Late Pleistocene Time Research Articles

A major seismogenic fault in a ‘silent area': the Castrovillari fault (southern Apennines, Italy)

SUMMARY Large historical earthquakes in Italy define a prominent gap in the Pollino region of the southern Apennines. Geomorphic and palaeoseismological investigations in this region show that the Castrovillari fault (CF) is a major seismogenic source that could potentially fill the southern part of this gap. The surface expression of the CF is a complex, 10-13 km long set of prominent scarps. Trenches across one scarp indicate that at least four surface-faulting earthquakes have occurred along the CF since Late Pleistocene time, each producing at least 1 m of vertical displacement. The length of the fault and the slip per event suggest M=6.5-7.0 for the palaeoearthquakes. Preliminary radiocarbon dating coupled with historical considerations imply that the most recent of these earthquakes occurred between 380 BC and 1200 AD, and probably soon after 760 AD; no evidence for this event has been found in the historical record. We estimate a minimum recurrence interval of 1170 years and a vertical slip rate of 0.2-0.5 mm yr-' for the CF, which indicates that the seismic behaviour of this fault is comparable to other major seismogenic faults of the central-southern Apennines. The lack of mention or the mislocation of the most recent event in the historical seismic memory of the Pollino region clearly shows that even in Italy, which has one of the longest historical records of seismicity, a seismic hazard assessment based solely on the historical record may not be completely reliable, and shows that geological investigations are critical for filling possible information gaps.

Cave archaeology in Southeast Asia

Archaeological research in caves and rockshelters has been the major source of data about cultural developments in Southeast Asia during late Pleistocene and early Holocene times. Despite the numerous cave excavations in nearly every Southeast Asian country over the last 100 years, however, the earliest general archaeological schemes were ironically based on rather cursory findings from poorly dated open air sites. In recent years ongoing excavations and analyses of archaeological materials from caves and rockshelters have finally come into their own, and newer interpretations have superseded some of the more speculative aspects of the earlier archaeological schemes. During Late Pleistocene times, caves were used only as brief campsites, often selected for their inaccessibility; in the early Holocene Epoch they were frequently used as dwelling sites; in the middle Holocene time the more accessible caves and rockshelters were often used as burial sites. One site, the Lang Rongrien Rockshelter, southwestern Thailand, serves as a case study for pointing out problems encountered in cave and rockshelter excavations in Southeast Asia. This 3500–>43000-year-old site is also used as the basis for exploring some more general issues concerning the lifeways of late Pleistocene Homo sapiens in the region and possibilities of preagricultural rainforest adaptation of their early Holocene descendants. © 1997 John Wiley & Sons, Inc.

Holocene aeolian activity in the southwestern Kalahari Desert, southern Africa: significance and relationships to late-Pleistocene dune-building events

Little is known about the timing of dune-building episodes in the southwestern Kalahari Desert. Previously, arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods. To redress this, optically stimulated luminescence (OSL) dating was applied to sediment samples taken from a range of aeolian landforms in the SW Kalahari. Four periods of dune building are identified in the late Pleistocene and Holocene. Holocene aeolian activity at 6 ka and 1-2 ka led to the construction of localised dune forms, in contrast to late Pleistocene times when the extensive field of linear dunes that dominates the region was emplaced. Late-Holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at Witpan. The record of aeolian activity is evaluated in the context of humid chronologies and an ocean-atmosphere model of Holocene climate change, with good agreement obtained. It is concluded that the application of optical dating to a range of aeolian landforms has the potential to establish the style as well as timing of dune-building. It is also demonstrated that the climate during dune-building periods must have differed significantly from the prevailing semi-arid climate with drought periods of today, which may have been the predominant climate regime through much of the Holocene.

