Early Oligocene Time Research Articles

Late Eocene and Early Oligocene Carbonate Sedimentation in the Deep Sea: ABSTRACT

The transition from carbonate-poor late Eocene sediments to carbonate-rich early Oligocene sediments undoubtedly reflects a major reorganization of the way in which carbonate was fractionated between shelf regions and deep basins. During Eocene time carbonate was being deposited on the shelves marginal to the extensive Tethys Sea, whereas in Oligocene time carbonate sedimentation on the continental shelves decreased, but increased in the deep sea. In the deep sea, this facies change represents a lowering of the carbonate compensation depth (CCD) with the magnitude of the drop varying in a systematic fashion from one ocean basin to another. The CCD drop was most dramatic in the equatorial Pacific, sinking nearly 1,500 m (4,900 ft) in less than 2 m.y. Both the magnitude and abruptness of this drop in the CCD decrease steadily away from the equatorial Pacific. In the South Pacific and South Atlantic, the CCD drop across the Eocene/Oligocene boundary was on the order of 1,000 m (3,280 ft) and 750 m (2,460 ft) respectively. Similarly, in the North Atlantic, not only was the magnitude of this drop reduced to only several hundred meters, but it appears to have occurred over a prolonged period of time. By early Oligocene time a distinct latitudinal trend in the depth of the CCD existed in the Atlantic Ocean. The maximum depth was centered around 10°N paleolatitude and most probably marked a zone of high productivity. Above 40° north and south lat., there was a definite shoaling of the CCD. This is a feature that also occurs in the present-day Atlantic and may be due to the production of cold bottom waters at high latitudes. Time series carbonate records from sites located above the CCD show varying responses across the Eocene/Oligocene boundary. DSDP Site 219 (Indian Ocean) and Site 292 (western equatorial Pacific) show no change in carbonate content across this boundary indicating that they were consistently above not only the CCD but the lysocline as well. In contrast, Site 77B (eastern equatorial Pacific), Site 277 (southwest Pacific), Site 363 (southeast Atlantic), and E128 (Gulf of Mexico) all show decreases of 10 to 25% across the boundary in conjunction with a distinct dissolution event. All of these sites may be recording a shoaling of the lysocline that accompanied the drop in the CCD across the Eocene/Oligocene boundary. End_of_Article - Last_Page 558------------

Cordilleran Overthrust Belt in Southern Canada--Its Regional Tectonic Implications, and Its Role in Hydrocarbon Generation and Entrapment: ABSTRACT

Palinspastic reconstructions based on balanced sections that are constrained by deep crustal structure, as outlined by seismic refraction, gravity anomaly, magnetic anomaly, and geomagnetic depth sounding studies, show that: (1) the Cordilleran miogeocline, a northeast-tapering wedge of craton-derived sedimentary strata, more than 15 km thick, accumulated outward from the rifted(?) edge of a 35-km thick slab of early Proterozoic continental crust, on a basement of oceanic crust and/or attenuated continental crust; (2) the miogeocline was compressed, detached from underlying crustal rocks, and displaced more than 200 km northeast as two successive collages of small allochthonous terranes from the adjacent ocean basin collided with North America; (3) the overthrust belt is tectonically prograded shallow accretionary prism that formed during the subduction of the basement of the miogeocline, as supracrustal rocks were scraped off the underriding continental craton and accreted to the overriding miogeoclinal prism; (4) subsidence and molasse sedimentation in the northeastward-migrating foreland basin were a result of isostatic flexure of the lithosphere in response to the weight of the encroaching accretionary prism, and of the molasse itself; and (5) burial of source rocks, and hydrocarbon generation, migration and entrapment are indirect results of the subduction of the lithosphere that formerly lay beneath the miogeocline. The first collision (Late Jurassic and Early Cretaceous Columbian orogeny) involved outward-verging thrusting and folding on either side of the uplifted core of the miogeocline, and produced a thick wedge of molasse (Kootenay-Blairmore) that extended over the western part of the craton. Mid-Cretaceous granitic plutons truncate Columbian structures. The second collision (latest Cretaceous and Paleocene Laramide orogeny) marked the final phase of convergence during which the reservoir structures associated with northeast-verging listric thrust faults and folds developed in the Canadian Rockies. Source rocks were buried to depths of 13 km under the Lewis thrust sheet in southeastern British Columbia, and 5 or 6 km under the plains, as a wedge of Laramide molasse (Brazeau-Paskapoo) was pr graded northeastward in front of the overthrust belt. Intracontinental transform faulting, involving 450 km of righthand strike slip on the Tintina-Northern Rocky Mountain Trench fault system, was partly taken up by thrust faulting in the Rocky Mountains south of 56°N lat. during the Laramide orogeny, but during the Eocene it was linked to the en echelon Fraser River fault zone by ductile stretching of the intervening lithosphere. This stretching is expressed, over an area of about 150,000 km2 in south-central British Columbia and adjacent parts of the United States, at a shallow level, by listric normal faults and Eocene dike swarms, and at a deep level, by boudinage of the whole crust. Supracrustal rocks moved into the necked zones between the boudins as the metamorphic core complexes emerged in northeast-trending domal culminations with K-Ar mica cooling ages of about 50 Ma. A different pattern of regional extension, involving uplift and partial unroofing of deeply buried source rocks in the southern part of the over-thrust belt and adjacent foreland basin, was established in early Oligocene time. End_of_Article - Last_Page 620------------

