Charles A. Sandberg

Devonian eustatic fluctuations in Euramerica

The Devonian System of Euramerica contains at least 14 transgressive-regressive (T-R) cycles of eustatic origin. These are separated into three groups (or depophases) and from Carboniferous cycles by three prominent regressions. Twelve post-Lochkovian T-R cycles are recognized, and they commonly appear to result from abrupt deepening events followed by prolonged upward shallowing. Deepening events in the western United States (especially Nevada), western Canada, New York, Belgium, and Germany have been dated in the standard conodont zonation and are demonstrably simultaneous in several or all five regions. This synchroneity indicates control by eustatic sea-level fluctuations rather than by local or regional epeirogeny. Facies shifts in shelf sedimentary successions are more reliable indicators of the timing of sea-level fluctuations than are strandline shifts in the cratonic interior, because the latter are more influenced by local epeirogeny. Strandline shifts are most useful in estimating the relative magnitude for sea-level fluctuations. Devonian facies progressions and the three prominent regressions are of a duration and an order of magnitude that could have been caused by episodes of growth and decay of Devonian oceanic ridge systems. The described T-R cycles could have formed in response to mid-plate thermal uplift and submarine volcanism. The latter process may have been a control on small-scale (1–5 m thick), upward-shallowing cycles within the major T-R cycles. Continental glaciation could have been a factor in sea-level fluctuations only in the Famennian and could not have been responsible for the Devonian facies progressions or the numerous T-R cycles. The Frasnian extinctions were apparently cumulative rather than due to a single calamity. Two rapid sea-level rises occurred just before, and one at, the Frasnian-Famennian boundary. It is probable that this series of deepening events reduced the size of shallow-shelf habitats, caused repeated anoxic conditions in basinal areas, and drowned the reef ecosystems that had sustained the immensely diverse Devonian benthos.

Devonian conodont biochronology in geologic time calibration

The conodont-based biochronology of the Devonian Period is calibrated herein on the basis of: (1) a recent, reliable radiometric dating (Claoué-Long et al. 1992) of a stratigraphic position just above the Devonian-Carboniferous boundary; and (2) new calculations of stage durations based on our estimates and those of a large number of other Devonian workers, which pertain to many stratigraphic sequences in several different paleotectonic settings globally. Using these tools we maintain the 46-m.y. duration for the Devonian Period given by the recent global timescale (GTS 89) ofHarland et al. (1990). The main changes to GTS 89 are accomplished by raising the dating of the end of the Devonian by 9 m.y. from 363 Ma to 354 Ma and considerably shortening the bizarre estimate given by GTS 89 for the length of the oldest two Early Devonian stages. We reiterate our earlier exposé (Ziegler & Sandberg 1994) of the inaccuracies and misestimates of the ages of stage boundaries inherent in GTS 89 because of its misuse of conodont zones, which it employed as chrons to subdivide the Devonian Period.Our revisions provide the following new ages: 354 Ma, end of Devonian Period; 364 Ma, start of Famennian Stage; 369 Ma, start of Frasnian Stage; 376 Ma, start of Givetian Stage; 381 Ma, start of Eifelian Stage; 389 Ma, start of Emsian Stage; 393 Ma, start of Pragian Stage; and 400 Ma, start of Lochkovian Stage and Devonian Period. Durations of stages are thus: Famennian, 10 m.y.; Frasnian, 5 m.y.; Givetian, 7 m.y.; Eifelian, 5 m.y.; Emsian, 8 m.y.; Pragian, 4 m.y.; and Lochkovian, 7 m.y.This paper is an expanded version of a talk (Sandberg & Ziegler 1996) presented in July 1996 at the ECOS-VI meeting of the Pander (Conodont) Society in Warsaw, Poland.KurzfassungDie auf Conodonten gegründete Biochronologie der Devon-Periode wird geeicht mit Hilfe einer verläßlichen radiometrischen Datierung (Claoué-Long et al. 1992) für eine stratigraphische Position direkt oberhalb der Devon-Karbon-Grenze. Außerdem wird die Dauer der Devon-Stufen aufgrund eigener Schätzungen und denen zahlreicher anderer Devon-Bearbeiter neu definiert. Diese Kalkulationen lassen sich in vielen typischen Devongebieten mit unterschiedlicher Fazies und Struktur weltweit erkennen. Bei der Festlegung dieser neuen Daten halten wir an der Gesamtdauer der Devon-Periode von 46 m.y. fest, wie sie auch im GTS 89 vonHarland et al. (1990) angegeben ist. Die Hauptveränderungen gegenüber GTS 89 werden erreicht, indem das Ende des Devons um 9 m.y. von 363 Ma auf 354 Ma verringert wird; zudem werden die kuriosen Schätzungen der beiden ältesten frühdevonischen Stufen durch GTS 89 jetzt beträchtlich gekürzt. SchonZiegler & Sandberg (1994) hatten auf die Ungenauigkeiten und Fehlschätzungen der Alter der Stufengrenzen durch GTS 89 hingewiesen. Diese waren entstanden, weil GTS 89 die Conodontenzonen hierarchisch falsch eingestuft und sie trotzdem als Chronen benutzt hatte, um das Devon zu untergliedern.Unsere Revisionen führen zu folgenden neuen, angepaßten Altern: 354 Ma, Ende der Devon-Periode (= Devon-Karbon-Grenze); 364 Ma, Beginn der Famenne-Stufe; 369 Ma, Beginn der Frasne-Stufe; 376 Ma, Beginn der Givet-Stufe; 381 Ma, Beginn der Eifel-Stufe; 389 Ma, Beginn der Ems-Stufe; 393 Ma, Beginn der Prag-Stufe; und 400 Ma, Beginn der Lochkov-Stufe und der Devon-Periode. Die Dauer der Stufen sind demnach wie folgt: Famennium, 10 m.y.; Frasnium, 5 m.y.; Givetium, 7 m.y.; Eifelium, 5 m.y.; Emsium, 8 m.y.; Pragium, 4 m.y.; Lochkovium, 7 m.y.Die vorliegende Arbeit ist die erweiterte Fassung eines Vortrags (Sandberg & Ziegler 1996), der im Juli 1996 während der ECOS-VI Tagung der Pander Society in Warschau, Polen, gehalten wurde.