Radiometric dating of submarine hydrocarbon seeps in the Gulf of Mexico

Research Article| May 01, 1997 Radiometric dating of submarine hydrocarbon seeps in the Gulf of Mexico Paul Aharon; Paul Aharon 1Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar Henry P. Schwarcz; Henry P. Schwarcz 2Department of Geology, McMaster University, Hamilton, Ontario L8S 4M1 Search for other works by this author on: GSW Google Scholar Harry H. Roberts Harry H. Roberts 3Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana 70803 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1997) 109 (5): 568–579. https://doi.org/10.1130/0016-7606(1997)109<0568:RDOSHS>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 Aharon, Henry P. Schwarcz, Harry H. Roberts; Radiometric dating of submarine hydrocarbon seeps in the Gulf of Mexico. GSA Bulletin 1997;; 109 (5): 568–579. doi: https://doi.org/10.1130/0016-7606(1997)109<0568:RDOSHS>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 Massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid Bathymodiolus sp. containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and U-series isotopes. Stable carbon isotopes and 87Sr/86Sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon-rich fluids. Samples from five seepage sites on the northern Gulf of Mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to >2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives.In general, paired 14C and 230Th dates of carbonate buildups consisting of aragonite, high-Mg calcite, and dolomite mineralogies are highly discordant. The cause of the discordance, established on the basis of paired Δ14C and δ13C values (Δ14C = −898‰ to −992‰ δ13C = −9.5‰ to −53.3‰, for n = 27), lies with the impairment of the radiocarbon dates resulting from dilution of the 14C pool with fossil-hydrocarbon–derived carbon. The validity of the ionium dates based on U-rich (2.4–7.6 ppm) samples is demonstrated by the concordance between 234U/238U and 230Th/234U evolution in time, and by the (234U/238U)o activity ratios that are generally within the range of sea-water value of 1.14 ± 0.04 (2 sigma). Some dolomite-rich samples are exceptional because their (234U/238U)o ratios are significantly higher (1.22–1.38) than sea water, suggesting deposition from anoxic pore waters where the soluble U reached anomalous 234U/238U ratios. The formation of the carbonates from sea-water–derived fluids, rather than from formation fluids advecting from deep aquifers, is supported by the 87Sr/86Sr composition of the samples (mean 0.709145 ± 19 × 10−6, n = 14) that compares well with modern nonseep marine carbonates and the ambient Gulf of Mexico sea water (0.709171 ± 8 × 10−6). Calcareous shells, the δ13C values of which indicate a carbon source in sea water (δ13C = −4.3‰ to −1.1‰), yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates. Isotope migration attested by the observed U uptake in the fossil shells is likely to affect the accuracy of their ionium dates.Radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late Pleistocene time (195–13 ka). Ages derived from nascent seep sites at mid-bathyal and abyssal depths (12.3–0.0 ka) indicate that currently vigorous seepage was initiated at the end of the last deglaciation. These radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea 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.

Quaternary history of sea ice and paleoclimate in the Amerasia basin, Arctic Ocean, as recorded in the cyclical strata of Northwind Ridge

The 19 middle-early Pleistocene to Holocene bipartite lithostratigraphic cycles observed in high-resolution piston cores from Northwind Ridge in the Amerasia Basin of the Arctic Ocean, provide a detailed record of alternating glacial and interglacial climatic and oceanographic conditions and of correlative changes in the character and thickness of the sea-ice cover in the Amerasia Basin. Glacial conditions in each cycle are represented by gray pelagic muds that are suboxic, laminated, and essentially lacking in microfossils, macrofossils, trace fossils, and generally in glacial erratics. Interglacial conditions are represented by ochre pelagic muds that are oxic and bioturbated and contain rare to abundant microfossils and abundant glacial erratics. The synglacial laminated gray muds were deposited when the central Amerasia Basin was covered by a floating sheet of sea ice of sufficient thickness and continuity to reduce downwelling solar irradiance and oxygen to levels that precluded photosynthesis, maintenance of a biota, and strong oxidation of the pelagic sediment. Except during the early part of 3 of the 19 synglacial episodes, when it was periodically breached by erratic-bearing glacial icebergs, the floating Arctic Ocean sea-ice sheet was sufficiently thick to block the circulation of icebergs over Northwind Ridge and presumably other areas of the central Arctic Ocean. Interglacial conditions were initiated by abrupt thinning and breakup of the floating sea-ice sheet at the close of glacial time, which permitted surges of glacial erratic-laden icebergs to reach Northwind Ridge and the central Arctic Ocean, where they circulated freely and deposited numerous, and relatively thick, erratic clast-rich beds. Breakup of the successive synglacial sea-ice sheets initiated deposition of the interglacial ochre mud units under conditions that allowed sunlight and increased amounts of oxygen to enter the water column, resulting in photosynthesis and biologic productivity, and strong oxidization of the pelagic sediment. The lithostratigraphy of Northwind Ridge suggests that during at least late Pleistocene time, glacial conditions in the Arctic Ocean were initiated abruptly and continued unabated until terminated, also abruptly, by onset of the succeeding interglacial warming. Variations in abundance of glacial erratics within the interglacial units of the late Pleistocene indicate that during at least most interglacial episodes northern North America was glaciated, but with generally diminishing severity, until onset of the succeeding continental glaciation. Magnetostratigraphy suggests that the glacial-interglacial cycles on Northwind Ridge had an average periodicity of approximately 93.5 k.y. during the Brunhes normal and approximately 105 k.y. during the latter part of the Matuyama reverse polarity zone. These average periodicities are close to the 100 k.y. temperature cycles found in North Atlantic deep-water sediments of the Brunhes normal polarity chron, which have been ascribed to forcing by a Milankovitch eccentricity cycle. They are also close, however, to the average interval (101 k.y.) between the aperiodic glacial terminations in the 500 k.y. Pleistocene continental climate record from Devil’s Hole, Nevada, which have been ascribed to nonlinear feedbacks within the Earth’s atmosphere-ice sheet-ocean system.