Age of emplacement of the schistes lustres nappe, Alpine Corsica

Structural studies in the Schistes lustrés nappe west of Bastia, Corsica, demonstrate the existence of a tectonic mélange in which km-scale blocks and smaller lozenges of basement granite gneiss, thick-layered marble and dismembered Mesozoic ophiolite are enveloped in a matrix of calc-schist and blueschist. The main ( S 1) foliation is developed in both block and matrix and is concordant with lithologie contacts. Blueschist facies metamorphism was syn-kinematic with the main foliation. The S 1 in the Schistes lustrés was refolded about ENE-WSW trending, tight similar and monoclinal fold axes ( F 2). These second folds verge to the southeast and show km-scale axial culminations and depressions that are reflected by topography and residual Bouguer gravity anomalies. Parautochthonous Hercynian basement (Tenda-Corte complex) beneath the western edge of the Schistes lustrés nappe contains a mylonitic foliation which is concordant with the main foliation in the Schistes lustrés. The intensity of deformation in the basement decreases away from this contact and undeformed granites are found 3 km to the west. Whole rock samples of the deformed basement immediately beneath the Schistes lustrés yield an Rb-Sr isochron diagram ( n = 4) which has an age of 105 ± 8 Ma (1σ) and initial 87Sr 86Sr ratio of 0.7228 ± 0.0005 (1σ). This result is more precise than our preliminary age and initial ratio estimate of 98 ± 14 and 0.7296 ± 0.0068, respectively (Cohen et al., 1979). It is similar to a recently published mid-Cretaceous (90 Ma) 40Ar- 39Ar age from glaucophane mineral separates. We interpret this date as the age of a metamorphic overprint related to the emplacement of the Schistes lustrés nappe and associated ophiolites, the formation of the main foliation and blueschist facies metamorphism. These results indicate that the mid-Cretaceous blueschist facies metamorphism documented in the Western Alps formerly extended farther south of its present terminus. The data are consistent with mid-Cretaceous obduction of Tethyan oceanic crust onto the present-day eastern continental margin of Corsica. We postulate that during Eocene—early Oligocene time a polarity flip occurred outboard of the obducted crust and a new, southfacing subduction zone developed. This change in polarity was responsible for the development of southeast-vergent second folds and for the resetting of 40Ar− 39Ar and K-Ar geochronologic clocks described in the literature.

