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Revision of ages of the Fusselman, Wristen, and Thirtyone Formations (Late Ordovician-Early Devonian) in the subsurface of West Texas based on conodonts and graptolites.

ABSTRACT. -- Conodonts obtained from the Socony Mobil Pegasus Unit 3 #7-20 core, Midland County, Texas, permit more reliable dating of Ordovician-Devonian West Texas subsurface units. The Fusselman Formation consists of two major lithostratigraphic units, a thin basal, Late Ordovician (Hirnantian) oolite that is overlain unconformably by a thicker Llandoverian (Silurian) complex of carbonates. The Wink Member of the Wristen Formation is Wenlockian (Silurian) in age. Overlying dark shales questionably placed in the Frame Member extend into the Early Devonian. Restudy of graptolite collections of Decker (1942, 1952) from nearby wells in Crane County, Texas, indicates that dark shales (Frame Member) previously assigned to the Silurian, range from the Ludlovian (Silurian) into the Pragian (Early Devonian). Key words: West Texas; conodonts; graptolites; Ordovician; Silurian; Devonian.

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Regional correlation of subsurface units of the "Siluro-Devonian" of the Permian Basin region of West Texas and southeastern New Mexico has been hindered by a lack of published data on biostratigraphically useful fossils. Although "Siluro-Devonian" strata have been extensively explored for hydrocarbons, most of the information available consists of cuttings and geophysical logs, supplemented by a smaller number of cores. This body of data has been important in determining the spatial relations of the major lithologic units, and more recently, in permitting detailed reconstructions of the environments of deposition of "Siluro-Devonian" strata. However, the paleontological control required to ascertain chronostratigraphic relations among the depositional sequences is lacking, and detailed reconstruction of the geological history for this interval of the Paleozoic has not been possible.

Herein we provide new information on the chronostratigraphy of the "Siluro-Devonian" succession in the subsurface of West Texas based on conodonts from a core from the Pegasus field in Midland County, Texas, and a reevaluation of graptolites originally described by Decker (1942, 1952) from wells in adjoining Crane County.

STRATIGRAPHY AND PREVIOUS WORK

Most workers recognize three major stratigraphic subdivisions of the subsurface Silurian and Lower Devonian in West Texas (Fig. 1), in ascending order: (1) Fusselman Formation, (2) Wristen Formation--the "upper Silurian shale" of older works, and (3) Thirtyone Formation--the "Devonian cherty limestone" of older works. McGlasson (1967) and Hills and Hoenig (1979) have summarized the general lithostratigraphic distribution of Silurian and Lower Devonian strata in West Texas, and Geesaman and Scott (1989), Garfield and Longman (1989), and Mazzullo et al. (1989) provide a modern understanding of facies relationships and depositional sequences. The most thorough discussion of biostratigraphic information and chronostratigraphic relationships was given thirty years ago by Wilson and Majewske (1960).

[FIGURE 1 OMITTED]

In the subsurface of West Texas, the Fusselman Formation comprises a complex series of carbonate facies, including light-colored ooid grainstones, green glauconitic and pink pelmatozoan grainstones and packstones, and sparse skeletal wackestones with minor shaly intercalations. Geesaman and Scott (1989) and Garfield and Longman (1989) divided the Fusselman into two informal units in the subsurface of the central Midland Basin, a lower Fusselman and an upper Fusselman, each of which represents a separate depositional sequence. The lower Fusselman consists of a thin (<20 feet thick) unit of oolitic grainstone in the center of the basin that becomes dominantly dolomitic mudstone, of inferred peritidal origin, in the eastern part of the basin. According to these authors, the base of the lower Fusselman is intimately associated with the underlying Upper Ordovician Sylvan Shale. Contrary to previous work (e.g., Hills and Kottlowski, 1983), Garfield and Longman (1989) concluded that no unconformity exists between the upper Sylvan and lower Fusselman, and therefore, either the upper Sylvan must be Early Silurian in age or the lower Fusselman is Late Ordovician in age. Diagenetic textures suggest that the top of the lower Fusselman was exposed to subaerial conditions before deposition of the upper Fusselman (Geesaman and Scott, 1989; Garfield and Longman, 1989).

