Early Paleocene Time Research Articles

Cenozoic evolution of the Faroe Platform: comparing denudation and deposition

Abstract Throughout Paleocene and Eocene time the Faroe-Shetland Channel and the eastern part of the Faroe Platform was a subsiding marine basin. In Early Paleocene time, basin-floor fans of a British provenance were deposited in the eastern part of the basin. In Late Paleocene time, c. 6 km of basalt entered the basin from the west and north, and the basin was constricted by the large volumes of basalt that entered the basin, creating the Faroe-Shetland Escarpment. In Eocene time subsidence continued in the basinal areas. Again, sediments of a dominantly eastern provenance were deposited. Throughout Eocene time, erosion products from the Faroe Platform were possibly deposited in the Faroe Bank Channel and the Norwegian Sea Basin, but only to a limited degree in the Faroe-Shetland Channel. The oldest sediments of documented western provenance on the eastern margin of the Faroe Platform are of Early Oligocene age. During a compressional phase commencing in Mid-Late Miocene time some basinal areas emerged and erosion took place on the top of emerged anticlines. However, denudation throughout Late Miocene and Early Pliocene time was apparently rather limited compared with a Late Pliocene phase of denudation. During this phase of denudation, a large progradational wedge was deposited on the eastern margin of the Faroe Platform. On the basis of a structural analysis of the Faroe Platform, the amount of basalt removed from it during Cenozoic time is estimated to be c. 46 000 km 3 (131 100 × 10 12 kg). Using 2900 kg m −3 as the density of basalt and 2300 kg m −3 as sediment density the estimated amount of removed basalt is in fair agreement with the estimate of the volume of sediments derived from the platform ( c. 56 000 km 3 , 114 800 × 10 12 kg). The greatest deposition rates on the eastern Faroe Platform and in the Faroe-Shetland Channel apparently occurred after two distinct inversion or compression events in Mid-Eocene and Mid-Late Miocene time. However, uplift of the Faroe Platform could have been forced by denudation rather than endogenous processes.

Development of the continental margins of the Labrador Sea: a review

Abstract The Labrador Sea is a small oceanic basin that developed when the North American and Greenland plates separated. An initial period of stretching in Early Cretaceous time formed sedimentary basins now preserved under the continental shelves and around the margins of the oceanic crust. The basins subsided thermally during Late Cretaceous time and a second episode of tectonism took place during latest Cretaceous and early Paleocene time, before the onset of sea-floor spreading in mid-Paleocene time. Around the northern Labrador Sea, Davis Strait and in southern Baffin Bay, voluminous picrites and basalts were erupted at and shortly after the commencement of sea-floor spreading. Volcanism occurred again in early Eocene time at the same time as sea-floor spreading commenced in the northern North Atlantic. Farther southeast, along the Labrador and southern West Greenland margins, oceanic crust is separated from continental crust by highly stretched but non-magmatic transition zones which developed before sea-floor spreading. A complex transform zone, which developed during sea-floor spreading in late Paleocene and early Eocene time, separates continental and oceanic crust along the Baffin Island margin. The Greenland and Labrador ocean-continent transitions are asymmetric across the only available conjugate cross-sections. However, a cross-section through the Labrador margin farther north resembles the Greenland cross-section in the conjugate pair more than it does the Labrador cross-section of this pair. Consideration of the geological history of the area suggests that the non-magmatic transition zones may have formed by slow extension of a few millimetres per year through a period of 53 Ma during Cretaceous and early Paleocene time.

Carbonate Platform to Basin Transition along an Upper Cretaceous to Lower Tertiary Syrian Arc Uplift, Galala Plateaus, Eastern Desert of Egypt

