Anatomy of the middle ordovician sevier shale basin, eastern Tennessee

Sedimentary Geology', 34 (1983) 315-337

315

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

A N A T O M Y OF THE M I D D L E O R D O V I C I A N SEVIER SHALE BASIN, EASTERN TENNESSEE G A N A P A T H Y S H A N M U G A M * and K E N N E T H R. W A L K E R

Mobil Field Research Laboratory, P.O. Box 900, Dallas, TX 75221 (U.S.A.) Department of Geological Sciences, Universi(v of Tennessee, Knoxville. TN 37916 (U.S.A.) (Received April 13, 1982; revised and accepted October 28, 1982)

ABSTRACT Shanmugam, G. and Walker, K.R., 1983. A n a t o m y of the Middle Ordovician Sevier Shale basin, eastern Tennessee. Sediment. Geol., 34: 315-337. The Sevier Shale basin in eastern Tennessee comprises one of the thickest clastic sequences (nearly 2500 m) of Middle Ordovician age in North America. The lower one-half of the sequence is composed of Lenoir, Whitesburg, Blockhouse and Sevier Formations, in ascending order. The sequence ranges in age from Whiterockian to lower Wilderness in North American stages. The Middle Ordovician sequence exhibits tidal flat (Mosheim Member of Lenoir Fm.), subtidal (main body of Lenoir Fm.), slope (Whitesburg Fm.), anoxic basin (Blockhouse Fm), turbidite and contourite (Sevier Fm.) facies. The Sevier basin evolved in five stages: First, a widespread marine transgression initiated carbonate-shelf deposition in the study area. Second, a major tectonic downwarping event caused the stable shelf to break and subside rapidly at a rate of 60-65 cm 1000 yrs i, and areas of shelf facies became areas of slope and basin facies. Third, global transgressions maintained the deep anoxic conditions for nearly 10 Ma. Fourth, turbidites began to fill the basin from a westward-prograding submarine fan system. Fifth, contour currents reworked the turbidites and progressively ventilated the Sevier basin. The basin-filling process terminated with shallow-water/subaerial clastics at the end of Middle Ordovician.

INTRODUCTION

A complex mosaic of carbonate and terrigenous clastic sequences comprise the Middle Ordovician Sevier Shale basin in eastern Tennessee. The terrigenous part (Blockhouse and Sevier Formations) alone has a thickness of more than 2500 m in some areas. The Sevier basin forms one of the thickest Middle Ordovician clastic sequences in North America. This paper is based on field study of more than 2000 m of measured sections from five localities and on petrographic study of 300 lithologic samples. Our primary purpose is to develop a systematic stratigraphic and sedimentologic framework for * Present address: Mobil Research and Development Corporation, P.O. Box 345100, Farmers Branch, TX 75234, U.S.A.

0037-0738/83/$03.00

© 1983 Elsevier Science Publishers B.V.

316

Middle Ordovician sequences in the Sevier basin. Age relationships have been established on the basis of conodont and graptolite biostratigraphy. Distinctive facies from shelf, slope and basin environments are described and evolutionary stages of the basin are considered in terms of tectonics and sedimentation. GEOLOGICAL

SETTING

More than a century has elapsed since the preliminary work of Safford (1869) on the Ordovician rocks of Tennessee. The basic structural and stratigraphic aspects of these rocks have been studied by Keith (1895), Rodgers (1953) and Neuman (1955). A regional facies pattern of Middle Ordovician sequences in the southern Appalachians (Fig. 1) was first proposed by Walker and Alberstadt (1973). Walker (1974, 1977, 1980) has suggested a facies pattern composed of three parts for the Middle Ordovician in east Tennessee: (1) a western shelf with a shelf edge skeletal sandb a n k / r e e f tract; (2) a deep-water basin southeast of the shelf; and (3) a shallow-water, nearshore environment southeast of the basin. A regional paleobathymetric analysis of the Middle Ordovician sequence is given by Benedict and Walker (1978). Depositional environments of coeval rocks in the Virginia Appalachians have been discussed by Read (1980, 1982). In this study we focus on the western shelf to basinal sequences exposed at the

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317

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MIDDLEORDOVICIAN o~

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Fig. 2. Location map showing Boyds Creek, Deep Springs, Nina-B, Nina-A and Nina-N study sections. Middle Ordovician rocks north of the Dumplin Valley fault are not shown.

