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At least 1,153 records · Page 64Linked to original sources
USGS2015

Context of ancient aqueous environments on Mars from in situ geologic mapping at Endeavour Crater

Using the Mars Exploration Rover Opportunity , we have compiled one of the first field geologic maps on Mars while traversing the Noachian terrain along the rim of the 22 km diameter Endeavour Crater (Latitude −2°16′33″, Longitude −5°10′51″). In situ mapping of the petrographic, elemental, structural, and stratigraphic characteristics of outcrops and rocks distinguishes four mappable bedrock lithologic units. Three of these rock units predate the surrounding Burns formation sulfate-rich sandstones and one, the Matijevic Formation, represents conditions on early Mars predating the formation of Endeavour Crater. The stratigraphy assembled from these observations includes several geologic unconformities. The differences in lithologic units across these unconformities record changes in the character and intensity of the Martian aqueous environment over geologic time. Water circulated through fractures in the oldest rocks over periods long enough that texturally and elementally significant alteration occurred in fracture walls. These oldest pre-Endeavour rocks and their network of mineralized and altered fractures were preserved by burial beneath impact ejecta and were subsequently exhumed and exposed. The alteration along joints in the oldest rocks and the mineralized veins and concentrations of trace metals in overlying lithologic units is direct evidence that copious volumes of mineralized and/or hydrothermal fluids circulated through the early Martian crust. The wide range in intensity of structural and chemical modification from outcrop to outcrop along the crater rim shows that the ejecta of large (>8 km in diameter) impact craters is complex. These results imply that geologic complexity is to be anticipated in other areas of Mars where cratering has been a fundamental process in the local and regional geology and mineralogy.

Journal of Geophysical Research E: Planets↗
USGS2015

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40 Ar/ 39 Ar and U-Pb dating of sanidine and zircon crystals

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km 3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40 Ar/ 39 Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40 Ar/ 39 Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206 Pb/ 238 U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206 Pb/ 238 U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 10 3 −10 4 year interval before eruption.

Wyoming↗
USGS2015

Late Pleistocene ages for the most recent volcanism and glacial-pluvial deposits at Big Pine volcanic field, California, USA, from cosmogenic 36 Cl dating

The Big Pine volcanic field is one of several Quaternary volcanic fields that poses a potential volcanic hazard along the tectonically active Owens Valley of east-central California, and whose lavas are interbedded with deposits from Pleistocene glaciations in the Sierra Nevada Range. Previous geochronology indicates an ∼1.2 Ma history of volcanism, but the eruption ages and distribution of volcanic products associated with the most-recent eruptions have been poorly resolved. To delimit the timing and products of the youngest volcanism, we combine field mapping and cosmogenic 36 Cl dating of basaltic lava flows in the area where lavas with youthful morphology and well-preserved flow structures are concentrated. Field mapping and petrology reveal approximately 15 vents and 6 principal flow units with variable geochemical composition and mineralogy. Cosmogenic 36 Cl exposure ages for lava flow units from the top, middle, and bottom of the volcanic stratigraphy indicate eruptions at ∼17, 27, and 40 ka, revealing several different and previously unrecognized episodes of late Pleistocene volcanism. Olivine to plagioclase-pyroxene phyric basalt erupted from several vents during the most recent episode of volcanism at ∼17 ka, and produced a lava flow field covering ∼35 km 2 . The late Pleistocene 36 Cl exposure ages indicate that moraine and pluvial shoreline deposits that overlie or modify the youngest Big Pine lavas reflect Tioga stage glaciation in the Sierra Nevada and the shore of paleo-Owens Lake during the last glacial cycle.