Exploration Guides for Uranium in Volcanic Environments: ABSTRACT

Exploration for volcanogenic uranium deposits in the United States is a relatively new endeavor, but it is not new in other countries in the world. The Soviet Union, Italy, Yugoslavia, Canada, Brazil, and Mexico have substantial reserves in volcanic host environments. Few volcanogenic uranium deposits have been discovered in the United States, but they may be indicators of larger, more favorable environments. Volcanogenic systems that evolve from mantle-derived, hydrous, alkaline, magmas that can be enriched in Li, Be, Mo, and Hg, are most likely to develop favorable host environments. Rocks that develop in these host environments exhibit regional enrichment of uranium and thorium. The tectonic settings of these systems are extensional and are related to rift systems and transverse zones in the Basin and Range province of the western United States. Uranium mineralization processes active in the volcanogenic system produce deposits in both high-temperature and low-temperature regimes. High-temperature deposits form from pneumatolytic, magmatic-hydrothermal, and meteoric-hydrothermal processes; the deposits may be fumaroles, breccia pipes, brecciated margins of intrusives, fractured and porous country rock, and porous zones along ash-flow contacts. Diagenetic and reduction-oxidation processes work in conjunction to form low-temperature deposits, commonly in sedimentary environments where oxidizing ground-water flow can introduce labile uranium into reducing environments. Sierra Pena Blanca, Chihuahua, Mexico, is a preserved part of an alkaline volcanic complex that formed in a rift system active in late Eocene through early Oligocene time. Although detailed mapping is still underway, the number of uranium and thorium enriched alkaline and peralkaline ash-flows suggests several eruptive centers nearby. Enrichment of other metals in these ash-flow sheets suggests regional enrichment of lithophilic elements. Sierra Pena Blanca has pneumatolytic uranium deposits in fumarolic ash flows, magmatic and meteoric hydrothermal deposits in interbedded ash flows and invaded country rock, and low-temperature deposits forming in closed basins adjacent to the volcanic complex. End_of_Article - Last_Page 567------------

Seismic stratigraphy of the western Venezuela Basin

Multi-channel seismic reflection records from the central Venezuela Basin reveal planar reflections within the upper half of the oceanic crust as well as within the overlying sediment layer. The high resolution of the reflection data allows a division of the sediment section into three seismic intervals bounded by strong laterally persistent amplitude peaks which can be traced throughout much of the basin. The underlying crustal reflection zone is divided into two seismic intervals based on an angular relationship between the reflections which constitute each zone. Deep-sea drilling permits local determination of age and lithology of the seismic intervals of the sediment section and the top of the uppermost crustal interval. The lateral continuity of high-amplitude peaks that separate the sedimentary seismic intervals suggests that the boundaries are for the most part time-synchronous throughout the Venezuela Basin. Thinning and local pinch-outs of the deeper sedimentary seismic intervals in several parts of the basin contrast with the continuity in thickness of the shallower sedimentary seismic intervals. This suggests that the deeper intervals were affected more than shallower intervals by bottom currents which scoured and redistributed sediment from Cretaceous to Eocene time, but which became much weaker in post-Eocene or Early Oligocene time. Perhaps the weakening of bottom currents in Late Eocene or Early Oligocene was related to the development of the Lesser Antilles island arc and the isthmus of Panama in Eocene time. Planar seismic reflections within the high-velocity crust of the Venezuela Basin extend 1.5 sec below the base of the sediment section. These planar reflections extend tens of kilometers laterally. The upper interval of this crustal section consists of fairly level internal reflections compared to the lower interval with more steeply dipping reflections. The boundary between these two seismic intervals is not a distinct reflection, but rather a zone of apparent merging of the two intervals. The reflections of the lowest seismic interval become lost in the noise at a depth approximately equivalent to the top of the region in the lower Venezuela Basin crust with refraction velocities of 7.2 km/sec.

Late Oligocene transgression of middle Atlantic Coastal Plain

The subsurface Piney Point Formation of the coastal plain of Maryland, Delaware, and New Jersey contains planktonic foraminifera of late Oligocene age. The transgressive Piney Point Formation was deposited in inner to middle shelf environments on an eroded Eocene surface. The disconformity that separates the Piney Point from the Eocene formations below resulted from a major lowering of global sea level in early Oligocene time. This eustatic regression correlates with severe climatic changes noted elsewhere in the geologic record of this time.

Hydrocarbon Potential of Sitkalidak Formation, Eocene Submarine Fan Complex, Kodiak Island Archipelago, Alaska: ABSTRACT

