Low Stands Research Articles

Depositional Environment of the Yates Formation in Kermit Field, Winkler County, Texas: ABSTRACT

The Yates Formation (Permian Guadalupian) is a series of alternating clastic and carbonate units deposited on a shallow marine shelf in repetitive cycles: (1) a mottled, anhydritic dolomite at the base, (2) an overlying oxidized wind-blown very fine-grained sand and silt, (3) a bioturbated, but otherwise massive, sandstone, and (4) an uppermost massive dolomite containing oolites or skeletal material. A typical cycle is 10-20 ft thick. Deposition of the Yates was controlled by fluctuations in sea level governed by glacial activity and local tectonics. During sea level low stands, carbonates were dolomitized and displaced by anhydrite. This was followed by deposition of wind-blown sediments creating planar horizontal laminae and exposure features (desiccation cracks, tepee structures, and adhesion ripples). During sea level high stands, dunes an the platform were deposited as sheet sands. Lastly, a carbonate layer was deposited before sea level dropped and another cycle began. Sands were transported to the Central Basin platform as eolian dunes from the Pedernal highlands of central New Mexico. Wind direction for the Permian is documented as south-southeast based on mapping of sand dunes in the southwestern US. Quartz size in the clastics ranges from 0.03-0.11 mm, averaging 0.07 mm. The sandstones are homogeneous,more » well sorted, and most quartz grains are frosted. Except for planar wind laminae, these units contain only indistinct primary sedimentary structures, constrained by small grain size and grain size uniformity within beds. The clastics are 52% monocrystalline quartz, 17% feldspar, 4% rock fragments, 5% other minerals, and 22% matrix (dolomite in the carbonates and authigenic clays in the clastics).« less

Ooid turbidites from the central western continental margin of India

Gravity displaced debris flows/turbidites have been observed in five box cores collected between water depths of 649 and 3,627 m from the central western continental margin of India. Studies on grain size, carbonate content, and coarse fraction revealed that the turbidites are mainly composed of ooids, shell fragments, and shallow water benthic foraminifera. Bioclastic sediments of the outer shelf and upper slope regions are considered the source of the debris flows/turbidity deposits. It appears that the flows were initiated by failure on the outer shelf and upper slope during late Pleistocene low stands of sea level.

Carbon isotope variations in the Upper Carboniferous - Pennian Mallemuk Mountain Group, eastern North Greenland

Isotope data from Late Palaeozoic limestones of the Wandel Sea Basin in eastern North Greenland show a variation of b13C from 0.0 %o to 5.7 %o vs PDB. Carbonates depleted in 13C occur in the basal part of lower Moscovian, upper Moscovian and middle Gzhelian transgressive sequences. 13C enriched limestones occur later in the cycles. The most 13C enriched limestones occur in the youngest (late Early Permian-early Late Permian) part of the sequence in Amdrup Land. The isotopic data is believed to represent changes in the global carbon cycle. Thus 13C enriched carbonates correlate to periods of burial of organic carbon mostly as coal, while 13C depleted carbonates formed as the result of erosion and oxidation of organic carbon during sea-level low stands.

Nitrogen mineralization in high elevation forests of the Appalachians. I. Regional patterns in southern spruce-fir forests

Annual and seasonal rates of net nitrogen mineralization were determined for 19 sites in the spruce-fir forests of the Southern Appalachian Mountains. These sites included high and low elevation stands of red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh.) Poir.) on east and west exposures on Whitetop Mountain, Virginia; Mt. Mitchell, North Carolina; and Clingman's Dome in the Great Smoky Mountains National Park. Mineralization rates were determined using in situ soil incubations in PVC tubes with ion exchange resin bags placed in the bottom of the tubes to collect leachate. Throughfall was collected in resin bags placed in the top of the tubes. Average initial NH4-N + NO3-N ranged from 0.6 to 4.8 kg N/ha across all plots, and average mineralization rates ranged from 26 to 180 kg-N ha−1 yr−1. Throughfall ranged from 18 to 32 kg-N ha−1 yr−1 with NH4-N accounting for about two-thirds of the throughfall N across all sites. Throughfall and mineralization rates were not related to elevation or exposure. The high rates of N mineralization and relatively high nitrate concentrations indicate that leaching losses of nitrogen and associated cations could be substantial.