Boyds Creek, Deep Springs, Nina-B, Nina-A and Nina-N sections (Fig. 2). These sequences are bounded by two major thrust faults, the Dumplin Valley fault to the northwest and the Great Smoky fault to the southeast. The Ordovician rocks of the Appalachian region are tectonically associated with the Taconic Orogeny (Rodgers, 1970). STRATIGRAPHY

In our study localities, the Middle Ordovician is composed of the Lenoir, Whitesburg, Blockhouse and Sevier formations, in ascending order. Each formation has distinct field and petrographic properties (Table I). The term "Sevier Shale" derives from the widespread occurrence of shale in Sevier County of eastern Tennessee which constitutes a major sedimentary fill in the basin.

Age relationships Conodonts (Table II) and graptolites (Fig. 3) are used to establish the age relationships between stratigraphic sections (Fig. 4). Based on conodont biostratigraphy, the thickest measured interval at Boyds Creek is assigned to the Pygodus serrus, Pygodus anserinus, and Amorphognathus tvaerensis zones. Bergstrom and Carnes (1976) discussed conodont biostratigraphy of related Middle Ordovician sequences in east Tennessee. Graptolite biostratigraphy suggests a Nemagraptus gracillis zone for the Boyds Creek section. The thick Boyds Creek section ranges in age from Whiterockian to lower Wilderness in North American stages.

318 TABLE l Characteristics of shelf, slope and basin facies Formation

Informal Division

Lithology

Color

Bedding

Sevier

Upper

Sandy shale, sandstone

Light gray

Medium to thick

Lower

Silty to sandy shale

Medium light gray

Thin to medium

Blockhouse

Homogeneous

Clayey shale, volcanic tuff

Dark gray

Thinly laminated

Whitesburg

Upper

Interbedded microsparite/ pebbly lime mudstone and shale Skeletal packstone and grainstone

Dark gray

Thin

Medium dark gray

Medium to thick

Lower

Lenoir

Upper main body

Medium dark gray

Thin to medium

Lower main body

Skeletal packstone and grainstone Skeletal wackestone

Medium dark gray

Thin to medium

Mosheim Member

Lime mudstone, dolostone

Medium light gray

Medium to thick

SEDIMENTOLOGY

Lenoir Formation T h e L e n o i r F o r m a t i o n has a d i s t i n c t l o w e r u n i t c a l l e d the M o s h e i m M e m b e r a n d a n u p p e r u n i t t h a t is the m a i n b o d y . T h e b a s a l p a r t of this f o r m a t i o n s h o w s a s h a r p b r e c c i a t e d e r o s i o n a l c o n t a c t (Fig. 5A).

Tidalflatfaeies. T h e M o s h e i m M e m b e r is c o m p o s e d o f m e d i u m - to t h i c k - b e d d e d , m e d i u m l i g h t - g r a y l i m e m u d s t o n e s a n d d o l o s t o n e s . T h e t h i c k n e s s of this u n i t varies f r o m 16 to 41 m. W e a t h e r i n g o f these r o c k s o f t e n results in a f l u t e d a p p e a r a n c e . T h e

319

Features

Fossils

Insol. res. (%)

Mn in carb. (ppm)

Facies

Beds w/sharp upper and lower contacts, Bouma divisions, burrows Bouma divisions, graded laminations

Trilobites, brachiopods, lingulids

37-73

1446 2 3 4 6

Contourite and turbidite (basin)

Graptolites, tintinnids, radiolarians

50-77

1408-3561

Turbidite (basin)

Fissile, weathers to thin chips,

Graptolites, tintinnids, radiolarians

61-78

6451-8794

Anoxic basin

Phosphatic c l a s t s in muddy m a t r i x , planar fabric, projected clasts Phosphate-coated echinoderm debris (phosphatic ooids)

Graptolites, radiolarians

11-26

346 667

Lower slope

Echinoderms, cephalopods

8-20

549-1321

Upper slope

Brachiopods, gastropods, trilobites Algae, brachiopods,sponges, gastropods Gastropods, ostracodes, trilobites

4 14

313 1364

Deep subtidal (shelf)

17-21

435-1394

Shallow subtidal (shelf)

Cross-stratification

Oncolites, nodular appearar~ce Fenestral f a b r i c , fluted appearance, erosional surface

