metamorphic

trends. The large thermal high in central southern Arizona (ruled area. Figure 2) outlines localities having CAI values consistently greater than 4.5 and includes an area that contains many exposures of igneous and metamorphic rocks. This regional thermal high may extend westward across southern Arizona, where the few Paleozoic outcrops sampled also produced very high CAI values. The Paleozoic and Mesozoic rocks in this entire area should hold little interest for hydrocarbon explorationists, but may be of interest to metallic-mineral explorationists. All other localities that produced high CAI values are adjacent to areas that have low thermal values. The high values are apparently related to relatively local igneous intrusions and associated hydrothermal activity and include such well-known mineralized areas as the Morenci and Bisbee copper districts. Our thermal-maturation study of Paleozoic rocks in Arizona confirms some of our speculations about CAI derived from observations from other metamorphic belts and contact-metamorphic rocks throughout North America. In general, conodonts from regional thermal highs have consistently high CAI values within and between samples, whereas conodonts from local thermal highs, such as mineralized areas or contact-metamorphic aureoles, have a wide range of CAI values within and between samples (cf. Figures 2, 3). Therefore, individual samples

1104


ARIZONA 1

MEXICO Figure 3—Map of Arizona showing localities having CAI values of 3 or less, selected isograds, and generalized total Ordovician through Permian isopachs (in feet). Isopachs modified from Armstrong et al (1980), Craig et al (1979), McKee et al (1969,1975), Poole etal (1977). or several samples within a section or small area that produce conodonts having a wide range in thermal values are good guides to hydrothermal activity, contact metamorphism, and possible mineralization.

Figure 3 shows localities having CAI values of 3 or less; these values generally reflect regional burial metamorphic trends. Isopachs on the map indicate thicknesses of Ordovician through Permian rocks, thicknesses that are great-

Bruce R. Wardlaw and Anita G. Harris

1105 UTAH

OTENTfAL Figure 4—Map of Arizona showing conodont-based thermal-maturation evaluation of oil and gas potential in Paleozoic rocks. Area of low hydrocarbon potential may be of interest for metallic-mineral explorationists.

est in southeast (Pedregosa basin) and northwest Arizona, Significant thicknesses of post-Paleozoic rocks (greater than 1,000 ft or 305 m) are restricted to northernmost and

southeasternmost Arizona (Cook and Bally, 1975). Because these maps are of particular value for explorationists interested in targeting large areas of hydrocarbon

1106


potential and also because the data are relatively sparse, only isograds for CAI values of 2 and 4.5 are shown. A CAI value of 2 is slightly below the upper thermal limit for most commercial oil generation and preservation, and a CAI of 4.5 is about the upper thermal limit for most commercial gas production. The CAI 2 isograd generally lies beyond the limit of the Colorado Plateau in which Paleozoic rocks have CAI values of 1 to 1.5. A southeast-widening belt through central to southeastern Arizona of faulted plateau-like Paleozoic rocks borders the Colorado Plateau and also has low CAI values of 1 to 2. These two areas appear to have relatively little surface expression of large-scale intrusive igneous activity because none of the rocks collected here produced conodonts having elevated thermal levels. In southeast Arizona, Pennsylvanian and Permian rocks thicken into the Pedregosa basin; the 5,000-ft (1,524-m) isopach (Figure 3) mimics the margin of the basin (Figure 4). CAI values increase from 2 outside and adjacent to the basin margin to as much as 4 near the basin depocenter in New Mexico and Mexico. This thermal gradient probably reflects southward-thickening upper Paleozoic and Lower Cretaceous deposits (Greenwood et al, 1977).

HYDROCARBON POTENTIAL The exposed Paleozoic rocks of Arizona are generally thin carbonate and clastic shelf deposits. In northern Arizona, these consist of nearshore marine and nonmarine rocks containing few potential hydrocarbon source beds except in extreme northeast Arizona. In southern Arizona, the rocks are chiefly marine carbonates together with widely separated clastic units and a few potential source beds in the Upper Devonian, Upper Mississippian, and Pennsylvanian. Figure 4 shows a hydrocarbon evaluation for Paleozoic rocks in Arizona based on thermal-maturation levels derived from CAI values. Paleozoic rocks in the Colorado Plateau, in part of the belt of faulted plateau-like rocks bordering it, and in a small area in southeast Arizona, have thermal potential for oil. Paleozoic rocks in the same areas (the belt of faulted plateau-like rocks, however, is broader), as well as in the Pedregosa basin, have thermal potential for gas. We caution that even in these favorable regions there are local areas of moderate to high thermal levels related to elevated geothermal gradients and the effects of oxidizing hydrothermal solutions.