Critical Depth for the Survival of Coral Islands: Effects on the Hawaiian Archipelago

Coral islands drown when sea level rise exceeds the maximum potential of coral reefs to grow upward (about 10 millimeters per year). During the Holocene transgression (18,000 years ago to present) sea levels rose at rates of up to 10 to 20 millimeters per year, and most coral island reefs situated deeper than a critical depth of30 to 40 meters below present day sea level drowned. Coral islands that did not drown during the Holocene transgression apparently all developed on antecedent foundations shallower than critical depth. During low stands in sea level during the Pleistocene, these islands were elevated and subject to subaerial erosion. Today, in the Hawaiian Archipelago, the depth of drowned banks is inversely related to summit area; smaller banks are progressively deeper, evidently because of erosional truncation during low sea level stands. Bank summit area may therefore be an important factor determining the failure or success of coral islands.

INFLUENCE OF TIME OF EMERGENCE AND PLANT DENSITY ON GROWTH OF HEMP-NETTLE (Galeopsis tetrahit)

The effects of time of emergence and plant density on hemp-nettle growth (Galeopsis tetrahit) were measured in natural field populations from the Québec City region and in a greenhouse experiment. In the field, more than 80% of the plants emerged during a peak period of about 2 wk in early May. Few hemp-nettle plants emerged after this peak period. Late-emerging plants produced relatively little biomass compared to plants that had emerged during the early emergence flush. Total biomass accumulation increased with density on three out of four sites. Stands from the lowest density (80 plants m−2) generally produced less biomass than stands of higher plant densities. Average plant biomass production decreased with increasing density. Plants from low density stands displayed a bushy profile compared to the etiolated, single-stemmed plants from high density stands. Mature hemp-nettle stands did not develop strong hierarchical population structures such as found for other annual broadleaf species. Hemp-nettle plants within a stand were distributed rather evenly over a number of height classes. A large proportion of the total biomass was produced by a relatively considerable number of plants 45–59 cm in height rather than by a few dominating individuals.Key words: Hemp-nettle, Galeopsis tetrahit, weed populations, emergence pattern, population density, population structure

Development of large submarine canyons in the Bering Sea, indicated by morphologic, seismic, and sedimentologic characteristics

Research Article| October 01, 1988 Development of large submarine canyons in the Bering Sea, indicated by morphologic, seismic, and sedimentologic characteristics PAUL R. CARLSON; PAUL R. CARLSON 1U.S. Geological Survey, 345 Middlefield Road; Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar HERMAN A. KARL HERMAN A. KARL 1U.S. Geological Survey, 345 Middlefield Road; Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1988) 100 (10): 1594–1615. https://doi.org/10.1130/0016-7606(1988)100<1594:DOLSCI>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PAUL R. CARLSON, HERMAN A. KARL; Development of large submarine canyons in the Bering Sea, indicated by morphologic, seismic, and sedimentologic characteristics. GSA Bulletin 1988;; 100 (10): 1594–1615. doi: https://doi.org/10.1130/0016-7606(1988)100<1594:DOLSCI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Seven large submarine canyons cut the Beringian continental margin. Three of these are among the world's largest submarine canyons. Bering is 400 km long, Navarinsky and Zhemchug are each 100 km wide at the shelf break, and volumes of sediment removed from these three canyons range from 4,300 to 5,800 km3 an order of magnitude larger than any submarine canyons incised in the margin of the lower 48 states.Two major events set the stage for the development of the Beringian margin and the dissection of these canyons: (1) the jump of the subduction zone to the Aleutian trench in Late Cretaceous-early Tertiary time that changed the margin from active to passive and (2) the low stands of sea level during the Cenozoic glacial stages. The position and configuration of these canyons have been determined, or strongly influenced, by structural features of the margin. Some of the Beringian canyon systems appear to occupy continental-margin embayments, perhaps residual structural configurations inherited from the time of subduction. The principal mechanisms responsible for cutting and shaping the canyons are extensive slumps and slides that carried large volumes of continental-margin sediment to the base of the slope. Second in importance are sediment gravity flows (debris flows, mud flows, and turbidity currents) that eroded and transported shelf and slope sediment to the rise and far into the Aleutian basin, contributing significantly to the 4- to 11-km-thick rise wedge and >2-km-thick sedimentary sequence of the basin floor. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Deposition and diagenesis of the windward reef of Enewetak Atoll