Cenozoic uplift of Fennoscandia inferred from a study of the mid-Norwegian margin

Studies of the mid-Norwegian margin reveal that the Fennoscandian continental uplift represents a flexural intraplate deformation event separated in time and space from the regional syn-rift uplift associated with crustal breakup at the Plaeocene-Eocene transition. In the area 64–68°N, the uplift occurred from late Oligocene through Pliocene. During Late Pliocene and Pleistocene times the tectonic uplift was amplified by isostatic rebound in response to the Northern Hemisphere glaciation. The tectonic uplift component reaches ∼ 1 km in the northern part of the study area decreasing to the south. The shelf stratigraphy and sediment composition record the combined effects of tectonic uplift, eustatic sea level changes and Neogene climatic deterioration. The coeval uplift and climatic change may suggest causal relations. The resulting depositional model has three stages: (1) late Miocene (∼ 10.5-5.5 m.y.) increased continental erosion and deposition of prograding wedges most of which were later removed; (2) early-middle Pliocene (5.5-2.6 m.y.) development of extensive local ice-sheets reaching the coastline and deposition of the prominent, oldest Pliocene wedges; (3) Northern Hemisphere glaciation (2.6-0.01 m.y.) resulting in the younger wedges farther west covered by Quaternary deposits. The model is consistent with the development of landforms on the adjacent mainland. Both the tectonic and isostatic components of the Fennoscandian uplift appear to vary in magnitude along the uplift axis, however separation of the syn-rift plate boundary related uplift and the intraplate event support the Neogene age of the main Fennoscandian uplift. We document a correspondence between structural and physiographic margin segmentation and uplift magnitude and suggest that the intraplate deformation has a thermal origin. A hot-cold asthenosphere boundary beneath the Caledonide-Baltic Shield transition combined with pre-Tertiary relief at the base of the lithosphere might induce small-scale convection and preferential volume expansion beneath the observed elongate uplift.

Comparative Taphonomy of Modern and Pleistocene Corals, San Salvador, Bahamas

The taphonomic condition of the corals Acropora cervicornis and A. palmata obtained from two facies of the late Pleistocene Cockburn Town fossil reef was compared to that of modern subfossil specimens of the same taxa collected from analagous environments. The extent of coverage by potentially preservable and preserved encrusting organisms (coralline algae, worm tubes, bryozoans and corals) and borer organisms (clionid sponges, lithophagid bivalves) was recorded. The degree to which the material had been abraded by a variety of processes was also recorded. Results revealed that the Holocene corals were significantly (a = 0.05) more degraded than those preserved in the Pleistocene facies, suggesting that the Pleistocene corals were exposed on the sea floor for less time than the Holocene material currently residing on the sea floor. The recent history of the reefs from which the Holocene corals were collected suggests this interval to be less than ten years; however, sedimentologic evidence indicates that a single storm event buried and killed the Cockburn Town reef during late Pleistocene time. The taphonomic analysis presented here supports this conclusion. INTRODUCTION