The upper Fusselman is dominated by widespread thick, crinoidal grainstones, and lesser amounts of dolomitic wackestone to skeletal packstone. These three lithofacies are interbedded such that they reflect minor differences in paleotopographic setting and degree of relative subsidence during deposition (Canfield, 1985; Geesaman and Scott, 1989). All available information indicates that the upper Fusselman in the Midland Basin was deposited in a spectrum of shallow-water, high-energy open marine to peritidal environments. The top of the upper Fusselman in a number of wells is characterized by diagenetic textures indicative of karstification and soil formation, both of which suggest a prolonged period of subaerial erosion prior to deposition of the overlying Wristen Formation (Garfield and Longman, 1989; Mazzullo et al., 1989).

The age of the subsurface Fusselman is poorly known due to a paucity of fossil material from only limited core studies. Wilson and Majewske (1960) concluded that insufficient macrofossil material (largely brachiopods and trilobites) existed to date the subsurface Fusselman any better than it possibly being Early Silurian in age (Fig. 1).

In the West Texas subsurface, the overlying Wristen Formation as defined by Hills and Hoenig (1979) includes strata often referred to as the "upper Silurian shale" (Fig. 1). From Andrews County, Texas, southward, the Wristen consists of two major lithofacies that Hills and Hoenig (1979) placed into separate members. The lower member, the Wink Member (15-130 feet thick), consists of argillaceous carbonates that grade upward and laterally into silty shales of the Frame Member (0-200 feet thick). A thin green shale or shaly mudstone, termed the "Lower Wristen Marker" by Garfield and Longman (1989), occurs at the base of the Wristen in Midland County. Elsewhere, dark greenish, nodular, peloidal ostracode mudstones and wackestones of the Wink Member of the lower Wristen rest directly on the Fusselman. The Wink Member is relatively more uniform in lithology and thickness than the underlying Fusselman, and is interpreted to represent a new transgressive phase of sufficient magnitude to establish relatively uniform environmental conditions over the low-relief Fusselman erosional surface (Garfield and Longman, 1989, p. 200 and figure 17).

The Frame Member of the Wristen Formation is restricted to the area extending southward from Andrews County, Texas. It consists of dark gray to gray-green shales that grade northward into thicker sections of carbonate-dominated undifferentiated Wristen Formation (700 feet thick). Hills and Hoenig (1979, p. 1518) included the thin dark shale unit that Jones (1953) placed at the base of the Devonian in the Midland Basin as a local facies of the uppermost Frame Member.

Wilson and Majewske (1960) recovered a shelly fauna from undifferentiated Wristen carbonates in the northern part of the Midland Basin that Berry and Boucot (1970) interpreted to be pre-Ludlovian (late Llandoverian-Wenlockian?) in age. Decker (1942, 1952) reported Ludlovian (Late Silurian) graptolites from the Frame Member from wells in Crane County, Texas. This sparse faunal information suggested that the Wristen Formation in the West Texas subsurface might be late Llandoverian to Ludlovian in age.

Jones (1953) and Wilson and Majewske (1960) recognized four subdivisions of the "cherty Lower Devonian" in the subsurface of the Midland Basin, in ascending order: (1) dark shale with conodonts and spores (0-45 feet thick); (2) dark chert and cherty limestone (100-300 feet thick); (3) fossiliferous calcarenitic limestone (450 feet thick); and (4) light-colored chert and limestone (200 feet thick). Hills and Hoenig (1979) combined the upper three units into the Thirtyone Formation (Fig. 1), and included the basal dark shale unit in the Frame Member of the underlying Silurian Wristen Formation. In the Midland Basin the Thirtyone Formation includes a variety of marine litho- and biofacies that suggest deepening conditions going from north to south. The proportion of siliceous strata in the Thirtyone Formation varies geographically as well, with cherts and tripolitic beds dominating in the south and limestones becoming more prevalent in the north (McGlasson, 1967; McWilliams, 1985; Speer, 1989).

Abundant shelly macrofossils were reported from the limestones of the upper two units by Wilson and Majewske (1960), who indicated a late Early Devonian age (Deerparkian to Onesquethawian). The lower two units of dark shale, chert, and cherty limestone did not yield diagnostic fossils, but were provisionally placed in the earliest Devonian (Helderbergian) by Wilson and Majewske (1960). Stainbrook (in Jones, 1953, p. 14) and Wilson and Majewske (1960) reported lowermost Devonian (Helderbergian) shelly macrofossils from unspecified beds near the base of the Thirtyone Formation in Andrews County.