2- 6

76

141

Tidal flat (shelf)

most diagnostic field characteristic of the Mosheim M e m b e r is its fenestral fabric or birdseye structures (Fig. 5B). Soft-sediment d e f o r m a t i o n a n d stylolites are c o m m o n in these rocks. Fossils include gastropods, trilobites a n d ostracodes (Fig. 5C). Birdseye structures form because of e n t r a p m e n t of gas b u b b l e s in sediment a n d because of desiccation p h e n o m e n a , a n d can be observed forming in m o d e r n tidal flat e n v i r o n m e n t s of Florida Bay a n d elsewhere (Shinn, 1968). T h u s the Mosheim M e m b e r is interpreted as representing tidal flat e n v i r o n m e n t s . Birdseye structures suggest an exposure index of 90-100% ( G i n s b u r g et al., 1977) for these sediments. Subtidal facies. The m a i n b o d y of the Lenoir F o r m a t i o n is composed of thin- to m e d i u m - b e d d e d , m e d i u m dark-gray skeletal wackestones, packstones and grain-

Lithology

Dismicrite

Wackestone

Packstone

Formation

Lenoir

Lenoir

Lenoir

B.20

B. 19

B. 14

Sample number

Distribution of conodonts in the Boyds Creek section

TABLE il

27.01

23.20

12.00

Sample location (m above base)

fibrous ff.

l~vgodus serrus Eoplacognathus foliaceus Periodon aculeatus Protopanderous varicostatus Walliserodous e hingtoni Dapsilodous mutatus

simple cones indeterminable

pvgodus serrus Zone, Eoplacognathus foliaceus subzone

Pygodus serrus zone

Polyplacognathus friendsvillensis Drepanoistodus sp. Belodella n. sp. A Panderodus sp.

-

Zone and subzone

Barren

Conodonts

Quartzose

Sevier

Calcarenite

Packstone

Whitesburg

B. 196

B.26

1373.78

58.19

Protopanderous varicostatus

simple cone indeterminable

Periodon aculeatus, Protopanderous varicostatus Walliserodous ethingtoni Phragmodus flexuasus Dapsilodus mutatus

Pygodus serrus Eoplacognathus sp.

Coelocerodontus digonius Polyplacognathus friendsvillensis

fibrous ff.

Drepanoistodus suberectus Belodella nevadensis, Panderodus sp. Staufferella sp.

Pygodus serrus

tvaerensis zone

No younger than lower A morphognathus

Eoplacognathus foliaceus subzone

Zone, and probable

322

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stones. The nodular and lumpy appearances of these beds (Fig. 6A) are distinct field characteristics. The thickness of this unit varies from 3 to 39 m. The lower part of the main body is argillaceous in composition, whereas the upper few meters of the main body are composed of cross-stratified pelmatazoan calcarenite. The argillaceous lower part is enriched in algal flora such as Girvanella (Fig. 6B) and Nuia (Fig. 6C). Other fossils include brachiopods, gastropods and sponges. Girvanella commonly occurs as oncolites (Fig. 6B). In modern carbonate environments oncolites form at a water depth of approximately 3 m (Ginsburg, 1971). The muddy wackestone lithology and oncolites suggest a shallow quiet subtidal environment for the lower part of the main body of the Lenoir Formation. The cross-stratified upper part of the main body is composed of packstones and grainstones. These beds show a general lack of algal flora. The presence of cross strata and a near lack of mud indicate a high-energy environment. A relatively deeper subtidal environment is indicated by the absence of algal flora for the upper

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part of the main body when compared to the lower part. Analogous environments have been reported from equivalent rocks north of our study area by Ruppel (1979).

Whitesburg Formation The Whitesburg Formation has a distinct basal contact with the underlying Lenoir Formation. It is composed of thin- to thick-bedded, dark-gray limestones and interbedded shales. Thickness of this formation varies from 3 to 29 m. Slope facies. The lower part of this formation is composed of skeletal packstone and grainstone. The grainstone is diagnostically rich in phosphate-coated echinoderm debris (phosphatic ooids). The absence of shallow-marine biota and sedimentary structures in these rocks suggests a deep marine environment. A sudden deepening in waterdepth is indicated by biofacies. The phosphatic grains were

324

Fig. 5. Tidal flat facies, Mosheim Member of Lenoir Formation: (A) brecciated erosional contact, Nina-B; (B) outcrop view of fenestral fabric, Nina-N, scale is 10 cm: (C) photomicrograph of lime mudstone with articulated ostracodes near bottom center, scale is I mm.