Implications of Latest Pennsylvanian to Middle Permian Paleontological and U-Pb SHRIMP Data from the Tecomate Formation to Re-dating Tectonothermal Events in the Acatlán Complex, Southern Mexico

Limestones in the highly deformed Tecomate Formation, uppermost unit of the Acatlán Complex, are latest Pennsylvanian—earliest Middle Permian in age rather than Devonian, the latter based on less diagnostic fossils. Conodont collections from two marble horizons now constrain its age to range from latest Pennsylvanian to latest Early Permian or early Middle Permian. The older collection contains Gondolella sp., Neostreptognathodus sp., and Streptognathodus sp., suggesting an oldest age limit close to the Pennsylvanian—Permian time boundary. The other collection contains Sweetognathus subsymmetricus, a short-lived species ranging only from Kungurian (latest Leonardian) to Wordian (earliest Guadelupian: 272 ± 4 to 264 ± 2 Ma). A fusilinid, Parafusulina c.f. P. antimonioensis Dunbar, in a third Tecomate marble horizon is probably Wordian (early Guadelupian, early Middle Permian). Furthermore, granite pebbles in a Tecomate conglomerate have yielded ~320-264 Ma U-Pb SHRIMP ages probably derived from the ~288 Ma, arc-related Totoltepec pluton. Collectively, these data suggest a correlation with two nearby units: (1) the Missourian—Leonardian carbonate horizons separated by a Wolfcampian(?) conglomerate in the upper part of the less deformed San Salvador Patlanoaya Formation; and (2) the clastic, Westphalian—Leonardian Matzitzi Formation. This requires that deformation in the Tecomate Formation be of Early—Middle Permian age rather than Devonian. These three formations are re-interpreted as periarc deposits with deformation related to oblique subduction. The revised dating of the Tecomate Formation is consistent with new data, which indicates that the unconformity between the Tecomate and the Piaxtla Group is mid-Carboniferous and corresponds to a tectonothermal event.