Six cores from the northeast reef of Enewetak Atoll reveal that 25 to 50 ft (7.5 to 15 m) of Holocene deposits unconformably overlie at least 200 ft (60 m) of Pleistocene limestone. The Holocene-Pleistocene boundary and four other unconformities within the Pleistocene section are distinguished by paleosols. The unconformities record repeated periods of subaerial emergence and death of the Pleistocene reef associated with low sea-level stands. During emergent periods, meteoric-water diagenesis altered the Pleistocene section. The resulting mineralogy and diagenetic textures are characteristic of the meteoric-vadose diagenetic environment. Aragonite generally composes from 30 to 70% by weight of the Pleistocene limestone and is absent only locally; magnesium calcite is not present. Cementation by calcite is variable but generally increases with depth. Much of the section is poorly cemented and retains high primary and secondary porosity. Micrite and mixed micrite-microspar are the most abundant cements throughout the section. They occur as meniscus, uneven, and irregularly distributed (“patchy”) pore-fill cement. In the upper parts of depositional units, the following paleosol-associated cements are common: caliche crusts, rhizocretions, needle-fiber cement, and lath-crystal cement. Phreatic-type cements are present, but generally are unimportant volumetrically. A reconstruction of the Pleistocene paleohydrology of the atoll indicates that the section has occupied the meteoric-vadose environment during emergent periods. The similarity of Pleistocene limestones beneath Enewetak and other Pacific atolls studied by drilling suggests that for many atolls, most alteration of shallow subsurface Pleistocene limestones has occurred in the meteoric-vadose diagenetic environment.

Mechanical models for correlation of ring-fracture eruptions at Pantelleria, Strait of Sicily, with glacial sea-level drawdown

Recent K-Ar dating of eruptions at Pantelleria, a peralkaline volcanic island in the Strait of Sicily, shows a correlation between eruption of pantellerite lavas from caldera ring fractures and low stands of sea level as determined fromδ 18O stratigraphy. Post-caldera pantellerite lavas associated with an ∼ 114-ky-old caldera erupted along the ring-fracture zone during a major low stand of sea level at about 67 Ka. The most recent episode of lava-flow emplacement began about 20 ky ago during the last glacial maximum. Magma vented along the ring fault of a 45-ky-old caldera, from fractures radial to the caldera, and along faults formed by intracaldera trapdoor uplift. Two mechanical models based on elasticity theory are presented to explain the correlation of post-caldera ring-fracture eruptions at Pantelleria with lowering of sea level. A simple analysis of a bending circular plate of thickness,T r, and radius,R, representing the magma-chamber roof block, shows that tensile stress is concentrated by a factor of 0.75R 2/T 2 at the lower perimeter of the plate when sea level drops. Stress changes may be even greater ifT r is effectively less than the stratigraphic thickness due to layering of rocks in the roof block. Calculated stress changes due to a 100-m drawdown of sea level are similar in magnitude to stresses associated with dike propagation. More realistic model geometries, including different chamber shapes, a conical volcanic edifice, and sea-level drawdown beyond the surface projection of the magma chamber, were tested using the boundary-element method. Lowering sea level generates a horizontal tensile stress above the chamber, even when sea water is removed outboard of the magma chamber. For some chamber geometries the magnitude of the tensile stress maximum is greater than the ∼ 1 MPa pressure of the 100 m of removed water and is of the right order of magnitude for dike propagation. Dikes initiated by the change of the stress field may originate and propagate along fractures inboard of the chamber margin. The magnitudes of tensile maxima along the top of the chamber decrease as original sea level is moved outboard of the chamber margin and as the chamber thickness decreases. When the depth to the top of the magma chamber reaches a critical value, dependent on chamber geometry, the propagation of dikes to the surface is inhibited.