From Andrews County northward, the Fusselman and Wristen Formations become extensively dolomitized and can not be reliably distinguished. In these areas the more general terms "Silurian" or "Siluro-Devonian" have been applied to the carbonate interval extending from the top of the Sylvan Shale to the base of the Woodford Shale.

CONODONT BIOSTRATIGRAPHY OF THE PEGASUS CORE

The most complete information on the conodont biostratigraphy and biofacies of the Fusselman and Wristen Formations was obtained from a core from the Socony Mobil Pegasus Unit 3 #7-20 well in Midland County, Texas (Fig. 2; sec. 30, Blk. 40, T-4-S, T & P RR Survey). The nearly complete core extends from the top of the Upper Ordovician Montoya Group into the Devonian Thirtyone Formation (Fig. 3). The stratigraphic sequence and lithofacies present in the core were described by Canfield (1985).

Montoya Group and Sylvan Shale

The upper part of the Montoya Group in the Pegasus core is brownish-gray cherty limestone (Fig. 3). Samples from the top of the Montoya yielded a typical Late Ordovician conodont fauna that is dominated by elements of Plectodina tenuis and P. florida. Pseudobelodina dispansa, Ps. kirki, and elements of Aphelognathus and Panderodus species are present. The fauna is relatively undiagnostic, and may range in age from late Edenian into the Richmondian. The Plectodina-dominated biofacies association is indicative of open marine shelf conditions of moderate depth (Sweet and Bergstrom, 1984). The overlying Sylvan Shale comprises four feet of greenish-gray shale that did not yield any conodonts.

Fusselman Formation

The lower Fusselman in the Pegasus well consists of eight feet of light-gray oolitic grainstone that rests with an abrupt contact on the Sylvan Shale. The grainstone is composed of about 80 percent ooids nucleated on a variety of bioclasts that include crinozoans, bryozoans, trilobites and ostracodes. At some levels peloids and grapestone textures occur. A small, but important, conodont fauna was obtained from the lower Fusselman. Several elements of Noixodontus girardeauensis were recovered in addition to a few specimens of Panderodus and Decoriconus. In the southern cratonic region of North America, N. girardeauensis is restricted to strata that contain a latest Ordovician, Hirnantian, brachiopod fauna (Amsden and Barrick, 1986).

[FIGURE 2 OMITTED]

The upper Fusselman in the Pegasus core comprises 93 feet of limestone. The contact between the lower Fusselman and upper Fusselman was missing from the core. The lower 75 feet of the upper Fusselman is dominated by pelmatozoan grainstones containing abundant bryozoans, trilobites, brachiopods, ostracodes, and mollusks (Canfield, 1985). The color of the grainstones varies irregularly from pink to white to gray, and a few glauconitic horizons occur. The upper 16 feet are characterized by light gray and tan wackestones and packstones with a similar fauna.

[FIGURE 3 OMITTED]

The lower grainstone unit of the upper Fusselman yielded only a sparse conodont fauna, averaging just 15 elements per kilogram of sample. The coniform taxa Panderodus unicostatus (67%) and Walliserodus curvatus (31%) constitute the vast majority of elements. This Panderodus/Walliserodus faunal association is characteristic of shallow water marine carbonate environments from the Llandoverian into the early Wenlockian (Silurian). The only diagnostic species recovered was a single Pa element of Distomodus kentuckyensis at 11,992 feet. It ranges from the uppermost Rhuddanian into the middle of the Aeronian Stage of the Llandoverian Series (Aldridge, 1975; Armstrong, 1990). Also, Walliserodus curvatus has not been reported from strata younger than the Aeronian.

The top of the grainstone interval and the overlying wackestones and packstones at the top of upper Fusselman contain a slightly more diverse and abundant conodont fauna (55 elements/kg). Panderodus unicostatus (39%) and a species of Walliserodus (39%) still strongly dominate the fauna. A few elements of a species of the Panderodus recurvatus group are present, Aspelundia and Oulodus species appear, and in the highest samples, Dapsilodus and Distomodus elements occur. The most diagnostic species is Distomodus staurognathoides, which ranges from the upper part of the Aeronian Stage into the base of the Wenlockian Series (Aldridge, 1975; Kleffner, 1989).

Wristen Formation

The Wristen Formation in the Pegasus core can be divided into two lithologic units. A lower unit of gray shale, limestone, and dolostone is assigned to the Wink Member. The overlying unit of black shale and carbonate is questionably assigned to the Frame Member.