Evolution of Devonian carbonate-shelf margin, Nevada

The north-trending, 550-km-long Nevada segment of the Devonian carbonate-shelf margin, which fringed western North America, evidences the complex interaction of paleotectonics, eustasy, biotic changes, and bolide impact–related influences. Margin reconstruction is complicated by mid-Paleozoic to Paleogene compressional tectonics and younger extensional and strike-slip faulting. Reports published during the past three decades identify 12 important events that influenced development of shelf-margin settings; in chronological order, these are: (1) Early Devonian inheritance of Silurian stable shelf margin, (2) formation of Early to early Middle Devonian shelf-margin basins, (3) progradation of later Middle Devonian shelf margin, (4) late Middle Devonian Taghanic onlap and continuing long-term Frasnian transgression, (5) initiation of latest Middle Devonian to early Frasnian proto-Antler orogenic forebulge, (6) mid-Frasnian Alamo Impact, (7) accelerated development of proto-Antler forebulge and backbulge Pilot basin, (8) global late Frasnian semichatovae sea-level rise, (9) end-Frasnian sea-level fluctuations and ensuing mass extinction, (10) long-term Famennian regression and continent-wide erosion, (11) late Famennian emergence of Antler orogenic highlands, and (12) end-Devonian eustatic sea-level fall. Although of considerable value for understanding facies relationships and geometries, existing standard carbonate platform-margin models developed for passive settings elsewhere do not adequately describe the diverse depositional and structural settings along the Nevada Devonian platform margin. Recent structural and geochemical studies suggest that the Early to Middle Devonian shelf-margin basins may have been fault bound and controlled by inherited Precambrian structure. Subsequently, the migrating latest Middle to Late Devonian Antler orogenic forebulge exerted a dominant control on shelf-margin position, morphology, and sedimentation.

Stratigraphic and Economic Significance of Mississippian Sequence at North Georgetown Canyon, Idaho

Recognition of the Mississippian sequence exposed at North Georgetown Canyon, Idaho, as a facies belt largely different from those already known in ranges on the east and west adds significantly to knowledge of the Mississippian stratigraphy and petroleum geology in the Overthrust belt of Idaho, Wyoming, and Utah. In the newly recognized facies belt in the Aspen Range, the Madison Group is represented by the Lodgepole Limestone and Mission Canyon Limestone, but only the lower part of the latter is present. The greatly thinned Mission Canyon is the westernmost known occurrence of the formation. The overlying beds of middle Osagean to middle Chesterian age are a unique combination of deep-basin shallowing upward to peritidal lithofacies that are included in a new formation, the Aspen Range Formation. An organic-rich phosphatic member at the base of the Aspen Range Formation is a possible petroleum source rock. Also, westward thinning of the Mission Canyon Limestone into the Aspen Range and its absence farther west suggest a westward pinch-out that may provide stratigraphic traps for petroleum beneath a seal formed by the phosphatic member of the Aspen Range Formation.

Mississippian Continental Margins of Conterminous United States: ABSTRACT

The paleogeographic, paleotectonic, and paleobathymetric reconstruction of continental margins around the present western, southern, and eastern sides of the conterminous United States can be defined for a brief span (about 1.5 m.y.) of Mississippian time. Interpretations are made by applying a biostratigraphic and sedimentological model for the Deseret starved basin of Utah and Nevada to recently published shelf-margin studies. The time span is that of the middle Osagean anchoralis-latus conodont zone. Precise dating and paleobathymetric interpretations are based on the biostratigraphy and paleoecology of conodonts, and also of corals, calcareous and agglutinate foraminifera, and radiolarians. At this time, a shallow tropical sea covered most of the southern North Americ n craton and was the site for sedimentation of a broad carbonate platform. Surrounding this carbonate platform was a starved trough comprising several bathymetrically distinct starved basins. These starved basins lay on the inner (continentward) sides of foreland basins that were bordered on their outer sectors by orogenic highlands. The highlands formed in response to convergences or collisions with the North American continent by an oceanic plate to the west, by South America to the south, and by Africa and Europe to the east. During a eustatic rise of sea level that accompanied the orogenies and reached its maximum during the anchoralis-latus zone, the carbonate platform prograded seaward and carbonate sediments cascaded over the passive shelf margins to intertongue with thin carbonat slope deposits and very thin (~ 10 m) phosphatic basinal sediments. Simultaneously, thick (~ 500 m) flysch and deltaic terrigenous sediments, such as the Antler flysch on the west and the Borden and Pocono deltaic deposits on the east, were shed into the outer parts of the foreland basins from active margins along the orogenic highlands. This Mississippian reconstruction provides a unique opportunity for comparing and contrasting shelf-slope boundaries in parts of contemporaneous passive and active margins on three sides of Paleozoic continent. End_of_Article - Last_Page 933------------