Low Lake Stands in Lakes Malawi and Tanganyika, East Africa, Delineated with Multifold Seismic Data

Seismic data reveal that water level in Lake Malawi, East Africa, was 250 to 500 meters lower before about 25,000 years ago. Water levels in Lake Tanganyika at that time were more than 600 meters below the current lake level. A drier climate appears to have caused these low stands, but tectonic tilting may also have been a contributing factor in Lake Malawi. High-angle discordances associated with shallow sequence boundaries suggest that these low stands probably lasted many tens of thousands of years. Because of its basement topography, the Lake Tanganyika basin had three separate paleolakes, whereas the Lake Malawi basin had only one. The different geographies of these paleolakes may be responsible in part for the differences in the endemic fish populations in these lakes.

Origin of depositional cycles in a Permian "saline giant": The Salado (McNutt zone) evaporites of New Mexico and Texas

Two types of metre-scale depositional cycles have been recognized within the McNutt Potash Zone of the Permian Salado evaporites. The cycles record progressive drawdown and concentration of brine in a shallow, marginal marine basin. cycles, made of basal carbonate-siliciclastic mudstone, then anhydrite-polyhalite pseudomorphous after primary gypsum, overlain by halite and capped by muddy halite, are interpreted as marine sea water dominated.They formed by massive spillover of sea water through a connection with the Permian Ocean at high relative sea-level stands (lagoonal mudstone and anhydrite-polyhalite after subaqueous gypsum), followed by relative lowering of sea level and gradual basin restriction (shallow saline-lake and salt-pan halite) and ending with a desiccated basin, isolated from the Permian Ocean (salt-pan halite and saline mudflat muddy halite cycle cap). cycles occur between the Type I cycles and contain halite overlain transitionally by muddy halite. They record a temporal evolution of environments from a shallow saline lake to an ephemeral salt-pan-saline mudflat complex and are interpreted as continental-dominated sequences sourced by meteoric inflow from surrounding land areas that mixed with variable amounts of sea water, either residual or introduced into the Salado basin by seepage. The vertical stacking of Type I cycles is best explained by periodic invasions of sea water into the Salado basin coincident with eustatic sea-level rises. The continental-dominated Type II cycles formed during intervening periods of eustatic sea-level fall and low stands. The maximum time interval between major marine incursions is an average of 10 5 yr.

Factors Controlling Depositional Patterns of Ebro Turbidite Systems, Mediterranean Sea

Tectonic control by subsiding grabens parallel to the Ebro continental margin created a steep narrow slope and rise that resulted in development of coexisting aprons, channel-levee complexes, and a fan, all fed by the same Ebro River source. In unstable steep-slope terrain, mass movement of material caused wide gullied canyons and deposition of nonchannelized base-of-slope aprons that extend 50 km across the rise. Within stable continental-margin areas, channelized turbidity currents formed individual narrow canyons and channel-levee complexes from north to south, successively, at different low sea level stands. Tectonically induced steep gradients (1:25-1:130) across the continental rise and northeastward in Valencia Trough caused sediment to bypass 200 km down Valencia alley and deposit on Valencia Fan at its end, thus preventing direct development of Ebro channel-levee complexes into fans. When a single source fed channel B for a relatively long period, however, (1) fan-valley sinuosity increased, (2) channel walls were modified through undercutting, slumping, and crevasse splays, and (3) channel bifurcation occurred, although lower fan lobes and a complete fan never developed. The dominance of local tectonic control thus aborted fan development in proximal regions on the continental margin near the Ebro River source and displaced fan formation to a distal site in the Valencia Trough basin. During low sea level stands, continued different local tectonic and sediment-source controls resulted in aprons, channel-levee complexes, and fans forming simultaneously from the same source and without evolving from one type of turbidite system to another. Complex ancient turbidite systems similar to those along the Ebro margin may be anticipated in the stratigraphic record of tectonically active settings like the alpine basins of Europe.