The middle upper Eocene Sitkalidak Formation is a 4,000-m thick sequence of interbedded sandstone, shale, and conglomerate. The formation crops out along the southeast side of the Kodiak Island archipelago and is believed to underlie the continental shelf for several hundred kilometers along a northeast structural grain. The Sitkalidak Formation, as used here, includes marine sandy flysch previously mapped as Sitkinak Formation. The Sitkalidak Formation is a complex of submarine fans deposited at mid- to upper-bathyal depths in a northeast-trending structural trough. This trough was probably an accretionary basin located on an oceanic trench inner slope. The submarine fan deposits were subsequently faulted and uplifted during late Eocene subduction along an ancestral Aleutian trench. Emplacement was complete by early Oligocene time as the Sitkalidak Formation is overlain with angular discordance by the nonmarine Sitkinak Formation of Oligocene age. Sitkalidak Formation sandstones were deposited as quartz-poor and mineralogically unstable feldspathic lithic arenites. Diagenesis has greatly reduced primary porosity and permeability to an average 2.5% ^phgr and 0.01 md k (101 samples - ^phgr = 1.0 to 7.5%; k = 0.0 to 1.3 md). Sitkalidak Formation shales are gray to black silty claystones which contain dominantly terrestrially derived organic matter. Total organic content averages 0.44% (217 samples - TOC = 0.01 to 2.38%), suggesting lean source-rock potential for liquid hydrocarbons. Vitrinite reflectance values average 0.80% (53 samples - Ro = 0.54 to 0.92%), suggesting thermal maturity levels sufficient for oil generation. Despite the presence of thick sandstone sequences and potential structural and stratigraphic traps, the Sitkalidak Formation has a low potential for hydrocarbon production because of the poor reservoir character of the sandstones and poor source potential of the shales. End_of_Article - Last_Page 671------------

Paleomagnetism of intrusive rocks in the Coast Range of Oregon: Microplate rotations in middle Tertiary time

Paleomagnetic study of intrusive rocks in the Oregon Coast Range shows clockwise rotation of the range relative to North America of 51.5° ± 30.0° since late Eocene–early Oligocene time, 44.5° ± 17.0° since middle Oligocene time, and possibly 32.0° ± 26.5° since middle Miocene time. The simplest interpretation of paleomagnetic data currently available for the Coast Range indicates that it rotated steadily at a rate of about 1.5°/m.y. for approximately the period 50 to 20 m.y. B.P. Events since the middle Miocene are entirely unresolved.

Origin of arc of the Western Alps

The arc of the Western Alps started to develop during the main Alpine orogenic phase in Late Eocene to Early Oligocene time. This occurred as a side effect of the general north-south compression in the Central and Eastern Alps. Before this time no arc existed. A dominant east-west structural grain, expressed by the axes of the folds which originated during the Late Cretaceous, characterized the entire belt from the Dauphine and Provence of southeastern France to the region which later became the Eastern Alps. The final and most powerful curvature was the result of an east-southeast to west-northwest compression caused by the motion of the Italian-Dinaric microplate along the right-lateral lnsubric line during Early Oligocene to Early Miocene time. The deformation produced in the Western Alps by these movements can best be explained by applying the slip-line field theory. Accordingly, the two systems of strike-slips faults observed in the Western Alps, in the Jura mountains and in the Dauphine, are more or less parallel to the slip-lines created in the rigid plastic medium of the Western Alps by the motion of the northwestern prong of the Italian-Dinaric microplate, which acted as the indenter. It is thus postulated that by the recogniqd style of deformation the rock masses forming the arc of the Western Alps flowed toward north-northeast and south-southwest from an axis through the centre of the arc.

K-Ar geochronology of Oligocene volcanic rocks, Davis and Barrilla Mountains, Texas

Volcanic rocks of the Davis and Barrilla Mountains constitute the largest contiguous segment of the mid-Cenozoic volcanic field of Trans-Pecos Texas. Salic volcanic rocks form a silica-oversaturated series ranging in composition from trachyte to peralkalic rhyolite. Mafic lavas are subordinate in volume to the salic, silica-oversaturated series. Stratigraphic relations and K-Ar age determinations allow a reconstruction of the evolution of the area. Major volcanic activity occurred in a period of about 3 m.y. in early Oligocene time (38 to 35 m.y. B.P.), during which several large lava shields were constructed and major ash-flow sheets emplaced. A late episode of mafic lava extrusion and intrusion of phonolite bodies remains undated, but may have occurred later than 31.5 m.y. ago. The silica-oversaturated volcanic rocks are the eruptive products of a group of highly evolved magmas emplaced into high crustal levels during early Oligocene time. In rapid succession, plutons vented to the surface, forming as many as five different eruptive centers, each of which was active for a period not exceeding 1 m.y. The eruptive centers are marked by concentrations of salic lava flows, dike swarms, intrusive masses, and at least one caldera.

Early Tertiary “Incaic” tectonism, uplift, and volcanic activity, Andes of central Peru

Potassium-argon age determinations on volcanic strata that overlie an extensive postorogenic erosion surface in the Western Cordillera show that the “Incaic pulse” of compressive deformation as well as subsequent uplift and erosion of the Andes of central Peru had taken place before 40 to 41 m.y. ago. Intense volcanic activity, which began before the end of uplift and erosion, continued for about 6 m.y. into early Oligocene time. The timing of these events suggests that uplift and volcanic activity, and perhaps also the preceding tectonic pulse, may have been initiated by the same global disturbance that produced the abrupt change in direction of movement of the Pacific lithospheric plate reflected by the bend in the Hawaiian-Emperor volcanic chain.