New Biostratigraphic and Paleotectonic Interpretation of Devonian and Mississippian Rocks in Southwestern Montana Thrust Belt: ABSTRACT

The structurally complex area of previously undifferentiated Mississippian rocks below the Meramecian Kibbey Sandstone in the southwestern Montana thrust belt of the northern Tendoy Range actually comprises an unusual combination of Devonian and Mississippian basinal, slope, platform, and nearshore deposits. The lower part of this sequence comprised two widespread western Montana formations: the Devonian part of the Three Forks Formation over 65 m (213 ft) thick, containing all three members, and the basinal Kinderhookian and Osagean Paine Limestone, 228 m (748 ft) thick. However, the Paine is succeeded by an eastern tongue of the older, Osagean part of the lower-slope Middle Canyon Formation, 266 m (872 ft) thick. This tongue consists of clinoform cherty micrite and bedd d chert, with some encrinite debris flows that increase upward in number and thickness. The overlying Osagean and Meramecian Mission Canyon Limestone comprises 102 m (335 ft) of an upper-slope encrinite lower member and 74 m (243 ft) of a mainly shelf-margin wackestone and encrinite upper member. Thus, the Mission Canyon here represents the distal part of a broad carbonate platform. Succeeding the Mission Canyon is 140 m (459 ft) of the newly named Meramecian McKenzie Canyon Limestone, which comprises a sabkha, back-mound, and lagoonal sequence of evaporite-solution breccia, micrite, dismicrite, and pelmicrite, with interbeds of crinoidal wackestone, encrinite, and bio-oopelsparite. This formation represents beds absent at an unconformity elsewhere in western Montana and western Wyoming. Thus, the sequence from the Paine through the McKenzie Canyon, constituting the newly named Tendoy Group is 810 m (2,657 ft) thick and represents one of the most complete records of Kinderhookian to Meramecian carbonate deposition in the northwestern United States, displayed in the predominantly upward-shallowing part of a eustatic megacycle. The reconstructed detailed biostratigraphy aids new structural interpretations and provides several new plays for petroleum exploration in the Montana thrust belt. End_of_Article - Last_Page 865------------

Structural Geometry of Newly Defined Blacktail Salient of Montana Thrust Belt: ABSTRACT

Complexly imbricated Upper Devonian and Mississippian rocks in the northeastern Tendoy Mountains, Montana, form the previously unrecognized McKenzie thrust system, which is south of and structurally above the south-plunging Armstead anticline and north of the Tendoy thrust sheet. The northern margin of the McKenzie system, east of Garfield Canyon, displays a minimum of 4 mi (6 km) of eastward displacement. The southeastern margin is south of Kelmbeck Creek, near McKnight Canyon. The eastern edge of the system is buried under Quaternary to Late Cretaceous cover at or east of Red Rock Valley. East of the McKenzie system, the front of the Montana thrust belt extends north-northeast from Dell, Montana, to the eastern Blacktail Range, on the basis of unpublished mapping by J. . Haley and W. C. Pecora, Jr. The convex eastward curvature of the thrust belt in this area, including the McKenzie thrust system, is herein designated the Blacktail salient. Imbricates of the McKenzie thrust system comprise two duplex fault zones between Bell and McKenzie Canyons. The lower duplex involves a unique suite of platform to basinal Kinderhookian to lower Meramecian (Mississippian) carbonate rocks as well as Upper Devonian rocks. The End_Page 858------------------------------ floor thrust of this imbricate stack appears to lie within the Upper Devonian Three Forks Formation; the roof thrust lies within the middle Meramecian Kibbey Sandstone. The upper duplex involves Upper Mississippian rocks above the Kibbey Sandstone. Its roof thrust closely follows bedding near the top of the Mississippian sequence. The geometry of imbricate stacks within the McKenzie plate demands shortening of greater than 100%, resulting in at least 2 mi (3 km) additional eastward displacement of its trailing edge. Recognition of the Blacktail salient with its complex structural patterns and unusual platform to basinal carbonate sequence provides new exploration targets in the southwestern part of the Montana thrust belt. End_of_Article - Last_Page 859------------