325

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photomicrograph

o f Nuia ( a l g a ) , s c a l e is 0.5 m m .

326 probably formed in an oxygen-deficient reducing environment (Richards, 1965). The upper part of the Whitesburg Formation is composed of alternating thin-bedded, dark gray microsparite and shale. At the Nina-N locality, pebbly-lime mudstone is interbedded with graptolitic shale (Fig. 7A). Pebbly-lime mudstones at Nina-N exhibit certain important sedimentologic properties; these are planar fabric, preservation of graptolites with delicate ornamentation, inverse grading, poor sorting, shale clasts in a muddy matrix and projected clasts (Fig. 7B). These features are attributed to deposition by fine-grained carbonate debris flow in a slope environment (Shanmugam, 1978; Shanmugam and Benedict, 1978). In these rocks, radiolarians (Fig. 7C) and graptolites are common. The complete absence of shallow-water fossils and presence of deep-water fossils such as radiolarians suggest a deep-water environment. Another important environmental clue comes from the composition of shale clasts in pebbly-lime mudstone. XRD analysis shows that apatite constitutes nearly 90% of these clasts. Phosphatic shale clasts were probably derived by reworking of phosphatic nodules on the sea floor. In modern environments, apatite forms under reducing conditions (Bentor, 1980) and, therefore, we infer similar environmental conditions for the clasts. The Whitesburg Formation is stratigraphically sandwiched between underlying shelf deposits and overlying basinal deposits. By applying Walther's "law", this sequence can be visualized in a lateral framework where the Whitesburg Formation would occupy a slope environment bounded by shelf on one side (west) and by basin on the other. This deep-water slope would provide an ideal setting for generation of debris flows (Stanley, 1973). A slope of 2.5 ° has been inferred for the Whitesburg environment based on flow strength of debris flows (Shanmugam, 1978). Blockhouse Formation

The Blockhouse Formation has a gradational contact with the underlying Whitesburg Formation (Fig. 8A). It is composed of dark-gray, fissile, clayey shale that commonly weathers to thin chips (Fig. 8B). It varies in thickness from 36 to 282 m. Laminations (Fig. 8C) are the only sedimentary structures associated with shales. A 2 m-thick altered volcanic tuff unit occurs near the base of this formation. The shales contain graptolites, protozoan tintinnids and radiolarians; benthic fossils are absent. A noxic basin facies. A deep-water environment is implied by the complete absence of shallow-water fossils and by the abundance of protozoan tintinnids that are usually associated with deep-water marine facies of ancient sediments (Loeblich and Tappan, 1968). The homogeneous shale lithology suggests a basinal setting far from the source. Graptolites of the Blockhouse Formation (see Fig. 3) represent the argillite-chert belt of Erdtmann (1976). The graptolitic assemblage indicates a pelagic, open-marine environment. Thin horizontal laminations of shale are the products of pelagic suspension settling in an open-marine environment. Trigonomet-

327

Fig. 7. Slope facies, Whitesburg Formation: (A) a pebbly-lime mudstone embedded in shale, Nina-N; (B) photograph of a thin section showing floating clasts (l), projected clasts (2), inverse grading and poor sorting; (C) photomicrograph of radiolarians, scale is 0.5 mm.

328

Fig. 8. Anoxic basin facies, Blockhouse Formation: (A) gradational contact (arrow) between Whitesburg (W) and Blockhouse (B) Formations, Deep Springs; (B) thin chips caused by weathering of fissile clayey shale, Nina-N; (C) photomicrograph showing thin laminations, scale is 1 mm.

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330

ric computation yields a water depth of 700 m or more for the Blockhouse Formation (Shanmugam and Walker, 1980). As mentioned before, this formation is characterized by the absence of benthic shelly fauna and by the presence of lanfinations. Bioturbation is present only in the uppermost part of this formation. Thus we infer an anoxic environment (see Byers, 1977) for these dark gray shales. The unusually high manganese content of the Blockhouse Formation (Table I) can also be attributed to prevailing reducing conditions in the deep-sea enviromnent (Bencini and Turi, 1974). Anoxic bottom conditions have been reported for coeval rocks in Tennessee (Benedict, 1983) and Virginia (Read, 1980) and probably reflect widespread anoxic conditions in the Early Paleozoic (Berry and Wilde, 1978). Sevier Formation