Wink Member. -- The base of the Wink Member consists of four feet of greenish-gray dolomitic shale that contains silt-sized quartz and minor amounts of glauconite(?) and pyrite. The remainder of the Wink is carbonate, grading from limestone near the base to dolostones at the top. The lower four feet of the carbonate unit is light gray, silty, dolomitic biopackstone and biowackestone. A diverse shelly fauna is recognizable in thin sections: brachiopods, crinozoans, trilobites, ostracodes, bryozoans, and mollusks. The Wink grades upward into darker gray biowackestones that bear only sparse remains of ostracodes, trilobites, brachiopods, and bryozoans. Dark gray, silty calcitic dolostones characterize the upper part of the Wink Member (11,910-11,900 feet), where skeletal remains are obscured by dolomitization.

The basal shale of the Wink member yielded only a few conodont elements, largely a result of our inability to completely disaggregate the shale. Among the few elements extracted from the shale were ramiform elements of a Distomodus species.

In contrast, the biowackestone and biopackstone in the lower four feet of the Wristen carbonate unit produced abundant conodonts, averaging 4750 elements per kilogram. The fauna is overwhelmingly dominated by elements of the coniform species Dapsilodus obliquicostatus (88%). Elements of Pseudooneotodus bicornis (7%), Panderodus unicostatus (2.5%), and Decoriconus fragilis (2%), all coniform taxa, constitute nearly all the remainder of the fauna. Other taxa present include Walliserodus sancticlairi, Dapsilodus sparsus, D. praecipuus, Ozarkodina excavata excavata, other Ozarkodina species, and Kockelella species. Microfossil residues are also characterized by abundant acrotretid brachiopods, the most common of which is Artiotreta parva.

In the overlying biowackestones conodont abundance falls to about 1200 elements per kilogram. Dapsilodus elements continue to dominate the faunas (73%), but in the interval 11,922-11,919 feet, D. praecipuus increases sharply in numbers to become nearly as abundant as D. obliquicostatus. Other conodont taxa become relatively more abundant with the decline in Dapsilodus elements: Panderodus unicostatus averages 11%, and Decoriconus fragilis averages 5.5% of the elements. At 11,917 feet, Belodella silurica appears in the Wristen, contributing about 3% of elements. No conodont elements were recovered from 11,910 to 11,900 feet. Although dissolution of the dolostones was incomplete, sufficient rock was dissolved to obtain conodont elements, even if they had been present in low numbers.

Despite the great abundance of conodont elements, few age-diagnostic species were obtained from carbonates of the Wink Member. The presence of Kockelella ranuliformis (11,930-11,927 feet) and Ozarkodina rhenana (11,930 feet) at the base of the carbonate section, above the last occurrence of Distomodus in the underlying shale at the base of the Wink Member, indicates the ranuliformis Zone for the lower Wink carbonates (Sheinwoodian Stage; Wenlockian Series; Barrick and Klapper, 1976; Kleffner, 1989).

Although the range of Dapsilodus praecipuus remains undetermined, faunas from the mid-Wenlockian amsdeni Zone of Barrick and Klapper (1976) in the Midcontinent region possess acmes in the abundance of this species like those found in the interval 11,922-11,919 feet in the Pegasus core (Amsden et al., 1980). Belodella silurica, which first occurs at 11,917 feet, consistently appears in the upper amsdeni Zone in the southern Midcontinent, above the acme in D. praecipuus (Barrick, 1977, 1983). Samples 11,917 and 11,913 feet contain small Pa elements probably best assigned to Ozarkodina bohemica, another species that appears in the upper amsdeni Zone (Barrick and Klapper, 1976) and which is indicative of the Homerian Stage of the upper Wenlockian (Aldridge, 1975).

Pa elements of Kockelella absidata and K. patula? occur in the highest sample of the Wink Member that yielded conodont elements, 11,910 feet. Kockelella absidata appears in the amsdeni Zone and ranges into the Ludlovian Series (Barrick and Klapper, 1976). The appearance of K. patula high in the Wenlockian, above the first occurrences of the taxa listed above, appears to be anomalous. The Pb element associated with the Pa element of K. patula? in this sample, however, more strongly resembles the Pb element called "Ozarkodina" crassa, a species used to indicate the base of the Ludlovian, than typical Pb elements associated with K. patula from lower in the Wenlockian (Barrick and Klapper, 1976). Because K. patula is the probable ancestor of both "O." crassa and the related species, Ancoradella ploeckensis, (Barrick and Klapper, 1976; Fordham, 1990), this association of a K. patula Pa element with a "O." crassa-like Pb element should be expected to occur in the upper part of the Wenlockian.