Palaeolithic landscapes of Europe and environs, 150,000-25,000 years ago: An overview

When considering the evolution and migrations of Neandertalers and early modem human beings, the harsh conditions of the last glacial maximum are often implicitly or explicitly assumed as their environmental background. This perception is false: the conditions of the high glacial apply to a small fraction of late Pleistocene time. Here we review the palaeoenvironmental history of Europe from 150,000 to 25,000 years ago with the aid of data from long cores of ice and marine and continental sediments. The results are displayed in four sketch maps that illustrate the landscapes of an interglacial-glacial cycle. The maps, connected by palaeoenvironmental histories, show that especially between 60,000 and 25,000 years ago, a critical part of the Palaeolithic, the glacial landscapes were for much of the time less barren than is generally assumed, but numerous climate changes on a scale of several millennia are evident, placing a premium on accurate dating of the co-evolution of humans and landscape. Moreover, during the glacial interval abrupt climatic changes lasting from a century to a few millennia were common. Their importance for landscape changes and their impact on human activity remain to be ascertained.

Quaternary biogeography of the Mustelidae family on the Mediterranean islands

Five taxa of the Mustelidae family now occur on the Mediterranean islands: the pine marten, Martes martes L., 1758, distributed on some of the Balearic and Tyrrhenian islands, the stone marten, M. foina Erxleben, 1777, which occurs on many islands of the basin, the badger, Meles nzeles Brisson, 1762, that has been only reported from some Greek islands, the weasel, Musteln nivalis L., 1766, common to many of the islands, and the otter, Lutra lutra L., 1758, recorded from the Greek islands. During Late Pleistocene times, only few carnivores seem to have occurred, however, on these islands and none is referable to the species that are present today. Paleontological evidence attests only to the occurrence of endemic elements, most of which were related to the Lutrinae sublamily. Apart from L. lutrn, the absence during Late Pleistocene and Early Holocene of the continental species present today suggests that these carnivores are allochthonous. They wcre imported by man, for various

Edifice collapse and related hazards in Guatemala

Guatemalan volcanoes have at least seven debris-avalanche deposits, associated with Cerro Quemado, Fuego, Pacaya, Tecuamburro and an unidentified volcano. The deposits range in size from less than 0.1 to in excess of 9 km 3 and from 2.5 to in excess of 300 km 2. The avalanches traveled 3 to 50 km from their sources in the Guatemalan highlands. Three of the avalanches occurred in Late Pleistocene time and four in Holocene time—two of them within the last 2000 years. The avalanches occurred at both andesitic and basaltic stratovolcanoes and at dacitic dome complexes. Laterally directed phreatic or magmatic pyroclastic explosions were associated with two of the debris avalanches. An evaluation of factors that might lead to an edifice collapse in Guatemala is based on the case studies presented in this report and a survey of the literature. Edifice collapses are more apt to occur if zones of weakness exist within the volcanic edifices, such as unwelded pyroclastic rocks and pervasively altered rocks. Further, the trench-ward side of volcano pairs like Fuego and Atitlán may be more likely to fail because it may have weak zones along the contact with the older back-arc volcano. The direction of failure may be influenced by regional slopes, which in Guatemala generally trend southward toward the oceanic trench, and by such structural factors as multiple vents or overly steep slopes reflecting previous activity or erosion. Debris avalanches are more likely to occur in drainages which have headwaters at two or more volcanoes. Domes are especially apt to produce small- to moderate-sized debris avalanches, and, further, if the domes form a coalescing chain, are most likely to fail in a direction normal to the chain. These factors are used at seventeen major volcanic centers in Guatemala to assess their potential for edifice collapse and most probable direction of failure.

Spatial and temporal patterns of late quaternary eolian deposition, Eastern Colorado, U.S.A.