Ancient Submarine Canyons of Tethyan Continental Margins, Czechoslovakia

Two large ancient submarine canyons buried below the Neogene foredeep and the Carpathian flysch belt are recognized within the ancient Tethyan margins. The features, as much as 10 km wide and 1,060 m deep, were cut into crystalline and sedimentary rocks of the Variscan platform and filled with late Eocene and early Oligocene sediments. The axes of both canyons follow the northwest-southeast structural trend of the platform. Farther downslope the two canyons probably join to form a single canyon-fan valley system. The submarine valleys are located within a northwest-southeast-trending tectonic depression which is filled with autochthonous Tethyan strata of Mesozoic and Paleogene ages. In terms of plate tectonics, this long-lived depression on the borderland between the Alps and the Carpathians, extending from the Tethyan seaway far into the platform, has been interpreted as an aulacogen. The sedimentary fill of the canyons is composed of silty mudstone in the upper part of the sequence, whereas sandstones and conglomerates are distributed mainly in the lower axial part of the valleys. Proximal and distal turbidites, grain-flow, and debris-flow deposits and pebbly mudstones have been recognized within the fill. The presence of pebbly mudstones, slump folds, and other mass-movement phenomena indicate relatively steep paleoslopes. The foraminiferal thanatocoenoses comprise: (a) indigenous canyon dwellers, (b) synsedimentary Foraminifera rewashed from estuaries and open-shelf environments, and (c) faunas reworked from older sediments. On the basis of indigenous planktonics, the duration of the canyon filling is estimated at 10 to 12 m.y. The fossil record, however, represents only the final stage of the canyon development; the early erosional stages have left little evidence within the canyons. The Tethyan canyons are considered to be submerged land valleys further developed by submarine processes. Their cutting and filling in late Eocene to early Oligocene time coincided with worldwide cooling of climate and possible eustatic sea-level changes, which also may have affected the canyon development. The Paleogene canyons also coincided with widespread accumulation of subsea-fan deposits in the Tethyan region (e.g., Annot Formation of the Alps, Krosno Formation of the Carpathians). The presence of ancient submarine canyons within the Tethyan continental margins provided means for coarse sediment to bypass the shelf environment. This strongly supports the writer's view that the tectonically fragmented marginal zone of the Variscan platform was much more important as a source of clastic materials than previously was believed. The canyon fill comprises both organic-rich petroleum source rocks and sandstone traps such as the proximal turbidites and grain-flow deposits. The tectonically controlled canyons also may have served as migration paths from the mobile belt to potential reservoirs in the foreland. Fossil submarine canyons can be identified elsewhere within the ancient continental margins; attention to the frontal zones of orogenic belts might thus prove fruitful in the search for petroleum.

Geochronology of basalts from the ninetyeast ridge and continental dispersion in the eastern Indian Ocean

Conventional K-Ar and 40Ar/ 39Ar total-fusion and incremental-heating ages from basalts recovered from the Ninetyeast Ridge confirm earlier indication from paleontology of basal sediments that basement crystallization ages increase systematically from south to north. A rate of migration of volcanism of 9.4 ± 0.3 cm/yr best fits the age-distance relationship determined from these basalts. The geometry, distribution of ages, paleomagnetism and geochemistry are most compatible with an origin of the Ninetyeast Ridge by northward movement of the Indian plate over a hotspot near Heard Island in the southern Indian Ocean, from mid-Cretaceous to Early Oligocene time. The Ninetyeast Ridge and nearly parallel Chagos-Laccadive Ridge provide an absolute frame of reference for the reconstruction of the eastern Indian Ocean.