ABSTRACT Biostratigraphic and facies analyses were made on Upper Campanian to Lower Eocene strata along a 58-kilometer-long dip transect across the Northern and Southern Galala plateaus, west of the Gulf of Suez. The analyses enabled us to reconstruct the evolution of a platform–slope–basin transition that is roughly parallel with the trend of the plateaus. We interpret individual sedimentary cycles as processes of a southward-prograding carbonate platform that developed along a branch of the Syrian Arc Fold Belt. The Northern Galala area was a structural high as shown by hiatuses spanning Late Cretaceous (Coniacian) to Early Paleocene times. To the south, carbonate platform progradation is evident from the Late Campanian onward. Late Cretaceous platform-derived slope deposits interfinger with basinal chalks and calcareous shales farther south. Early Tertiary carbonates were deposited in platform, slope, and basin settings. The evolution of the platform–slope–basin transition is documented by the changing large-scale depositional geometry. The evolution occurs within sedimentary sequences that are interpreted by means of a high-resolution biostratigraphic framework. The facies architecture reflects the evolution from a rimmed shelf (Late Cretaceous) to a distally steepened ramp (latest Cretaceous to Paleocene) and eventually to a homoclinal ramp (Early Eocene). The reconstruction of the imprint of fluctuating sea levels on the sedimentary record has been used to establish comparisons with age-equivalent cycles from neighboring regions.

An early Paleocene palaeanodont (Mammalia, ?Pholidota) from New Mexico, and the origin of Palaeanodonta

ABSTRACT We describe a skeleton from the late early Paleocene (Torrejonian) of the San Juan Basin, New Mexico, which represents the oldest and most primitive known member of the Palaeanodonta. It is here named Escavadodon zygus, gen. et sp. nov., and is assigned to the new family Escavadodontidae. The specimen is one of the most complete skeletons of a small mammal known from the San Juan Basin Paleocene. The skeleton combines fossorial postcranial adaptations closely similar to, but somewhat less specialized than, those of early Eocene palaeanodonts, with teeth reminiscent of those of Leptictidae and to a lesser extent Pantolestidae. It thus appears to be morphologically intermediate between Leptictidae and Palaeanodonta, suggesting that Palaeanodonta evolved from Leptictidae or a leptictid-like mammal, and pushing back the origin of Palaeanodonta to at least early Paleocene time.

Remanent magnetization of oolitic ironstone beds, Hazara area, Lesser Himalayan thrust zone, Northern Pakistan: Its acquisition, timing, and paleoenvironmental implications

Chamosite-hematite type oolitic ironstone is distributed in the Cretaceous-Tertiary (K-T) boundary zone of the Hazara area, Lesser Himalayan thrust zone, in the northwestern margin of the Indo-Pakistani subcontinent. A total of 52 oriented samples were collected from 6 beds in two open pit mines, Nathia Gali and Bagnotar. Ferromagnetic properties of the oolitic ironstone are dominated by fine-grained hematite that possesses a characteristic remanent magnetization (ChRM). The ChRM is a chemical remanent magnetization (CRM) acquired by the hematite which formed from Fe-rich initial materials (iron hydroxides) during early diagenesis of the ironstone bed. The direction of the remanent magnetization indicates that the magnetization was acquired in an equatorial region (average inclination = 2°, paleolatitude = 1°N), which is consistent with the paleoposition of the Indian subcontinent during early Paleocene time, as estimated by previous data. The initial Fe-rich sediments were deposited under anoxic conditions, probably in a non-marine or brackish environment. During early diagenesis, after development of oolitic textures, the paleoenvironment became arid, where the CRM was acquired through the conversion of amorphous hydroxides or goethite to fine-grained hematite. The paleomagnetic results reveal complicated local rotational movements of oolitic ironstone blocks about vertical axes, which might be a result of intense thrusting within the terrane after early Tertiary collision between India and Asia.

Marine Cretaceous-Paleogene biofacies and ichnofacies in southern Tibet, China, and their sedimentary significance