Mississippian Shelf Margin and Carbonate Platform from Montana to Nevada: ABSTRACT

The Kinderhookian to middle Meramecian history of a carbonate platform and shelf margin, extending from Nevada to Montana, is documented through four time-rock correlation charts and five successive maps that are synchronized by foraminiferan, conodont, and coral zonations. The platform was bordered on the west by a starved basin, a flysch trough, and orogenic highlands. The history of platform development is an integral part of the sedimentary cycle of the deep-water Deseret starved basin. Antler orogenic activity produced epeirogenic movements on the craton, which affected sea level and caused episodic progradation and retreat of the carbonate shelf margin. The sequential history is: (1) in earliest Mississippian time a narrow, northeast-trending seaway bordered by low oastal plains received mostly fine clastic sediments; (2) during late Kinderhookian time, a carbonate platform and shelf margin formed as a result of eastward expansion of the seaway; (3) during early Osagean time, the shelf margin retreated and a broad, gentle (less than 0°5^prime) clinoform ramp developed; (4) during middle Osagean time, lowering of the basin and craton and rise of sea level changed the pattern and sedimentary regime of the carbonate platform. Progradation of the shelf margin over the former ramp resulted in maximum expansion of the platform concurrent with maximum deepening of the starved basin. The foreslope attained a maximum steepness of 5°; (5) in middle Meramecian time, uplift of the craton and lowering of sea level caused shoaling of the carbonate plat orm and development of a sabkha landward. With increased uplift a karst plain developed over most of the former carbonate platform, and some cratonic sands were transported westward by streams into the basin. Meanwhile, filling of the flysch trough allowed eastward spillover of distal-flysch sediments to almost completely fill the basin. End_of_Article - Last_Page 716------------

Use of Devonian Conodonts in Petroleum Exploration, Western United States: ABSTRACT

Forty-eight Devonian conodont zones are now recognized worldwide: 11 zones in the Lower Devonian, 10 zones in the Middle Devonian, and 27 zones in the Upper Devonian. Only five of these zones have not yet been recognized in the western United States. Conodonts, which range from Cambrian to Triassic, attained their maximum faunal diversity and abundance during the Late Devonian, when each conodont zone lasted about 0.5 m.y. Because each zone represents such a short interval of geologic time, conodonts provide an indispensable tool for petroleum exploration in the Rocky Mountain, Overthrust belt, and Great Basin regions. Conodont color-alteration index (CAI) values have been used successfully to predict cool areas of potential production in otherwise thermally overcooked regions. Their use has also been demonstrated as follows. 1. Conodonts, by providing virtual time planes, suggest the complex relations between source beds in the Pilot basin and reservoir rocks in the enclosing carbonate platform. 2. Conodonts have been used to determine rates of sedimentation of source rocks and other synorogenic sediments of the Antler orogeny. These rates range from 1 to 400 m/m.y. They also demonstrate that deposition was episodic and that there were times and areas of nondeposition within marine basins. 3. Conodonts precisely date Antler orogenic events, which governed eustacy and caused marine transgressions and regressions on the craton. For example, the emplacement of the Roberts Mountains thrust took approximately 8 m.y. 4. Conodonts are used for biofacies analysis and for paleotectonic reconstructions. For example, within the Late Devonian Polygnathus styriacus Zone, eight different conodont biofacies, with almost mutually exclusive faunas, have been used to reconstruct five paleotectonic settings ranging from peritidal to offshore pelagic. End_of_Article - Last_Page 780------------