Devonian conodont biogeography: Quantitative analysis of provinciality

Cluster analysis of conodont faunas from each of 17 Lower‐lower Upper Devonian zones and subzones (data as reported by Klapper and Johnson, 1980) reveal changing patterns of provinciality. Provinciality, expressed by a differentiation into western Laurussian and proto‐Tethyan biogeographic regions, is moderate in the lower Lochkovian but is low or absent in the upper Lochkovian‐lower Pragian. Provincialism returns in the Pragian and reaches its maximum development during the Emsian. Most Australian faunas are distinct from those of western Laurussia and the proto‐Tethys. Conodont faunas from suspect terranes of western North America display no unusual biogeographic affinities. Provincialism declines during the Eifelian and is only weakly developed in Givetian‐lower Frasnian faunas. Changing global sea level during the Devonian may explain the development of Devonian conodont provinciality. As proposed by Klapper and Johnson (1980), low provinciality is associated with low stands of sea level. Endemic faun...

Solution conduits as indicators of late Quaternary sea level position

Solution conduits or caves can provide a measure of glacio-eustatic sea level position for the Quaternary. In high latitudes, cave deposits contain artifacts, fossils and sediments that reflect surface conditions with respect to climate. Secondary calcite deposits, most notably stalagmites, record growth episodes during ice minima and non-growth during ice maxima. They provide an indirect estimate of areal ice cover at high latitudes, and are an indirect measure of sea level. In low latitudes, stable carbonate platforms contain caves that provide a direct measure of past sea level position. Solution conduits developing in a fresh water lens will record the position of the lens and therefore sea level. As sea level fluctuates, fresh water lens horizons and conduit development horizons will shift, preserving a record of past sea level position. Cave wall rock and cave deposits can produce a chronology for given conduit positions. Secondary calcite deposits, notably stalagmites, record growth periods during sea level low stands (or ice maxima) and record growth hiatuses during flooding by sea level high stands (or ice minima); the inverse of high latitude stalagmites. Lighthouse Cave, San Salvador Island, Bahamas, records a sea level high stand at +1 to +6 m for some time period between 85,000 and 70,000 years ago, based on position and timing of conduit development; and records sea level at present elevation for some time period between 49,000 and 37,000 years ago, based on a stalagmite with a marine overgrowth.

Plio-Pleistocene Planktonic Foraminiferal Biostratigraphy, Paleo-oceanography, and Sediment Accumulation Rates, Northeastern Gulf of Mexico: ABSTRACT

The planktonic foraminiferal biostratigraphy has been analyzed for Ocean Drilling Program's hole 625B (Leg 100), drilled to a total subbottom depth of 235 m south of De Soto Canyon in the northeast Gulf of Mexico. The hole penetrated to at least early Pliocene (zone N19), as indicated by the Globigerina nepenthes LAD (approx. 3.7 Ma) at 182 m. Preliminary seismic section of the Plio-Pleistocene sediments of the continental slope in the northeast Gulf suggested a relatively continuous depositional record. However, sediment accumulation rates have fluctuated in parallel with paleo-oceanographic changes. Percent coarse (approx. 63 ..mu..m) carbonate fraction (mainly planktonic foraminiferal tests) in hole 625 B shows high-frequency, low-amplitude fluctuations from the Pliocene to the Jaramillo paleomagnetic event (approx. 900,000 y.B.P.), whereupon low-frequency, high-amplitude fluctuations occur to the Holocene. Low-frequency, high-amplitude fluctuations correspond primarily to increased amplitude of sea level fluctuations and terrigenous dilution during low sea level stands. Percent abundance of Globorotalia menardii and percent coarse carbonate fraction in the uppermost 18 m reflects oxygen isotope stages 1-5 and therefore, primarily, ice volume and, secondarily, water temperature. However, the lack of a strict correspondence between G. menardii abundance, percent coarse carbonate fraction, and planktonic-benthic ratios (presumed dissolution indicator) lower in more » the hole indicates that G. menardii abundance peaks reflect not only eustatic sea level changes and water temperature but possibly also productivity (upwelling) and/or differential dissolution. Preliminary results of factor analysis of foraminiferal assemblages are also presented. « less