The study of biofacies is of great significance not only for biostratigraphy, but also for the interpretation of sedimentary environments and the reconstruction of paleogeography. The areas of Tüna, Gamba and Tingri of southern Tibet, China, were located in the north margin of the Indian Plate in the Cretaceous and Paleogene. The marine Cretaceous and Paleogene biofacies there can be divided into four groups using the following criteria: (a) the benthic biota and biogenic structures in the sediments; (b) the paleoecology of the main fossils; (c) the genesis of fossils. The four groups are euxinic biofacies, nektonic and planktic biofacies, benthic biofacies and redeposited biofacies, which include 24 main biofacies types. Like biofacies, ichnofacies are important indicators of paleoenvironments. The Cretaceous-Paleogene trace fossils in the Tüna and Gamba areas are attributable to Nereites, Zoophycos, Cruziana and Skolithos ichnofacies. Nereites ichnofacies indicates an open marine basin environment. Zoophycos and Cruziana ichnofacies indicate shelf margin and open sea shelf environments. The Skolithos ichnofacies reflects the environments of sand barrier deposited in the intertidal zone and the upper part of subtidal zone. The Zoophycos ichnofacies can further be subdivided into Thalassinoides-Planolites and Zoophycos-Chondrites subichnofacies which reflect aerobic and disaerobic conditions, respectively. The development of biofacies and ichnofacies in vertical profiles shows the sedimentary history from the Early Cretaceous to the Paleogene in Tüna, Gamba and Tingri areas. A regression occurred in Tuna and Gamba from the Neocomian to the Early Paleocene. The sedimentary environments went from euxinic basin and open marine basin, shelf margin, open sea margin, platform slope, platform margin, open marine platform to the sand barriers deposited in the intertidal zone or the upper part of subtidal zone. A transgression took place at the beginning of the middle Paleocene. The carbonate deposits formed in the open marine platform and open sea shelf environments overlay the sandstones of the Jidula Formation. The termination of marine deposits in these areas began in the Thanetian. The sedimentary history in the Tingri area differed from that in the Tüna and Gamba areas especially during middle Maastrichtian to Early Paleocene times. The sedimentary process in Tingri during this period took place in the lower part, the middle part of a fan and seaward prograding delta plains. The marine deposition in the Tingri area ended in Lutetian later than in the Tuna and Gamba areas.

Apatite fission-track evidence for Laramide and post-Laramide uplift and anomalous thermal regime at the Beartooth overthrust, Montana-Wyoming

The crystalline rocks of the Beartooth overthrust in southwest Montana and northwest Wyoming were uplifted and thrust over the northwest margin of the Bighorn basin during the Laramide orogeny. The timing and geometry of that event is well documented in the synorogenic Paleocene and early Eocene sedimentary rocks that were deposited ahead of the rising Beartooth basement block and partially over-ridden as the block advanced. The absence of post-Laramide sedimentary rocks on the block and in the adjacent Bighorn Basin has, however, precluded the reconstruction of the post-orogenic tectonothermal history of this region. We report here the results of fission-track analysis of apatite from the three components of the Beartooth over-thrust with the aim of reconstructing the Laramide and post-Laramide tectono-thermal history of the southeast margin of the Beartooth Mountains. Apatite fission-track ages and corresponding horizontal confined track length distributions (HCTLDs) from Precambrian basement rocks constituting the upper plate of the Beartooth overthrust indicate that from 7 to 12 km of uplift of the Red Lodge corner of the Beartooth block has occurred since early Paleocene time. This amount of uplift occurred in two stages, with an intervening mid-Tertiary period of tectonic quiescence. The latter was a period of either (1) Oligocene and/or Miocene deposition or (2) tectonic quiescence. Uplift of 4 to 8 km occurred during the first phase of cooling, which lasted from early Paleocene time (∼61 Ma) to early Eocene time (∼52 Ma). During the second phase, which began in late Miocene-early Pliocene time and continues to the present day, about 4 km of uplift occurred. Our fission-track data suggest that the thermal regime in rocks above the Beartooth overthrust was relatively stable during Tertiary time. The maximum geothermal gradient permitted by model thermal histories generated from our observed fission-track data during post-Laramide time is 17 °C/km. This value is the same as that of the present-day geothermal gradient measured in the Amoco Beartooth well and suggests that a low geothermal gradient prevailed throughout Tertiary time. Apatite fission-track ages and HCTLDs from Jurassic and Cretaceous sedimentary rocks beneath the Beartooth overthrust indicate that these rocks have remained 10-20 °C cooler than overlying rocks in the shear zone and the lowermost part of the upper plate since ∼61 Ma, an observation that is not consistent with the stratigraphic position of these rocks. We interpret this temperature to be the result of a persistent thermal regime in which ground water circulating through the sedimentary rocks of the lower plate cooled them and insulated them from conductive heat transfer from hotter overlying rocks; this ground-water circulation may have been responsible for flushing hydrocarbons out of the rock column explored by the Beartooth well. Shear-zone rocks experienced a higher temperature during Cenozoic time than did rocks of the upper and lower plates; this condition was maintained by flow through the shear zone of ground water heated to higher temperatures at deeper levels along the thrust.