Geochemistry, Geophysics, Geosystems↗
USGS2016

A revised surface age for the North Polar Layered Deposits of Mars

The North Polar Layered Deposits (NPLD) of Mars contain a complex stratigraphy that has been suggested to retain a record of past eccentricity- and obliquity-forced climate changes. The surface accumulation rate in the current climate can be constrained by the crater retention age. We scale NPLD crater diameters to account for icy target strength and compare surface age using a new production function for recent small impacts on Mars to the previously used model of Hartmann (2005). Our results indicate that ice is accumulating in these craters several times faster than previously thought, with a 100 m diameter crater being completely infilled within centuries. Craters appear to have a diameter-dependent lifetime, but the data also permit a complete resurfacing of the NPLD at ~1.5 ka.

Geophysical Research Letters↗
USGS2009

Evidence for late Holocene relative sea-level fall from reconnaissance stratigraphical studies in an area of earthquake-subsided intertidal deposits, Isla Chiloé, southern Chile

At Río Pudeto and Quetalmahue, two estuaries along the northern shore of Isla Chiloé that subsided as much as 2 m in the great 1960 earthquake, reconnaissance stratigraphical studies reveal evidence of a regressive, nearshore marine sequence. The intertidal deposits include a peat-bearing, high-intertidal marsh sequence as thick as 1.4 m overlying shell- and foraminifera-bearing silt and clay layers presumed to represent a deeper water, low-intertidal environment. Stratigraphy indicates a relative sea-level fall since about 5000 years BP as evidenced by radiocarbon ages that constrain the peat-bearing sequences. Locally, low-intertidal silt and clay overlie high-intertidal peat layers, but such minor transgressions cannot be correlated from site to site. At Río Pudeto, the youngest foraminifera-bearing silt deposit is no younger than 1200 years BP. The youngest age of shells at Quetalmahue is about 2600 years BP. The oldest peat-bearing deposits that are not overlain by silt deposits are about 1350 years BP at Río Pudeto, and as old as 4900 years BP at Quetalmahue. At Río Pudeto, peat-bearing deposits, which are overlain by silt and clay, range in age from 760 to 5430 years BP, and at Quetalmahue from 290 to 5290 years BP. A beach terrace on the northwest coast of the Isla is estimated to have been emergent since 1150 + 130 years ago. Although some relatively abrupt transgressions may be due to sudden coseismic subsidence, data are not sufficient to document regional subsidence during individual plate-interface earthquakes. Seven earthquakes in south central Chile since 1520, especially those that occurred in 1575, 1737, and 1837, are thought to have been of a magnitude comparable to that of the 1960 earthquake. Although the sedimentological effects of the 1960 earthquake on the intertidal zone were dramatic, only limited evidence of possible historic earthquakes is found on Isla Chiloé and nearby islands; the ages and displacements of these earthquakes are indeterminable. Dead forests still mark some locations that subsided into the intertidal zone during the 1960 earthquake, particularly at Río Pudeto and southern coastal Chiloé. There is little evidence of post-1960 growth in any of these subsided areas. Tree-ring counts and tree-diameter measurements provide evidence that these trees survived the 1837 earthquake, and probably survived the 1737 earthquake, strongly suggesting that these earthquakes were of smaller magnitude than the 1960 event, or that the epicentre locations were further removed than the 1960 epicentre from Isla Chiloé, and that earthquake-induced relative sea-level changes differed from those occurring in 1960.

Isla Chiloé↗
USGS2004

Catastrophic precipitation-triggered lahar at Casita volcano, Nicaragua: Occurrence, bulking and transformation