Frame Member? -- The sequence of black siliceous shale and carbonate that extends from about 11,900 to 11,870 feet is questionably assigned to the Frame Member of the Wristen Formation. The limestones are silty, dolomitic biowackestones with sparse, poorly preserved calcitic and phosphatic bioclasts. The contact between the dark gray dolostones at the top of the Wink Member and the beds at the base of the Frame? is indistinct, and is placed where the first definite black strata and shaly layers appear in the core.

The limestones and shales of the Frame? could not be readily processed for conodonts. Typically less than 25 percent of the sample could be disaggregated, using a succession of treatments of formic acid, bleach and Stoddard's solvent. Alternation of bleaching and Stoddard's solvents was the most effective in breaking down the rock, but the sparse conodont elements were badly broken as a result of such processing. Three samples, though, did yield fragments of Pa elements of Icriodus (Fig. 3). In the best sample, 11,877 feet, two complete spindles of Pa elements were preserved. These specimens clearly possess a middle denticle row, excluding the oldest species of the genus, I. woschmidti. The specimens, however, could belong to any of several other Early Devonian species of Icriodus.

Thirtyone Formation

The base of the Thirtyone Formation is placed where the first chert beds occur in the core at 11,840 feet. Strata of the Thirtyone Formation are too well silicified to permit processing for conodonts, and the age of this unit in the Pegasus core could not be determined.

GRAPTOLITE FAUNAS

Decker (1942, 1952) described Silurian graptolites from dark shales from two wells on the Central Basin Platform in West Texas. The dark shales bearing the graptolites are assigned to the Frame Member of the Wristen Formation, based on their position above the Fusselman Formation and the limestones of the Wink Member of the Wristen Formation, and below the cherty limestones of the Thirtyone Formation (Decker, 1952). At the time of Decker's work, however, graptolites were considered to range no higher than the Silurian. For that reason alone, Decker could not interpret a younger age for the graptolite-bearing intervals in the black shale. In the early 1960's, studies of highest Silurian and lowest Devonian strata in Belgium, Podolia, and Czechoslovakia lead to the realization that graptoloid graptolites ranged well into the Early Devonian (see Berry, 1968, for discussion and bibliography). In addition, the 1960's and 1970's were a time in which considerable new light was shed on graptolite morphology as a result of the discovery and description of very well preserved specimens. Graptolite taxonomy was significantly revised and many new late Silurian and early Devonian species were described. In light of these facts, the three segments of cores studied by Decker were borrowed from the University of Oklahoma and re-examined in order to determine correct identifications and age interpretations.

Core segment OU 699 was obtained from the Gulf University No. 1 F well (Fig. 2; sec. 22, University Land Block, 31, Crane County). The graptolites occur in a dense, black shaly limestone at 9,340 feet. Decker (1942) described Monograptus (Monoclimacis) vomerinus from this segment, but most specimens are too poorly preserved or are oriented in such a way that distinctive morphological features cannot be seen. The features that are preserved indicated a generic assignment of Pristiograptus, which suggests a Late Silurian, probably no older than Ludlovian, age.

Core segments OU 1423 and OU 1424 originated from the Gulf TXL No. 1 "EE" well (Fig. 2; sec. 19, Block 43, T & P RR Survey, Crane County). The graptolites occur in a black shale in two zones at depths of 10,941-10,942 feet (OU 1423) and 10,927-10,928 feet (OU 1424). None of the species reported by Decker (1952) could be recognized in the core segments.

Core segment OU 1423 contains species indicative of latest Silurian to earliest Devonian age; the graptolite-bearing sample is, no doubt, at or close to the Silurian-Devonian boundary. Species recognized upon reexamination include Monograptus sp. cf. M. uniformis, Saetograptus sp., and Pristiograptus sp. Core segment OU 1424, which originated 14 feet higher in the well, contains a definitely younger fauna than present in OU 1423. A specimen of Monograptus sp. cf. M. thomasi was discovered; most other specimens are too poorly preserved to be identified or are preserved in such an orientation that diagnostic features are not visible. This specimen of Monograptus is one of the youngest species of graptoloid graptolites known. Its presence indicates that the core segment is correlative with the Pragian Series (Lower Devonian), and thus to a horizon substantially above the Silurian-Devonian boundary.