Tectonic development of Upper Cretaceous to Eocene strata of southwestern Utah

Upper Cretaceous to Paleogene nonmarine sedimentary rocks of southwest Utah record Sevier foreland basin sedimentation, Laramide-style folding and intermontane sedimentation, and cessation of Laramide deformation. The formations that record this tectonic evolution are, from oldest to youngest, the Iron Springs, Kaiparowits, Canaan Peak, Grand Castle (informal name), Pine Hollow, and basal part of the Claron. The upper part of the Santonian to lower Campanian(?) Iron Springs Formation represents synorogenic, fluvial deposits derived from the Wah Wah and Blue Mountain thrust sheets of southwestern Utah. The middle to upper Campanian Kaiparowits and upper Campanian(?) to lower Paleocene Canaan Peak Formations are an upward-coarsening sequence derived from southeastern California and southern Nevada. Initial Laramide-style deformation occurred during latest Cretaceous or early Paleocene time, influencing the depositional pattern of the Canaan Peak fluvial system. The lower Paleocene Grand Castle formation represents an east- to southeast-flowing, braided-river system with the same source as the Iron Springs Formation (the Wah Wah and Blue Mountain thrust sheets). Conglomerate of Grand Castle onlaps the easternmost Sevier thrusts and is folded by Laramide structures. Although strata of the Grand Castle formation represent post-thrust and, in part, pre-Laramide deposition, initial development of a south-southwest-trending, Laramide-style upwarp controlled the geometry of the Grand Castle basin. The lower Paleocene to middle Eocene Pine Hollow Formation unconformably overlies post-Sevier conglomerates and records Laramide partitioning of the foreland basin. The Pine Hollow basin received sediment from both the west and northeast, and it is associated with development of the Johns Valley anticline and possibly the Circle Cliffs uplift. Small alluvial fans developed on the limbs of these structuresand graded laterally into playa mudflats. Fluvial, deltaic, and lacustrine deposits of the basal part of the Claron Formation are coeval with the Pine Hollow Formation, but they developed in a separate basin to the southwest. Lacustrine deposits of the Claron Formation overlap paleotopographic highs of the Pine Hollow basin and indicate cessation of Laramide deformation by middle Eocene time.

Sedimentation and tectonism in the Laramide tornillo basin of West Texas

The Tornillo Basin of west Texas and adjacent Mexico is the southernmost Laramide-age intermontane basin in the North American cordillera. The basin is bounded on the west by the Chihuahua Tectonic Belt, a southern extension of the Cordilleran Fold and Thrust Belt, and on the east by the Marathon Uplift, a Laramide-style basement-cored uplift. Sedimentological evidence indicates that tectonism began in this region during Late Cretaceous time (ca. 70 Ma) and ended during middle Eocene time (ca. 50 Ma). The basin experienced two major pulses of fluvial sedimentation, the first during middle to late Maastrichtian time, and the second during late Paleocene to early Eocene time. High sedimentation rates during these periods correspond to the influx of coarse extra-basinal detritus, high mudstone content in the basin alluvium, and thick tabular fluvial sand bodies. These episodes probably reflect pulses of tectonism. A lull in sedimentation during early Paleocene time resulted in a thin sequence of alluvium with very high mudstone content, thin lenticular fluvial sand bodies, and well-developed paleosols. The basin experienced left-lateral transpressive deformation during the second phase of tectonism. The sedimentary sequence in the Tornillo Basin is compatible with the two-phase model of Laramide tectonism developed for the central and southern Rocky Mountains.

Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska

The Trans‐Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra‐mafic‐mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid‐ to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid‐Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite‐pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc‐derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time. A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism.