The measured lower one-half of the Sevier Formation at Boyds Creek (909 m) may be divided into lower and upper parts on the basis of lithology and bed

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331

thickness. The basal contact of this formation is gradational with the underlying Blockhouse Formation. The shaly sequence is megascopically monotonous; when studied by means of thin sections, peels and X-radiographs, however, a great wealth of environmentally sensitive sedimentary features is evident (see Shanmugam and Walker, 1978). Turbidite facies. The lower part is composed of thin- to medium-bedded, medium light gray, calcareous silty and sandy shale. Beds are uniform in thickness and show thinning-upward cycles (Fig. 9A). Bouma (1962) divisions (Fig. 9B), graded laminations and animal escape burrows are common. Fossils include protozoan tintinnids, graptolites, radiolarians and ostracodes. Fossils suggest a bathyal environment for the lower part. The presence of Bouma divisions, graded laminations and animal escape burrows suggesting rapid deposition are evidence for turbidity current deposition. The stratigraphic distribution of sedimentary structures reveals that the influence of turbidity currents increases from the base to the middle of the measured interval. Thickening- and coarsening-upward turbidite sequences at Boyds Creek (Shanmugan, 1980) are indicative of progradational lobes (Ricci Lucchi, 1975) of a submarine fan system. Progradation has resulted in the deposition of lower fan lobes (Sevier Fm.) on a basin-plain environment (Blockhouse Fm.). A submarine fan model exhibiting the lateral relationships of Middle Ordovician sequences in the Sevier basin is shown in Fig. 10. Contourite facies. The upper part of the measured interval at Boyds Creek is composed of medium- to thick-bedded, light-gray, calcareous sandy shale and sandstone. In addition to turbidite beds, there are beds that exhibit sharp basal and sharp upper contacts (Fig. 9C). These beds are laterally discontinuous and grains are moderately well sorted. Such features are indicative of reworking of sediments by contour-following ocean bottom currents (Hollister and Heezen, 1972; Stow and Lovell, 1979). In comparison to the anoxic environments of the Blockhouse Formation, during Sevier time the ocean was progressively ventilated as entrapped oxygen was brought into the basin by deep-sea bottom currents. Shelly fauna such as trilobites, brachiopods and lingulids are present in situ near the uppermost part of the measured interval at Boyds Creek. This suggests a shoaling upward nature of the sequence. BASIN EVOLUTION

The Sevier basin evolved in five stages (Fig. 11). Using biostratigraphic age relationships and inferred rates of sedimentation for each environment (Shanmugam, 1978), absolute ages (Shanmugam and Walker, 1980) have been assigned to each evolutionary stage in the following discussion.

332

Stable shelf stage The study area consisted of emergent Knox land prior to the deposition of carbonate rocks during Lenoir time (469-467 Ma). A widespread marine transgresWEST

EAST

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1

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Fig. 1I. Evolutionary stages of the Sevierbasin.

sion during Whiterockian time (early Middle Ordovician) is inferred by features of progressively deep-water carbonate shelf environments of the Lenoir Formation. Widespread transgressions m a y be explained either by melting of ice sheets (Donovan and Jones, 1979) or by reducing of ocean volume due to increased rate of spreading along the mid-oceanic ridges (Hays and Pitman, 1973). Because of lack of evidence for glaciation in the early Middle Ordovician (Leggett, 1978), the inferred transgressive sequence was probably a manifestation of a pulse of sea-floor spreading in the Ordovician seas. It is conceivable that a transgressive imprint might have been caused by a local tectonic event farther to the east within the basin.

333

Downwarping stage The Whitesburg Formation was deposited on a slope environment of tectonic origin. A major tectonic event (467 466 Ma) had caused the stable shelf to break (Shanmugam and Lash, 1982) and subside rapidly at a rate of 60-65 cm 1000 yrs 1 (Shanmugam and Walker, 1980) and thereby the shelf facies became areas of slope/basin facies due to bathymetric reversal. Although no normal faulting has been recognized in the study area, the abrupt lateral increase in sediment thickness and sudden deepening of the basin suggest a fault-bounded basin. An alternative hypothesis to explain a sudden deepening would be a rapid rise in sea level. Such a hypothesis is not tenable because even during the great transgression of the Late Cretaceous, the maximum rate of sea-level rise was only 9 cm 1000 yrs i (Hancock and Kauffman, 1979). The abnormally high subsidence rate of 60-65 cm 1000 yrs ~ cannot be explained by eustatic causes in the absence of glaciation. Hallam and Bradshaw (1979) noted that anaerobic conditions were maintained in regions with greater subsidence due to local inhibition of bottom circulation. Anaerobic conditions of the Whitesburg Formation are suggested by phosphatic ooids in the upper slope and by debris flows with phosphatic clasts in the lower slope. Phosphatic clasts that occur only in the Whitesburg Formation imply primary phosphatization. Similar debris flows have been reported from the South African continental margin (Parker, 1975).