[FIGURE 4 OMITTED]

DISCUSSION

The Hirnantian (uppermost Ordovician) oolitic limestone of the lower Fusselman Formation of the West Texas subsurface correlates directly with the Keel and Petit Oolites of Oklahoma (Fig. 4), the Cason Shale oolite of Arkansas, and the Noix Formation and related units in the Mississippi Valley region. Collectively, these units form a thin, discontinuous sheet of extremely shallow water carbonates of Hirnantian age that stretches across the southern part of the North American craton (Amsden and Barrick, 1986). The presence or absence of an unconformity at the base of these carbonates is difficult to demonstrate, but in the Oklahoma subsurface, as well as the Texas subsurface, it is possible that deposition was continuous from the underlying Sylvan Shale. Regionally, an unconformity separates Hirnantian strata from overlying Silurian carbonates. This unconformity corresponds to the eustatic fall in sea level associated with the terminal Ashgillian event (Berry and Boucot, 1973). Physical evidence of exposure at the top of the lower Fusselman is the result of this fall in sea level.

Shallow water carbonates like those found in the upper Fusselman Formation of the West Texas subsurface occur in Oklahoma (Cochrane and Blackgum Formations), the Mississippi Valley (Sexton Creek Formation), and across the remainder of the southern craton (Amsden and Barrick, 1988). Lowermost Llandoverian (Rhuddanian) strata appear to be absent at most sections on the southern cration, as is the case with the base of the upper Fusselman. Based on community studies of shelly fossils, Johnson (1987) has inferred that four major eustatic highstands of sea level produced transgressive sequences on the North American craton, as well as on other parts of the globe during the Llandoverian (Johnson et al., 1991): a late Rhuddanian-early Aeronian event, a late Aeronian event, an early Telychian event, and a late Telychian event.

Based on our sparse data, the upper Fusselman appears to range in age from the Aeronian into the Telychian. It should represent at least the middle two transgressive events of Johnson (1987), and may also include the oldest one. However, no one has demonstrated any regional transgressive or regressive events in the upper Fusselman. The extensive exposure surface at the top of the upper Fusselman corresponds to the major low stand in sea level that occurred near the middle Telychian that left an unconformity at the top of Llandoverian deposits across North American (Johnson, 1987).

Strata of the lower part of the Wink Member of the Wristen Formation are typical of Wenlockian strata that extend across the southern craton. A thin basal shale rests on the unconformable surface in West Texas. Oklahoma (Prices Falls Shale Member, Clarita Member), as well as in the Mississippi Valley region (Seventy-Six Shale Member, Bainbridge Formation) (Amsden and Barrick, 1988). This shale and the overlying thin sequence of offshore carbonates represent a regional flooding event on the North American craton that corresponds to a eustatic rise in sea level at the end of the Llandoverian (Jeppsson, 1987; Johnson, 1987; Johnson et al., 1991). The appearance of Dapsilodus-dominated conodont faunas, in association with abundant acrotretid brachiopods, forms an obvious ecostratigraphic marker for the lower Wenlockian in southern North America (Barrick and Biggers, 1985).

It is more difficult to interpret the topmost beds of the Wink Member and the Frame Member in terms of sea level events. The regional pattern, as exemplified by the Henryhouse-Haragan sequence in Oklahoma, suggests more or less continuous carbonate deposition on the southern cration. Sequences elsewhere on the craton show a gradual shallowing event starting as early as the late Wenlockian and continuing into the Pridolian. A brief break in deposition separates uppermost Silurian from lowermost Devonian (Lochkovian) deposits in most areas (Amsden and Barrick, 1988; Barrick and Klapper, 1992).