Eolian sediment covers about 60% of Colorado east of the Rocky Mountains; about 30% of the sediment is sand and 70% is loess. Initially, flood plains were the principal sources of eolian sediment, but during the Holocene, dunes formed from older eolian sand and alluvium on uplands. Since latest Pleistocene time, dominant dune-forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part. At present, sand sheets and dunes, mainly parabolic types, are stable and covered with vegetation. In dunes, sand is commonly 20–30 m thick but elsewhere averages < 10 m. Three sand units are recognized on the basis of bedforms, topographic expression, and soil development. Preliminary age limits for the three units, based on 26 numerical ages, are 22.5–9 ka, 8−1 ka, and l.0−0. 15 ka. The middle unit is the product of multiple episodes of eolian activity that are not yet accurately dated. Loess is widespread but thin (generally < 2.4 m). Three units — middle Pleistocene, late Pleistocene. and Holocene — are recognized on the basis of differences in soil-profile development and stratigraphic position; late Pleistocene loess is by far the most common loess.

The New Zealand Flora-Entirely Long-Distance Dispersal?

The present New Zealand flora is popularly seen as a living example of «Gondwanan» vegetation isolated by sea-floor spreading in the Late Cretaceous. However, living plants on purely oceanic islands give clear evidence of many New Zealand genera which could have arrived this way. The geology of Norfolk, Lord Howe, Fiji and the Kermadec Islands is reviewed. They are shown to be oceanic, and their flora therefore dispersed. A good palynological record indicates that almost all of New Zealand's flora can be seen to have arrived'post-drift'. The macrofossil record supports this showing complete change since the Late Cretaceous. Throughout the Tertiary New Zealand's flora had an «Australian» character-the character of the present evergreen forests of New Zealand probably does not predate the late Tertiary or early Pleistocene times. The present New Zealand vegetation can not be called «Gondwanic», either in character or in the sense that it has evolved in isolation, directly from Gondwana-period lineages. It is probable that the entire forest-flora of New Zealand arrived by long-distance dispersal

Kinematics of arc‐continent collision in the eastern Central Range of Taiwan inferred from geomorphic analysis

Active arc‐continent collision in Taiwan is an example of a typical collisional orogen, with a deformed continental margin (Foothills Belt, Hsueshan Range, and Central Range) separated from an accreted island arc (Coastal Range) by a collapsed forearc basin (Longitudinal Valley). The geomorphology of the eastern Central Range reflects spatial and temporal variations in uplift rate and pattern along the orogen during late Quaternary time. Digitized drainage basin perimeters, stream channels, and the mountain‐piedmont junction of a 200‐km‐long segment of the eastern Central Range provide data for calculating quantitative geomorphic parameters. Statistical analyses of these parameters define four domains of distinct landscape character. The northern domain extends northward along the coast from the northern end of the Longitudinal Valley (LV) and has landscape characteristics suggesting uplift near the mountain front. The central two domains are adjacent to the LV and show evidence of uplift concentrated in the interior of the range, away from the front. The boundary between the two central domains occurs near the middle of the LV. The southern domain extends southward along the coast from the southern end of the LV and has characteristics suggesting a transient response to increased uplift rate. Analysis of fluvial terrace profiles in the Central Range adjacent to the southern half of the LV indicates a change in uplift pattern through time, with migration of maximum uplift rate from the mountain front to the interior of the range. This change probably occurred just prior to late Pleistocene time.

Structural styles, chronology rates of deformation, and time‐space relationships in the Umbria‐Marche thrust system (central Apennines, Italy)

Structural interpretation of geological and geophysical data available for the central Apennines (Italy) allowed us to draw, balance, and restore three geological profiles across the external zones of the Umbria‐Marche thrust system and to evaluate the timing and rates of deformation in a roughly 4 Ma time window (from late Messinian to late Pleistocene time). From this data set we established a general correlation between spacing L and thickness D of the thrust sheets (which was used as a depth correction factor for deriving local versus regional depths to the sole thrust) and a time‐space relationship between the closing time of activity of each thrust and the distance X from a reference point within the system.