The late Pleistocene and Holocene stratigraphy and paleolimnology of Lakes Kivu and Tanganyika

Evidence from diatoms, chemistry, and sedimentology shows that Lakes Kivu and Tanganyika experienced late Pleistocene low stands, −86 to −300 m for Kivu and −250 to −350 m for Tanganyika. Both lakes had higher (2–10 fold) sedimentation rates prior to 11,500 yr B.P. About 9900 yr B.P., upwelling became important in southern Lake Tanganyika, which had been deepening for at least several millenia. By 9400 yr B.P., Kivu had become deep and dilute, with overflow via the Ruzizi River to Lake Tanganyika. Southern Lake Tanganyika became more stably stratified about 5200 yr B.P. Volcanism and hydrothermal activity in Kivu began about 5000 yr B.P., and the salinity reached 3 ppt and the lake became strongly stratified. The Ruzizi River outflow was interrupted from 3500 yr B.P. until recently. The sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in Kivu has probably been the cause of its impoverished fauna. These results show that high lake levels in the Eastern Rift of Africa from 10,000 to 7000 yr B.P. were paralleled in the Western Rift. Given the great volumes of these lakes, climate has not caused great variations in salinity; the diatom floras, however, have been remarkably sensitive to changes in stratification and nutrient supply. Volcanism has been a major force in the limnological and biological evolution of Lake Kivu, while both Kivu and Tanganyika have responded to changes in precipitation and wind stress.

Seismic Stratigraphy and Salt Tectonics of Upper Continental Slope, East Breaks Flexure Trend, Gulf of Mexico: ABSTRACT

A study of more than 1200 of 72-fold, migrated, seismic reflection profiles, from the eastern part of the East Breaks flexure trend, Gulf of Mexico, indicates that, although individual seismic reflectors cannot be traced across the entire area, seismic depositional units can be recognized and correlated from basin to basin in most instances. Three tentative conclusions are evident from this study: (1) salt diapirs appear to have grown through a series of intermittent movements to form structurally separate intraslope basins; (2) sedimentation is probably channeled by differential dome growth, which creates local stratigraphic differences between basins; and (3) packages of distinctive seismic depositional units can be successfully used to map sediment cycles between separate basins. Fluctuations of Pleistocene sea level appears to be the dominate depositional control on the upper continental slope in this area. During high sea level stands, turbidity currents were confined to depressions between diapirs, whereas pelagic deposition blanketed the slope. But during low sea level stands, sediment bypassed the shelf and deposited directly onto the upper slope, which increased the sediment load and helped initiate diapirism. Repeated sea level fluctuation would then lead to the cyclic pattern of seismic depositional packages that is commonly seen onmore » upper slope seismic records.« less

Sedimentary processes on the Amazon continental shelf: past, present and future research

Previous research, and particularly research since the early 1960s, has examined many aspects of the dispersal system associated with the Amazon River. This system extends a distance of about 5000 km from western South America far into the Atlantic Ocean. Amazon particulate and dissolved materials are dispersed during low stands of sea level from the Andes to the flanks of the Mid-Atlantic Ridge, and during high stands of sea level from the Andes to the Caribbean Sea. The point of juncture between these two modes of dispersal is the continental shelf near the mouth of the Amazon River. Previous research has demonstrated how sedimentation on the Amazon shelf is tied to the rest of the dispersal system. Present research in this volume is able to recognize that many of the sedimentary processes occurring on the Amazon shelf cannot be extrapolated from observations of other shelves. The unique character of processes is largely the result of the extreme magnitude of numerous aspects of the dispersal system. Although the sedimentary processes active on the Amazon shelf have been delineated, a detailed understanding of their operation requires future research.