Uranium-daughter migration and U/Pb isotope apparent ages of uranium ores, Shirley Basin, Wyoming

Migration of uranium daughters has profoundly affected the U/Pb isotope ages of Tertiary uranium ores and constituent minerals sampled from three mines and three ore types in Shirley Basin, Wyoming. U/Pb ages of pitchblende grains are typically low, show moderate to extreme normal discordance, and are very sensitive to the presence of pyrite impurities. Pyrite separates give typically high U/Pb ages, ranging in discordance from moderate-normal to moderate-reverse. Total ore samples give U/Pb ages intermediate between pitchblende and pyrite and are slightly to moderately discordant. Observed apparent age ranges were (m.y.): 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 PbPitchblende 6-30 14-35 270-2,400Pyrite 40-280 39-270 -90-+610Total ore 18-23 21-28 82-810Other key findings are that all pyrite separates from ores contained 130 to 620 ppm unsupported radiogenic lead, and a separate of coalified wood from an ore contained 800 ppm of unsupported radiogenic lead having the astonishing 207 Pb/ 206 Pb value of 0.012 (-5,850 m.y. 207 Pb/ 206 Pb age).These U-Pb isotope systematics are the result of the loss of both lead and radioactive daughters of 238 U from pitchblende and the subsequent migration of these uranium daughters. Migrating lead was both lost from the total ore and incorporated by pyrite, whereas migrating radioactive daughters were both lost from the total ore and trapped by material such as coalified wood.If young uranium ores such as the Shirley Basin ores have experienced neither uranium migration nor a net gain of extraneous uranium daughters, then geochronometry is possible even though the ores have lost lead. Even some ores open to both lead and radioactive uranium-daughter loss can be dated if the time-averaged leakage of radioactive daughters can be estimated. The main requirement of such open system dating is that the isotopic composition of the lead lost by the total ore be similar to that of the lead gained by the pyrite.The time of pitchblende formation in the Shirley Basin for the youngest ore sample analyzed is inferred to be 24 + or - 3 m.y. ago, whereas the oldest sample was apparently formed before 35 m.y. ago. These limits require source rocks for the uranium in the Shirley Basin orebodies to have existed from at least early Oligocene time.

Relationship of Pinnipeds to Other Carnivores (Mammalia)

Tedford, R. H. (Department of Vertebrate Paleontology, The American Museum of Natural History, New York, New York 10024) 1976. Relationship of pinnipeds to other carnivores (Mammalia). Syst. Zool. 25:363–374.—Classical studies in comparative anatomy in the last century led to the recognition of a tripartite subdivision of the living Carnivora, modified in this century into a bipartite subdivision formalized in most classifications at superfamily rank (Canoidea and Feloidea). Osteological characters, particularly those of the cranium, support the sister-group relationship of the Canoidea and Feloidea and permit their recognition into early Oligocene time. Analysis of the Canoidea further supports the sister-group relationship of the Canidae (Cynoidea) with the Ursidae-Procyonidae-Mustelidae (Arctoidea) as classically perceived. These groups are also recognizable on osteological criteria in the Oligocene. Recent studies in the comparative anatomy, paleontology and immunology of the pinnipeds have reinforced their phylogenetic affinity with the Arctoidea. Paleontology and comparative anatomy in particular have presented a convincing case of sister-group relationship of the Ursidae (especially the Amphicyno-dontinae) with the Otarioidea (including the extinct Enaliarctinae). A major gap in the early history of the Phocidae limits paleontological contribution to the question of their precise sister-group within the Arctoidea. The early Miocene aquatic arctoid Potamo-therium fulfills many of the morphological criteria deduced for a phocid sister-group when autapomorphous characters of the living and fossil phocids are removed. Postcranial resemblances are particularly strong reflecting the reliance of Potamotherium on body flexion in swimming as in the phocids. Potamotherium retains a differentiated dentition and cranial characters which can be compared with other arctoids and these agree most unequivocably with those of the mustelids suggesting a specific sister-group relationship of the Mustelidae with the seals and Potamotherium. Thus the osteological evidence supports a specific sister-group relationship of the otariids with the ursids and of the phocids (including Potamotherium and its close relative Semantor) with the mustelids within the Arctoidea. By contrast immunological comparisons provided to date suggest a sister-group relationship of the pinnipeds as a whole with the remaining Arctoidea. A cladogram (Fig. 5) summarizes the criteria for recognition of the sister-groups within the Canoidea and suggests a new classification which would express the phylogenetic conclusions of this paper.