REFERENCES CITED Anschutz, P. E, 1980, The Overthrust belt: will it double U.S. gas reserves?: World Oil, January, p. 111-116. Armstrong, A. K., B. L. Mamet, and J. E. Repetslci, 1980, The Mississippian System of New Mexico and southern Arizona, in T. D. Fouch and E. R. Magathan, eds.. Paleozoic paleogeography of the westcentral United States: Rocky Mountain paleogeography symposium 1: SEPM Rocky Mountain Section, p. 82-99. Blackstone, D. L., Jr., 1980, Tectonic map of the Overthrust belt, western Wyoming, southeastern Idaho and northeastern Utah showing current oil and gas drilling and development: Wyoming Geological Survey Map Series 8A, scale 1:316,800. Cook, T. D., and A. W. Bally, eds., 1975, Stratigraphic atlas of North and Central America: Princeton, N.J., Princeton University Press, 272 p. Craig, L. C , and C. W. Connor, coordinators, 1979, Paleotectonic investigations of the Mississippian System in the United States: U.S. Geological Survey Professional Paper 1010, pt. 3,15 pis. Dixon, J. S., 1982, Regional structural synthesis, Wyoming salient of western Overthrust belt: AAPG Bulletin, v. 66, p. 1560-1580. Epstein, A. G., J. B. Epstein, and L. D. Harris, 1977, Conodont color alteration—an index to organic metamorphism: U.S. Geological Survey Professional Paper 995,27 p. Greenwood, E., E E. Kottlowski, and S. Thompson, III, 1977, Petroleum potential and stratigraphy of Pedregosa basin—comparison with Permian and Orogrande basins: AAPCJ Bulletin, v. 61, p. 14481469. Harris, A. G., 1979, Conodont color alteration, an organo-mineral metamorphic index, and its application to Appalachian basin geology, in P. A. SchoUeand P. R. Schluger, eds., Aspects of diagenesis: SEPM Special Publication 26, p. 3-16. L. D. Harris, and J. B. Epstein, 1978, Oil and gas data from Paleozoic rocks in the Appalachian basin—maps for assessing hydrocarbon potential and thermal maturity (conodont color alteration isograds and overburden isopachs): U. S. Geological Survey Miscellaneous Investigations Map I-917-E, scale 1:2,500,000. B. R. Wardlaw, C. C. Rust, and G. K. Merrill, 1980, Maps for assessing thermal maturity (conodont color alteration index maps) in Ordovician through Triassic rocks in Nevada and Utah and adjacent parts of Idaho and California: U.S. Geological Survey Miscellaneous Investigations Map 1-1249, scale 1:2,50O,OOO. Harris, L. D., A. G. Harris, W. de Witt, Jr., andK. C. Bayer, 1981, Evaluation of the southern eastern Overthrust beh beneath the Blue Ridge-Piedmont thrust: AAPG Bulletin, v. 65, p. 2497-2505. McKee, E. D., and E. J. Crosby, 1975, Paleotectonic investigation of the Pennsylvanian System in the United States, parts l-lll: U.S. Geological Survey Professional Paper 853, 3 volumes. et al, 1969, Paleotectonic maps of the Permian System: U.S. Geological Survey Miscellaneous Investigations Map 1-450, 22 pis. Poole, E G., C. A. Sandberg, and A. J. Boucot, 1977, Silurian and Devonian paleogeography of the western United States, in J. G. Stewart, C. H. Stevens, and A. E. Fritsche, eds.. Paleozoic paleogeography of the western United States: Pacific Coast paleogeography symposium 1: SEPM Pacific Section, p. 39-65. Wardlaw, B. R., A. G. Harris, and K. S. Schindler, 1984, Thermal maturation values (conodont color alteration indices) for Paleozoic rocks in Arizona: U.S. Geological Survey Open-File Report 83-819,13 p. Wilson, E. D., R. T. Moore, and J. R. Cooper, 1969, Geologic map of Arizona: Arizona Bureau of Mines and U.S. Geological Suvey, scale 1:500,000.