Miocene and Pliocene lacustrine and fluvial sequences, Upper Ramparts and Canyon village, Porcupine river, east-central Alaska

Cenozoic strata exposed along the Porcupine River between the Upper Ramparts and Canyon Village, Alaska, can be divided into five unconformity-bounded units (sequences) which are: lower and middle Miocene unit A, the white sandy fluvial sequence with peat beds; middle Miocene unit B, the basalt sequence—part B1 is basalt, and part B2 is organic-rich sedimentary beds; upper Miocene unit C, mudrock-dominated lake sequence; late Miocene or Pliocene to Pleistocene unit D, terrace gravels, detrital organic matter and associated sediments, and Holocene unit E, mixed sand and gravel-rich sediment and other sedimentary material including peat and eolian silt. The sequence (unit A) of lower and middle Miocene fluvial deposits formed in streams and on flood plains, just before the inception of local volanism. Fossil pollen from unit A suggests conifer-dominated regional forests and cool temperate climates. Peat beds and lake deposits from unit B contain pollen that indicates a warmer temperate climate coinciding with the middle Miocene thermal maximum. The lake deposits (unit C) downstream from the basalts accumulated in a small basin which resulted from a hydrologic system that was dammed in the late Miocene but breached soon thereafter. The lower part of the terrace gravels (unit D) expresses breaching of the dammed hydrologic system (of unit C). The Porcupine River became a major tributary of the Yukon River in late Pleistocene time when Laurentide ice blocked drainage from the Yukon interior basins causing meltwater to spill over the low divide separating it from the Porcupine River drainage initiating erosion and capture of the Yukon interior basins.

A voluminous avalanche-induced lahar from Citlaltépetl volcano, Mexico: Implications for hazard assessment

During the late Pleistocene the ancestral edifice of Citlaltépetl volcano (also known as Pico de Orizaba) collapsed to form a clay-rich deposit that extends 85 km from its source, has a volume of 1.8 km 3, and covers an area of 143 km 2 east of the volcano. The deposit has clay content ranging from 10 to 16% and contains secondary alteration minerals such as smectite and kaolinite. The deposit's features suggest that it had an origin as a sector collapse of hydrothermally altered rock that transformed from a debris avalanche to a cohesive lahar very close to its source. The presence of glacier ice and a hydrothermal system during late Pleistocene times apparently provided a source of pore water which enhanced the hydrothermal alteration of the summit of Citlaltépetl and was the origin of most of the water for the lahar. This deposit and several others suggest that glaciated volcanoes are sites where hydrothermal alteration and resulting cohesive lahars are most likely. Although cohesive lahars and debris avalanches both have origins as sector collapses, cohesive lahars are more mobile than similar-sized debris avalanches. Thus potential hazard of edifice collapse at glaciated volcanoes, especially those with large volumes of hydrothermally altered rock, includes the possibility of large-volume cohesive lahars.

Archaeological trends in the Southern Cone of South America

Current major research directions and findings in three areas of Southern Cone archaeology are discussed: (1) the entry of humans in Late Pleistocene times and early hunter-gatherer society, (2) the emergence of chiefdom societies, and (3) the Inca state. These themes are considered broadly in terms of local and interregional models of economic mobility and sociopolitical organization. Although emphasis is given primarily to the Andean regions of Argentina, Chile, and Bolivia, new studies and developments in the Argentinian Pampa and Patagonia are discussed.

Geochemical evidence for groundwater behavior in an unconfined aquifer, south Florida

Five well sites have been investigated along an east-west transect across the surfical aquifer system (SAS) of south Florida. Differences between rainfall during wet seasons (June–October) and evaporation during dry seasons (November–May) give surface waters of this region isotopically light ( δ 18 O −22‰ and δ D −7.6‰ ) and heavy ( δ 18 O +4.2‰ ) compositions, respectively. Surface waters and shallow groundwaters are enriched in 18O and D to the west, which is consistent with westward decrease in equal excess of rainfall. In the shallow portion of the SAS (less than 20 m, Biscayne sub-aquifer) heterogeneous stable isotopic compositions occur over short spans of time (less than 90 days), reflecting seasonal changes in the isotopic composition of recharge and rapid flushing. Homogeneous stable isotopic compositions occur below the Biscayne sub-aquifer, marking the zone of delayed circulation. Surface evaporation calculated from a stable isotope evaporation model agrees with previously published estimates of 75–95% by physical evaporation measurements and water budget calculations. This model contains many parameters that are assumed to be mean values, but short-term variability in some of these parameters may make this model unsuitable for the application of yearly mean values. For the Everglades, changes in the isotopic composition of atmospheric vapor during the dry season may cause the model to yield anomalous results when annual mean values are used. Chloride-enriched waters (more than 280 mg 1 −1) form a plume emanating from the bottom central portion of the transect. Elevated chloride concentration and light stable isotopic composition ( δ 18 O ≈ −2‰ , δ D ≈ −8‰ ) suggest this plume is probably caused not by salinity of residual seawater in the aquifer, but by leakage from the minor artesian water-bearing zone of the Floridan aquifer system. Stable isotope values from Floridan aquifer groundwater plot close to the meteoric water line, in the same area as Everglades rainfall. These Floridan waters are interpreted to have originated in central Florida some 25 000–132 000 years ago, indicating that meteoric conditions in the Florida peninsula have changed little since late Pleistocene time.