Shale Diagenesis in the Bergen High Area, North Sea

AbstractIllite diagenesis in Tertiary and Mesozoic shales in the Bergen High area, northern North Sea, was studied using mineralogic, isotopic, and computerized thermal modeling techniques. The Tertiary shales are dominated by smectite, with lesser amounts of illite, kaolinite, and chlorite. At present burial temperatures of >70°C smectite is absent, and the shales contain abundant lath-shaped illite which yields a mixed-layer illite/smectite (I/S) X-ray powder diffraction (XRD) pattern. Transmission electron microscopy (TEM) indicates that the illite laths increase in abundance and thickness with increasing depth; XRD patterns indicate a progressive increase in the illite component of the I/S. The deepest samples were found to contain long-range ordered (R=3) I/S, which showed platy particle morphology with the TEM. K-Ar ages of most of the <0.1-μm-size illite separates imply that illitization was a relatively brief event affecting a thick sequence of sediments during late Cretaceous to early Paleocene time (65–87 Ma); however, measured ages were affected by trace levels of detrital Ar contamination and do not represent the true age of diagenesis.Several methods of quantifying Ar contamination were used to correct measured ages to obtain a reasonable estimate of the true age of diagenesis. The corrected ages are imprecise due to uncertainties in quantifying the levels of sample contamination, but generally suggest a Paleogene (38–66 Ma) period of illitization. In contrast, simple kinetic models of smectite-illitization suggest much younger ages of diagenesis (0–40 Ma at the Veslefrikk field; 0–60 Ma at the Huldra field). The timing of the diagenesis and the morphologic aspects of the authigenic illite suggest that illite precipitated before late Tertiary compaction and resulted in a decrease in fluid permeability. Low trapping efficiency of early Tertiary sediments, vertical escape of warm fluid from the Brent sandstone, and high heat flow may have promoted illite diagenesis in the shales prior to deep late-Tertiary burial.

Note on a further occurrence of Soudanella laciniosa Apostolescu in northeastern Brazil

A new occurence of the West African (Mali, Nigeria) ostracod species Soudanella laciniosa Apostolescu from the Lower Paleocene Maria Farinha Formation of northeastern Brazil is reported on briefly. This confirms and strengthens evidence for interfaunal exchange between West Africa and Brazil in early Paleocene time. The genus Soudanella occurs in Libya, Mali, Nigeria, Argentina and Brazil.

Overthrust emplacement of New Caledonia Ophiolite: Geophysical evidence

Geophysical studies support inferences from outcrop geology that during the Late Eocene an ophiolite sheet exposed on New Caledonia was thrust southward over a rock complex consisting of sedimentary, volcanic, and metamorphic rocks that range in age from Pre‐Permian to Eocene. The outcropping ultramafic complex consists of a layered sequence, approximately 3000 m thick, including harzburgite, dunite, wehrlite, serpentinite, and gabbro. The absence of pillow basalts and sheeted dikes on land suggests that during or after overthrusting, these units were removed either by thrust faulting or by erosion. Seismic refraction profiles, collected over the adjacent Loyalty Basin, show that the lower crust and upper mantle are characterized by velocities of 4.7, 5.8, 6.8, and 8.1–8.4 km/s, suggesting the presence of a complete oceanic section that includes sediment, basalt, gabbro, and peridotite. The lower crust is probably 6.5 km thick beneath the Loyalty Basin. A seismic reflection profile that extends across the Loyalty Basin is interpreted to show the oceanic crust rising toward New Caledonia. We suggest that the ophiolite on New Caledonia is continuous with this rising oceanic crust. Studies of gravity anomalies observed both on land and offshore indicate the presence of a short‐wavelength, high‐amplitude (+180 mGal), asymmetrical, free‐air gravity anomaly along the northeastern coast of New Caledonia. A gravity anomaly profile, calculated for a geological model characterized by a 10‐km‐thick slab of oceanic crust and mantle material extending continuously from the ophiolite on New Caledonia to the oceanic crust of the Loyalty Basin, successfully matches the observed gravity data. We interpret the extension of the high‐gravity anomaly over the whole length of the eastern lagoon as evidence for the lateral extension of the root zone of the ultramafic complex. A tentative geodynamic reconstruction suggests that the proto‐Loyalty Basin crust formed a marginal basin along the eastern margin of Gondwanaland prior to Upper Cretaceous time; after the opening of the New Caledonia Basin during Upper Cretaceous to early Paleocene time a subduction zone developed along the western margin of New Caledonia. While this zone was active, the Loyalty Basin lithosphere overthrust New Caledonia in a southeasterly direction and locally rotated clockwise. This compressional tectonic event was terminated by upper Eocene time.