Because no diagnostic conodonts were obtained from the top of the Wink and the lower part of the Frame?, the presence or absence of Ludlovian and Pridolian strata can not be demonstrated in the Pegasus core. Based on our restudy of Decker's material, some graptolites from the Frame Member in Crane County (Gulf University No. 1 F well), may be as old as the Ludlovian. Both conodonts from the Frame? Member in the Pegasus core and graptolites from the Frame in the Gulf TXL No. 1 "EE" well in Crane County clearly indicate that the base of the Devonian lies within the black shales and limestones near the top of the Frame Member. If deposition was more or less continuous from the Ludlovian into the Lochkovian in the southern part of West Texas, then this period of time must be represented by the thin sequence of black shale and carbonate in the Pegasus core and in Crane County. Facies relationships (Ruppel, 1991), and the presence of graptolites in some of the black shales suggest deposition in deep water environments, where slow rates of deposition, punctuated by nondepositional hiatuses are likely to occur. To the north, in Andrews County, Texas, strata interpreted to be equivalent to the upper Wristen formed a thick carbonate platform on which a series of carbonate build-ups and related shallow water carbonates accumulated and prograded into deeper water environments at the margin (Ruppel, 1991).

Because Pragian (Early Devonian) graptolites occur in the underlying black shales in the Gulf TXL No. 1 "EE" well in Crane County, the Thirtyone Formation is inferred to be Pragian or younger in age. Preliminary study of conodonts from other wells suggests that the Thirtyone may be equivalent in age to the Frisco Formation of Oklahoma, which is interpreted to be Pragian in age (Barrick et al., 1990). In their analysis of North American Devonian transgressive-regressive cycles, Johnson and Klapper (1992) show the Frisco as part of the late Pragian cycle Ia. The Thirtyone Formation may be part of the same transgressive-regressive cycle.

ACKNOWLEDGMENTS

Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research (24191-AC8). Work on conodonts was supported in part by a Texas Tech Graduate School Summer Research Award to Jill Haywa-Branch. We thank Patrick K. Sutherland, University of Oklahoma, for permitting the loan of Decker's graptolite collections. The authors thank the reviewers, S. J. Mazzullo and S. C. Ruppel, for their helpful comments.

LITERATURE CITED

Aldridge, R. J. 1975. The stratigraphic distribution of conodonts in the British Silurian. J. Geol. Soc. London, 131:607-618.

Amsden, T. W., and J. E. Barrick. 1986. Late Ordovician-Early Silurian strata in the central United States and the Hirnantian Stage. Oklahoma Geol. Surv. Bull., 139, 95 pp.

______. 1988. Late Ordovician through Early Devonian annotated correlation chart and brachiopod range charts for the southern Midcontinent Region, U.S.A., with a discussion of Silurian and Devonian conodont faunas. Oklahoma Geol. Surv. Bull., 143, 66 pp.

Amsden, T. W., D. F. Toomey, and J. E. Barrick. 1980. Paleoenvironment of Fitzhugh Member of Clarita Formation (Silurian, Wenlockian), southern Oklahoma. Oklahoma Geol. Surv., Circular 83, 54 pp.

Armstrong, H. A. 1990. Conodonts from the Upper Ordovician--Lower Silurian carbonate platform of North Greenland. Gronlands Geologiske Undersogelse Bull., 159, 151 pp.

Barrick, J. E. 1977. Multielement simple-cone conodonts from the Clarita Formation (Silurian). Geologica et Palaeontologica, 11:47-68.

______. 1983. Wenlockian (Silurian) conodont biostratigraphy, biofacies, and carbonate lithofacies, Wayne Formation, central Tennessee. J. Paleo., 57:208-239.

Barrick, J. E., and B. Biggers. 1985. Paleoenvironmental distribution of Wenlockian (Silurian) conodonts and inarticulate brachiopods. Geol. Soc. Amer. Abst. with Programs, 17:79.

Barrick, J. E., and G. Klapper. 1976. Multielement Silurian (late Llandoverian--Wenlockian) conodonts of the Clarita Formation, Arbuckle Mountains, Oklahoma, and phylogeny of Kockelella. Geologica et Palaeontologica, 10:59-100.

______. 1992. Late Silurian-Early Devonian conodonts from the Hunton Group (upper Henryhouse, Haragan, and Bois d'Arc Formations), south-central Oklahoma. Oklahoma Geol. Surv. Bull., 145:19-65.

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JAMES E. BARRICK, STANLEY C. FINNEY, AND JILL N. HAYWA-BRANCH

Department of Geosciences, Texas Tech University. Lubbock, Texas 79409-1053, Department of Geological Sciences, California State University, Long Beach, California, 90840, and Soil Conservation Service, U.S. Department of Agriculture, 865 Oilfield Avenue, Shelby, Montana 59474
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Author:Barrick, James E.; Finney, Stanley C.; Haywa-Branch, Jill N.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Aug 1, 1993
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