Influence of Recurrent Laramide Structural Growth on Sedimentation and Petroleum Accumulation, Lost Soldier Area, Wyoming

Recurrent anticlinal growth during Late Cretaceous and early Tertiary (Laramide) time in the Lost Soldier area of south-central Wyoming profoundly affected local patterns of sedimentation, erosion, and petroleum accumulation. Uplift was accomplished by folding at Lost Soldier and by arching in the adjacent Granite Mountains area. During earliest Campanian time, shale beds thinned depositionally across the rising areas. Abnormally thick sequences of sandstone beds accumulated across shoals of structural origin at Lost Soldier, during both early Campanian and earliest Maestrichtian time. But during intervening late Campanian time, two episodes of abrupt local anticlinal folding resulted in erosional truncation of successively older beds beneath unconformities. Strong pulses of uplift in Maestrichtian and middle Paleocene time produced angular unconformities adjacent to the Granite Mountains uplift, gentle arching at Lost Soldier, and increasing amounts of arkosic sediment in the adjacent basin, culminating during the early Eocene in uplift and a flood of arkosic debris. Subsequent folding in late Eocene time segmented the ancestral Lost Soldier anticline, and a culminating pulse of mountain-flank thrusting followed, probably in early Oligocene time. Episodes of deformation lasted approximately 1-2 m.y., whereas intervals of quiescence and subsidence lasted from 2.5 to 5 m.y. Subsidence progressed fairly uniformly, except for sharp increases during Maestrichtian and early Eocene time; amounts of uplift on structures were nearly uniform during each pulse until latest Paleocene time when uplift of the Granite Mountains accelerated. Throughout Laramide time the ancestral Lost Soldier anticline continuously occupied an intermediate structural position between the Great Divide basin and the Granite Mountains uplift. By middle Paleocene time, successive pulses of deformation had developed nearly sufficient closure on the ancestral anticline to trap as much petroleum as is now in the area. Later folding during early late Eocene time increased the closure and segmented the ancestral fold. Folds formed later, possibly during late Tertiary and Quaternary time, are barren of petroleum. New prospects for exploration in the Great Divide basin can be inferred from the facies changes, variations in patterns of sedimentation, and relation of beds at unconformities, all resulting from recurrent structural growth in the Lost Soldier area.

Spreading history of the eastern Indian Ocean and Greater India's northward flight from Antarctica and Australia

Recent information from magnetic surveys and from deep-sea drilling allows Sclater and Fisher's Late Cretaceous and Cenozoic reconstructions of the eastern Indian Ocean to be extended back almost to the beginning of the Cretaceous Period. After a short phase of spreading off northwestern Australia in Middle and Late Jurassic time, Greater India and Antarctica-Australia dispersed near the beginning of Cretaceous time (130 m.y. B.P.) with the opening between them of a landlocked sea; between 110 to 120 and 105 m.y. B.P., Greater India cleared Antarctica-Australia, and the sea floor generated between them became continuous with the rest of the Indian Ocean. In Santonian time (80 m.y. B.P.), a new pattern of rapid spreading (as much as 17.5 cm/yr) began and caused an oceanic part of the Indian plate to be transferred to the Antarctic-Australian plate. Rapid spreading continued nearly to the end of Paleocene time (53 m.y. B.P.). With the inception at this time of spreading between Antarctica and Australia, three plates (Indian, Antarctic, and Australian) spread at a slow rate until the end of early Oligocene time (32 m.y. B.P.). At 32 m.y. B.P., the separate Indian and Australian plates became united, as they are today, while Antarctica remained a separate plate. Together with paleomagnetic and other determinations that show that Southeast Asia lay at or north of the Equator, the trail of sea floor generated during Greater India's northward flight implies that Southeast Asia rotated westward across this trail to its present position no earlier than middle to late Miocene time (10 m.y. B.P.).

Zeolitic Diagenesis in Oligocene Volcanic Sediments, Trans-Pecos Texas

Volcanic sedimentary rocks of the Veija Group of Trans-Pecos Texas contain zeolite, montmorillonite, and silica minerals that formed during diagenesis in an open hydrologic system. Volcanic glass shards dissolved in ground water to provide constituents for authigenic minerals. Diagenic mineral zones, from top to bottom, are (1) montmorillonite-opal-glass, (2a) montmorillonite-opal-clinoptilolite, (2b) montmorillonite-quartz-clinoptilolite, (3a) montmorillonite-quartz-analcime, and (3b) analcime-quartz. During diagenesis the original vitroclastic texture of volcanic sediment was preserved. Montmorillonite formed coatings on glass shards that preserved their outline during replacement by clinoptilolite. Clay or clinoptilolite cement filled much of the remaining inter-granular space. Analcime replaced both pseudomorphs of shards and interstitial cement that first formed as clinoptilolite. All authigenic minerals formed at low temperature and low pressure at burial depths no greater than a few hundred meters. Rocks in each zone were buried less deeply, and boundaries between zones are stratigraphically higher in the northeastern than in the southwestern part of the area. Distribution of clinoptilolite and analcime were controlled locally by permeability of the host rocks. Diagenesis converted sediment with an original composition similar to that of rhyolite into rock composed of Na2O, CaO, Al2O3, and SiO2, but relatively depleted in K2O. Field evidence shows that diagenesis occurred in early Oligocene time, but zeolites began to form only after a critical thickness of a few hundred meters of sediment had accumulated.