Upper Cretaceous–Paleocene biostratigraphy and magnetostratigraphy, Hell Creek and Tullock Formations, northeastern Montana

Fossils from the Hell Creek and Tullock Formations in northeastern Montana provide detailed documentation of terrestrial faunal and floral evolution during latest Cretaceous (Lancian) and early Paleocene (Puercan) time. Here the replacement of Lancian faunas by those of Puercan age, most obviously signaled by the extinction of dinosaurs, and the changes in pollen floras sometimes used to mark the Cretaceous-Tertiary boundary occurred during a period of reversed magnetic polarity. Paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the Red Deer Valley area, Alberta, during the same period of reversed polarity. Furthermore, also on the basis of paleontological correlations, the extinction of dinosaurs in the San Juan Basin, New Mexico, appears to have occurred either during the same period of reversed polarity or, possibly, during the preceding period of normal polarity.

Towards a paleogeography and tectonic evolution of Iran

Maps of the paleography of Iran are presented to summarize and review the geological evolution of the Iranian region since late Precambrian time. On the basis of the data presented in this way reconstructions of the region have been prepared that take account of the known major movements of continental masses. These reconstructions, which appear at the beginning of the paper, show some striking features, many of which were poorly appreciated previously in the evolution of the region. They include the closing of the 'Hercynian Ocean' by the northward motion of the Central Iranian continental fragment(s), the apparently simultaneous opening of a new ocean ('the High-Zagros Alpine Ocean') south of Iran, and the formation of 'small rift zones of oceanic character' together with the attenuation of continental crust in Central Iran.With the disappearance of the Hercynian Ocean, the floor of the High-Zagros Alpine Ocean started to subduct beneath southern Central Iran and apparently disappeared by Late Cretaceous – Early Paleocene time (65 Ma). From this time the compressional motion between Arabia and Eurasia has been accommodated in Iran by shortening and thickening of the continental crust. This crustal thickening is accompanied by a progressive, though eventful, transition from marine to continental conditions over the whole region.A striking feature highlighted in this study is the existence of extensive alkaline and calc-alkaline volcanics, which appear to be unrelated to subduction. The intrusion of these rocks started in Middle Eocene time (45 Ma) and extended to the present. It is clear that some major fault systems have played a continuous but varied role from the Precambrian until the present, and whatever controlled the original fold orientation at the onset of continental compression (65 Ma) apparently still controls the orientation of contemporary folding.

Tertiary Sea-Level Movements around Southern Africa

Sedimentological, micropaleontological and seismic-profiling data elucidate the history of Tertiary sea-level movements around southern Africa. These new data show that landward movement of the sea began in early Paleocene time and continued into the early Eocene. The sea probably reached its maximum Paleocene height during the early Eocene, and is today represented by outcrops up to at least 204 m, and probably as high as 360 m, above modern sea level. A brief regressive pulse occurred during the middle Eocene, with renewed transgression in the late Eocene. A major regression followed, probably spanning all of Oligocene and early Miocene times. This regression exposed much of the continental shelf and is clearly represented on seismic-reflection profiles by a widespread unconformity. The major Neogene transgression began in the middle Miocene and probably reached its greatest extent in the late Miocene or early Pliocene. The overall middle Miocene to early Pliocene transgression was interrupted by a brie...