Starved (anoxic) basin stage The tectonic deepening event led to the formation of a starved anoxic basin where sediment influx was exclusively from pelagic suspension. Because of the slow rate of pelagic deposition (1-3 cm 1000 yrs l; Shanmugam, 1978), the Blockhouse Formation represents the longest time span (466-456 Ma) in the basin. The presence of a nearby volcanic island-arc source is indicated by an altered tuff unit in these pelagic deposits. Computed bathymetry of the basin reveals bathyal depths during this time. The Blockhouse Formation corresponds with times of global transgressions (McKerrow, 1979) and the formation also suggests pelagic regimes of the Ordovician (Leggett, 1978, 1980; Jenkyns, 1980), and a "polytaxic" mode (Fischer and Arthur, 1977). The Sevier basin was initially deepened by tectonic downwarping, but later maintenance of deep anoxic conditions in the basin was due to global transgressions.

Turbidite fill stage Turbidites became the dominant sedimentary fill during the time of Sevier Formation deposition (456-453 Ma). Coeval polymictic conglomerates that occur east of the study area at Indian Creek Embayment (Kellberg and Grant, 1956) have been interpreted to represent a major submarine fan complex (Keller, 1977; Walker

334

and Keller, 1977; Raymond et al., 1979; Bowlin and Keller, 1980). The conglomeratic beds suggest an easterly source for the turbidites. The thickening- and coarsening-upward turbidite sequences reflect the gradual westward encroachment of submarine fan lobes from an eastern source. Contour current stage

A mature basin developed due to cessation of tectonic forces during the last one million years (454-453 Ma) of Sevier Formation deposition. Bottom currents were generated because of thermohaline contrasts in the Ordovician seas. The bottom currents were presumably flowing parallel to bathymetric contours (contour currents) along the western margin of the basin. These currents were responsible for reworking of turbidites and for ventilating the basin. At the end of the Middle Ordovician, the basin-filling process terminated with shallow-water/subaerial clastics. CONCLUSIONS

(1) The Middle Ordovician sequence in the Sevier Shale basin is composed of the Lenoir, Whitesburg, Blockhouse and Sevier Formations, in ascending order. The sequence ranges in age from Whiterockian to lower Wilderness in North American stages. (2) The Lenoir Formation represents tidal flat, shallow and deeper subtidal carbonate shelf environments. (3) The lower part of the Whitesburg Formation represents a deep-water, oxygen-deficient upper-slope environment, and the upper part of this formation represents a deeper, oxygen-deficient lower-slope environment with debris-flow deposits. (4) The Blockhouse Formation represents an anoxic basinal environment of bathyal depths where pelagic sediments accumulated from suspension settling. (5) The Sevier Formation is predominantly composed of turbidites of the lower .fan environment. Reworking of turbidites by contour-following bottom currents resulted in minor contourite deposition. (6) The Sevier basin evolved in five stages, namely: (a) stable shelf; (b) downwarping; (c) starved (anoxic) basin; (d) turbidite fill; and (e) contour current stages. The basin-filling process terminated with shallow-water/subaerial clastics at the end of the Middle Ordovician. ACKNOWLEDGEMENTS

This paper is based on a Ph.D. dissertation completed at the University of Tennessee. Grants from NSF, GSA and Sigma Xi supported this research. We thank

335 D r . D . W . K i r k l a n d for his e d i t o r i a l c o m m e n t s a n d M . F . J a c k s o n for t y p i n g t h e m a n u s c r i p t . D r s . Stig B e r g s t r o m a n d S t a n F i n n e y h a v e i d e n t i f i e d c o n o d o n t s a n d graptolites, respectively. Field assistance provided by Jean Shanmugam

is g r e a t l y

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