A late Pliocene to middle Pleistocene pluvial lake in Fish Lake Valley, Nevada and California

The question of whether a pluvial lake existed in Fish Lake Valley, Nevada and California, has been debated for more than 100 yr. New stratigraphic evidence indicates that a lake did exist in this valley at intervals during late Pliocene to middle Pleistocene time. This lake may have drained northward, or it may have been periodically contiguous with a pluvial lake to the north in Columbus Salt Marsh. Proof of the existence of this lake, informally named Pluvial Lake Rennie, is derived from three principal outcrops of shallow-water deposits, two outcrops of deep-water deposits, and several drilling logs. The deposits contain beds of silicic tephra, which provide age control. On the basis of thickness, grain size, major-oxide chemistry of glass shards, and paleomagnetism, three of the shallow-water deposits, including deltaic(?), beach, and siliceous hot-spring sediments, consist mainly of Bishop ash derived from the 0.77 Ma eruption of the Long Valley caldera. A fourth shallow-water deposit(?) is associated with ∼1 Ma Glass Mountain tephra beds. The exposed deep-water deposits consist of green claystone, siltstone, and fine-grained sandstone containing tephra derived from the eruptions of the ∼2.1 Ma tuff of Taylor Canyon and the ∼2.0 Ma Huckleberry Ridge Tuff. The drilling logs record numerous thick beds of clay and sandy clay inferred to be deep-water lacustrine deposits. Pluvial Lake Rennie fluctuated in size and depth beginning prior to 2 Ma and continuing until sometime alter 0.77 Ma. At about 0.77 Ma, the lake had a highstand at an elevation of ∼1,460 m, covered an area of 400-500 km2, and had a maximum depth of ∼250 m. The lake level dropped just after the eruption of the Bishop ash, but the lake may have persisted at a lower level until ∼0.5 Ma. No large, long-lived lake existed in Fish Lake Valley in late Pleistocene time, probably due to the increasing rain-shadow effect caused by the relative uplift of the White Mountains and Sierra Nevada in the Pleistocene. These results indicate that the late middle to late Pleistocene history of Pluvial Lake Rennie is similar to that of Lake Tecopa but is quite different from those of Lake Lahontan and Searles Lake.

The Bocono Fault Zone, Western Venezuela

The Bocono Fault Zone, the western part of the Bocono Moron-El Pilar Fault System of the southern Caribbean plate boundary, consists of aligned valleys, linear depressions, pull-apart basins and other morphological features, which extend for about 500 km in a N45[degrees]E direction, between the Tachira depression (Venezuela-Colombia border) and the Caribbean Sea. It crosses obliquely the Cordillera de Merida and cuts across the Caribbean Mountains, two different geologic provinces of Late Tertiary-Quaternary and Late Cretaceous-Early Tertiary age, respectively. Therefore, the maximum age that can be assigned to the Bocono Fault Zone is Late Tertiary (probably Pliocene). A total maximum right-lateral offset rate of 3.3 mm/a. The age of the sedimentary fill o[approximately] the La Gonzalez pull-apart basin suggests that the 7-9 km right-lateral offset necessary to produce it took place in Middle to Late Pleistocene time. The majority of seismic events are well aligned with the main fault trace; minor events are distributed in a belt several kilometers wide. Focal depth is typically 15 km and focal mechanisms indicate an average east-west compression across the zone. Return periods of 135-460 a (Richter M = 8), 45-70 a (M = 7), and 7-15 a (M = 6) have been calculated. Geodeticmore » studies of several sites along the zone indicate compressive and right-lateral components; at Mucubaji the rate of right-lateral displacement observed is about 1 mm every 5 months (15 a of measurements).« less