Tertiary Sea-Level Movements Around Southern Africa: ABSTRACT

Sedimentologic, micropaleontologic, and seismic-profiling data elucidate the history of Tertiary sea-level movements around southern Africa. These new data show that landward movement of the sea began in early late Paleocene time and continued into the early Eocene. The sea probably reached its maximum Paleogene height during the early Eocene, and is today represented by outcrops up to at least 204 m, and probably as high as 360 m, above sea level. A brief regressive pulse occurred during the middle Eocene, and renewed transgression in the late Eocene. A major regression followed, spanning all of Oligocene and early Miocene times. This regression exposed much of the continental shelf. It is clearly represented on seismic-reflection profiles as a widespread unconformity. > The major Neogene transgression began in the middle Miocene but probably only reached the present coastline by late Miocene time. This transgression continued into the early Pliocene, but was interrupted by a brief regressive pulse in the earliest Pliocene. Seas withdrew again in the late Pliocene. Units deposited during the Miocene-Pliocene transgression are today found up to at least 300 m above sea level. This scheme should be viewed as showing only the gross movements of the seas around southern Africa during the Tertiary. Local subsidence or uplift may have caused one area of the coast to submerge or emerge earlier than another area. Nevertheless, the timing of these southern African transgressions and regressions closely parallels the timing recently established for sea-level movements in other parts of the world. End_of_Article - Last_Page 529------------

Big Piney-La Barge Producing Complex, Wyoming: ABSTRACT

The Big Piney-La Barge complex has been the major gas producing area in Wyoming since 1957, and during the past 10 years it also has become a leading oil producing area. Recoverable natural gas is estimated at 3 Tcf and recoverable oil exceeds 100 million bbl. The area is along the western margin of Wyoming's Green River basin, occupying a zone of transition between the overthrust belt and the basin. This position of transition typifies much of the depositional history for this part of Wyoming, whose stratigraphic record reflects both the influence of the craton on the east and the Cordilleran geosyncline on the west. Productive reservoirs include the Triassic-Jurassic Nugget Sandstone, the Cretaceous Dakota, Frontier, and Mesaverde Formations, and sandstones of Paleocene age. Trapping mechanisms are diverse, and structural, structural-stratigraphic, and purely stratigraphic accumulations are represented. Commencing in Late Triassic and extending into Early Jurassic time, the Nugget was deposited as a massive blanket of largely eolian sand. In Early Jurassic time, seas spread southward across the Cordilleran front and the Wyoming craton during a major marine pulse that deposited the Twin Creek Limestone on the west, and Gypsum Springs and Sundance Formations on the east. The Gypsum Springs appears to be recognizable, but the Twin Creek terminology is better applied at La Barge. Jurassic time closed with continental deposition of the Morrison sequence. On the Wyoming shelf the transition from Jurassic to Cretaceous is commonly a straightforward transition from continental Morrison through transitional Lakota-Dakota and into marine Thermopolis Shale. La Barge appears to have been a hingeline in Early Cretaceous time and relations are not clear cut. All shelf units are recognizable and shelf terminology may be applied on the east side of the complex, but in a distance of 12 mi these units become nearly unrecognizable. In early Paleocene time the platform underwent folding, thrusting, and erosion. The Hogsback thrust was moving eastward and the newly formed La Barge anticline was deformed further. North of the platform a major drainage system developed that flowed southward and southeastward through the Green River basin. As Paleocene progressed, the Hogsback thrust reached its present position and aggradation commenced across the Big Piney-La Barge area. Deposition was dominated by a coarse, commonly conglomeratic facies adjacent to the Hogsback thrust, which graded abruptly through an alluvial, piedmont, and small stream fluvial unit. This low-energy facies graded basinward into a high-energy fluvial and paludal facies End_Page 793------------------------------ of the large river environment. As the basin subsided further, the major drainage facies encroached onto the flanks of the La BArge platform, resulting in buildups of thick channel, bar, and point-bar sands into the lower energy muds and silts. It is at this transition between the high- and low-energy environments the Paleocene is most commonly productive. In early Eocene time the disturbed belt gave a last shudder, reflected by the La Barge thrust and other faults that break Paleocene strata. Of particular interest are two east-dipping thrusts, and the tendency for the young thrusts to terminate as tear faults which assist in controlling accumulation. Tectonic activity since early Eocene time has been confined primarily to regional elevation and slight basinward tilting. The Frontier sandstones are the major gas reservoirs on the platform, and demonstrate a marked thinning from west to east. The second Frontier appears to reflect parallic depositional environments and strikingly variable reservoir conditions. Following deposition of the second Frontier, transgression resulted in the deposition of a few thousand feet of marine shale, except over the western part of the La Barge area, where a major drainage from the west built a delta into the shallow Coloradan sea. This unit is locally called the first Frontier. Regression from west to east in Montanan time deposited the fluival and paludal sediments of the Mesaverde Formation. Littoral transitional sands at the base of the Mesaverde commonly are productive both on subcrop and across structure. Isopach maps of Cretaceous units suggest the existence of the Moxa arch, a structural feature that now plunges southward from the La Barge platform. It was a northward-plunging positive feature during Cretaceous deposition. In latest Cretaceous and early Paleocene times, the Big Piney-La Barge area was uplifted and folded into a large anticlinal feature, with erosion stripping away newly deposited Mesaverde. End_of_Article - Last_Page 794------------