The frequency and periodicity of preserved turbidites in submarine fans as a quantitative record of tectonic uplift in collision zones

The frequency and periodicity of preserved graded turbidite cycles in submarine fans in the Coral Sea and Sea of Japan are correlated to times of tectonic uplift in response to major collisions in the Owen-Stanley Range of Papua and the Hida Range of Japan, respectively. Large frequencies and shorter-term periodicities of turbidites at DSDP Site 210 were coeval with early Pliocene maximum tectonic-uplift rates which occurred in the Owen-Stanley Range in response to obduction. Similarly, large frequencies and shorter-term periodicities of turbidites at Site 299 (Toyama Submarine Fan) were coeval with the late Pleistocene uplift in the Hida Range; this uplift of 1000 to 1500 m occurred in response to collision tectonics. In both cases, trends of increasing frequencies and towards shorter-term periodicities of preserved turbidite depositional events correlate to trends of increasing rates of tectonic uplift. The role of sea-level fluctuations on changing denudation rates in these two collision zones is secondary. At Site 210, larger frequencies and short-term periodicities of preserved turbidites were coeval with early Pliocene high stands of sea level, whereas at Site 299, Pleistocene sea-level fluctuations are considered minor because at low stands of sea level, both relief and denudation rates were increased by about ten to 14%. At Site 286 (New Hebrides Basin), Eocene turbidite deposition is coeval with high stands of sea level, whereas at Site 297 (Northern Shikoku Basin), turbidite deposition was coeval with both rising and falling sea level. Analysis of both frequency and periodicity of turbidites by fan subenvironment at Site 299 indicates a record of continuous deposition, and maintainance of frequency and periodicity trends controlled by tectonic uplift. Late Pleistocene channel and overbank deposits showed periodicity differences of less than 28% of an order of magnitude, whereas Miocene-Pliocene overbank and distal turbidite periodicities differed by a 19% order of magnitude. Greater differences in magnitude occurred between distal turbidites or early Pleistocene age and Pliocene age than between Miocene-Pliocene overbank and distal turbidite deposition with a magnitude difference of 860%. These findings suggest that shifting depocenters and differences in sedimentation history in subenvironments of submarine fans are secondary to the role of tectonic uplift in this particular case. The minimal rate of tectonic uplift required to generate deep-sea fan turbidities appears to be approximately 400 m/million years. This figure is tentative and is based on very few observation points. Frequency and periodicity of preserved turbidite cycles in submarine fans in active continental margins and ancient counterparts should provide an independent measurement of rates and timing of tectonic uplift, particularly in collision terrains. Because this sediment parameter is a record of a single process from a single source and a record of “event stratigraphy”, its usage is preferable over standard and bulk sediment accumulation rates determined from age depth curves.

Missourian (early late Pennsylvanian) climate in Midcontinent North America

The abrupt decrease in mineable coals from Desmoinesian to Missourian rocks in Midcontinent North America has been related by several lines of evidence to the probability that Missourian climate became at least seasonally drier than Desmoinesian climate. This represents a transition from the equatorial Desmoinesian rainforest climate to the arid Permian climate, as North America moved northward from the equator. This change is reflected in the progression of evaporites from western Colorado in the Desmoinesian to Kansas in the Permian. Direct climatic evidence from soils in two Missourian shales deposited at low stands of sea-level includes caliche horizons, incompletely leached mixed-layer clays (in contrast to intensely leached Desmoinesian kaolinite), and soil profiles resembling Vertisols, which develop under semi-arid conditions. Less direct climatic evidence includes the greater proportion of marine limestone deposited at intermediate sea-level stands in the Missourian than in the Desmoinesian part of the sequence, in conjunction with the greater abundance and thickness of oolite and shoreline facies in Missourian limestones than in Desmoinesian counterparts. This probably reflects increasing dryness of the climate, which would have led to decreased detrital influx and increased salinity as rainfall and runoff diminished. Indirect climatic evidence in offshore black phosphatic shales deposited at highest sea-level stands involves possible seasonality in organic and phosphatic laminations, related to periodicity of upwelling in the tropical trade-wind belt under the strong monsoonal influence of Pangaea. Midcontinent Missourian climates probably ranged from tropical monsoon to tropical savanna or hot steppe, all having wet-dry seasonality. The coincident extinctions of conodont, brachiopod, and other marine genera as well as the loss of swamp lycopods at the end of the Desmoinesian suggest the possibility that a greater than usual drop in sea level affected both realms at this time, while the climate was becoming drier.