Petrology of Basalt and Andesite of the Black Range, New Mexico

The Black Range, located in southwestern New Mexico on the margin of the Basin and Range physiographic province, consists mainly of Tertiary volcanic rocks, which include mafic, intermediate, and silicic lavas, and silicic ash-flow tuffs. This report treats the petrology of basalt, basaltic andesite, and andesite. Basaltic rocks are alkalic and fall into two groups — late Oligocene and Miocene, which are quartz normative and contain quartz inclusions, and Pliocene basalts, which are SiO 2 undersaturated and contain mafic and ultramafic inclusions. The andesite is Oligocene and Miocene in age and is potassic, having about 2.8 wt percent K 2 O at 60 wt percent SiO 2 ; Oligocene latite is also present and has about 4.0 wt percent K 2 O at 62 wt percent SiO 2 . The Oligocene rocks have lower alkali and higher MgO and CaO contents than the Miocene rocks. The total alkali, FeO, and MgO proportions (AFM) of the middle Tertiary suite of basalt-andesite are like those of calc-alkalic suites. Clinopyroxene and orthopyroxene coexisting in basaltic andesite and andesite also suggest that these rocks are calc-alkalic. Microprobe analyses show that average plagioclase compositions in the basalt and andesite are labradorite and that the high alkali contents of these rocks are manifested in the mode by interstitial alkali feldspar. Initial volcanism in early and middle Oligocene time was predominantly andesitic and was succeeded in late Oligocene and Miocene time by a suite of basalt, basaltic andesite, and andesite. On the basis of field relationships, the Oligocene andesite is interpreted as primary magmas, whereas younger andesite is differentiates of basaltic magma. Finally, only basaltic volcanism occurred in Pliocene time. The change in volcanism from intermediate to mafic is similar to the general trends of middle Cenozoic volcanism in the western United States. Compositional changes may have been related to physical changes in a subduction regime beneath New Mexico and progressive devolatilization.

The St. Eugene Formation and the Development of the Southern Rocky Mountain Trench

The Tertiary history of the southern Rocky Mountain Trench is inferred from a study of the distribution, stratigraphy, fabric, lithologic composition, structure, and palynology of the Miocene St. Eugene Formation in southeastern British Columbia.The St. Eugene Formation consists of flood-plain and fan facies and represents the upper part of up to about 1500 m of sediments which accumulated in the proto-Rocky Mountain Trench upon cessation of Laramide deformation and after initiation of extension and block faulting in the eastern Cordillera during Eocene or early Oligocene time. Deep Tertiary basins in the southern Rocky Mountain Trench are bounded on the east and west by high-angle faults parallel to the Trench margins and on the north and south by faults transverse to the trend of the Trench. Block faulting of a half-graben style was probably contemporaneous with sediment deposition, but at least 600 m of displacement on the east boundary fault postdates deposition of the St. Eugene Formation. Although there is no present seismic activity along the Rocky Mountain Trench north of latitude 49°N, Holocene fault scarps and earthquakes in a zone along the Rocky Mountains of the United States attest to the continuation of block faulting south of 49°N.The St. Eugene microflora includes at least 39 genera of ferns, gymnosperms, and anthophytes. Phytogeographic reconstruction based upon the habitats of extant counter-parts indicates floral elements growing on poorly drained lowlands, adjacent slopes, and montane uplands; thus, there was moderate to high relief in southeastern British Columbia during St. Eugene time. The climate apparently was temperate, with warm summers, mild winters, and abundant, uniformly distributed precipitation. This contrasts with the present climate of the southern Rocky Mountain Trench which is semiarid with hot summers and cold winters, and suggests that the mountain barriers which presently restrict cool, moist, Pacific maritime air masses to the coast were lower during the Miocene, or that the polar seas were relatively warm.