Rice Valley Outlier—New Sequence of Cretaceous-Paleocene Strata in Northern Coast Ranges, California

Lower Cretaceous through Paleocene strata are exposed in a small outlier within the Eastern Belt of the Franciscan Complex (Berkland and others, 1972) near Rice Valley in the northern Coast Range of California. This outlier, and three others known in the region, are synclinally folded, downfaulted remnants of an upper thrust plate of Great Valley sequence rocks. The Rice Valley synclinal remnant is separated by faults from surrounding rocks of the Franciscan Complex. Two belts of serpentinized dunite and harzburgite, converging toward the south, border the Rice Valley sequence on the east, west, and south. The northern border is a reverse fault that brings blueschist facies metaclastic rocks of the Eastern Belt over unmetamorphosed, mildly deformed strata of the Rice Valley sequence. Basal beds of the sequence are Lower Cretaceous (Hauterivian) sandstone, shale, and limestone that are steeply tilted and faulted against the ultramafic rocks. About 3,500 ft of Cretaceous marine strata are unconformably overlain by more than 1,150 ft of shallow-marine lower Tertiary beds. Fossils are generally scarce throughout the sequence, but are abundant in limestone-rich horizons of Hauterivian and Cenomanian age as well as in a calcareous sandstone bed with more than 30 genera of late Paleocene (Meganos Stage) invertebrates. Special significance is attached to a 150-ft-thick bed of polished-pebble conglomerate which underlies the Paleocene fossils. The conglomerate is composed chiefly of Franciscan detritus, with pebbles of red radiolarian chert, lawsonitic metagraywacke, and serpentinite. This is the earliest known appearance of coarse Franciscan debris in an upper plate (Great Valley sequence) environment. Time restrictions on underthrusting and isostatic rebound of deeply subducted Franciscan rocks imply rapid rates of uplift and denudation during early Paleocene time. The suggested rate of 240 cm per 1,000 yrs is comparable with the highest known rates in the world today.

Electrical Accessory Curves--New Tool for Subsurface Correlation: ABSTRACT

Electrical accessory curves facilitate recognition of subsurface formations from electrical logs and more accurate physical correlation of rock units. Two types of electrical accessory curves, which show the variation in percentage of thickness of sandstone (SP), and low resistivity mudstone as a function of depth, have been used to examine Laramide deformation in the Powder River basin of Wyoming during deposition of the thick (8,000 ft), nonmarine Late Cretaceous and Tertiary deposits which filled the basin. The Laramide orogeny did not start in this part of Wyoming in late Maestrichtian, during deposition of the Lance Formation, and was only weakly active in early Paleocene time, during deposition of the Tullock Formation. Strong deformation started in middle Paleocene with subsidence along the axis of the basin and deposition of fine-grained Lebo mudstones. Other Laramide structures associated with basin deformation probably were started at this time. Strong subsidence continued into late Paleocene when coarse clastics of the Tongue River Formation first were deposited, indicating uplift and erosion of the adjacent mountains. Deformation continued through part of the Eocene, but ceased before Oligocene time. Cross sections using electrical accessory curves illustrate their use in other thick nonmarine rock sequences and for detailed studies of subtle marine shale correlations. Electrical accessory curves are a new tool to help solve difficult subsurface rock correlation problems and they should be helpful in many other areas. End_of_Article - Last_Page 612------------