A catastrophic lahar began on 30 October 1998, as hurricane precipitation triggered a small flank collapse of Casita volcano, a complex and probably dormant stratovolcano. The initial rockslide‐debris avalanche evolved on the flank to yield a watery debris flood with a sediment concentration less than 60 per cent by volume at the base of the volcano. Within 2·5 km, however, the watery flow entrained (bulked) enough sediment to transform entirely to a debris flow. The debris flow, 6 km downstream and 1·2 km wide and 3 to 6 m deep, killed 2500 people, nearly the entire populations of the communities of El Porvenir and Rolando Rodriguez. These ‘new towns’ were developed in a prehistoric lahar pathway: at least three flows of similar size since 8330 14 C years bp are documented by stratigraphy in the same 30‐degree sector. Travel time between perception of the flow and destruction of the towns was only 2·5–3·0 minutes. The evolution of the flow wave occurred with hydraulic continuity and without pause or any extraordinary addition of water. The precipitation trigger of the Casita lahar emphasizes the need, in volcano hazard assessments, for including the potential for non‐eruption‐related collapse lahars with the more predictable potential of their syneruption analogues. The flow behaviour emphasizes that volcano collapses can yield not only volcanic debris avalanches with restricted runouts, but also mobile lahars that enlarge by bulking as they flow. Volumes and hence inundation areas of collapse‐runout lahars can increase greatly beyond their sources: the volume of the Casita lahar bulked to at least 2·6 times the contributing volume of the flank collapse and 4·2 times that of the debris flood. At least 78 per cent of the debris flow matrix (sediment < −1·0Φ; 2 mm) was entrained during flow. Copyright © 2004 John Wiley & Sons, Ltd.

Casita volcano↗
USGS2009

Sediment storage and transport in Pancho Rico Valley during and after the Pleistocene-Holocene transition, Coast Ranges of central California (Monterey County)

Factors influencing sediment transport and storage within the 156??6 km 2 drainage basin of Pancho Rico Creek (PRC), and sediment transport from the PRC drainage basin to its c. 11000 km 2 mainstem drainage (Salinas River) are investigated. Numeric age estimates are determined by optically stimulated luminescence (OSL) dating on quartz grains from three sediment samples collected from a 'quaternary terrace a (Qta)' PRC terrace/PRC-tributary fan sequence, which consists dominantly of debris flow deposits overlying fluvial sediments. OSL dating results, morphometric analyses of topography, and field results indicate that the stormy climate of the Pleistocene-Holocene transition caused intense debris-flow erosion of PRC- tributary valleys. However, during that time, the PRC channel was backfilled by Qta sediment, which indicates that there was insufficient discharge in PRC to transport the sediment load produced by tributary-valley denudation. Locally, Salinas Valley alluvial stratigraphy lacks any record of hillslope erosion occurring during the Pleistocene-Holocene transition, in that the alluvial fan formed where PRC enters the Salinas Valley lacks lobes correlative to Qta. This indicates that sediment stripped from PRC tributaries was mostly trapped in Pancho Rico Valley despite the relatively moist climate of the Pleistocene-Holocene transition. Incision into Qta did not occur until PRC enlarged its drainage basin by c. 50% through capture of the upper part of San Lorenzo Creek, which occurred some time after the Pleistocene-Holocene transition. During the relatively dry Holocene, PRC incision through Qta and into bedrock, as well as delivery of sediment to the San Ardo Fan, were facilitated by the discharge increase associated with stream-capture. The influence of multiple mechanisms on sediment storage and transport in the Pancho Rico Valley-Salinas Valley system exemplifies the complexity that (in some instances) must be recognized in order to correctly interpret terrestrial sedimentary sequences in tectonically active areas. ?? 2009 John Wiley & Sons, Ltd.

Earth Surface Processes and Landforms↗
USGS2024

Impounded sediment and dam removal: Erosion rates and proximal downstream fate

Sediment management is an important aspect of dam removal projects, often driving costs and influencing community acceptance. For dams storing uncontaminated sediments, downstream release is often the cheapest and most practical approach and can be ecologically beneficial to downstream areas deprived of sediment for years. To employ this option, project proponents must estimate the sediment quantity to be released and, if substantial, estimate how long it will take to erode, where it will go and how long it will stay there. We investigated these issues when the Bloede Dam was removed from the Patapsco River in Maryland, USA, in 2018. The dam was about 10 m high, and its impoundment was nearly filled with an estimated 186 600 m 3 of sediment composed of 70% sand and 30% mud. After removal, using elevation surveys generated by traditional methods as well as structure-from-motion (SfM) photogrammetry at high temporal resolution, we documented rapid erosion of stored sediments in the first 6 months (~60%) followed by greatly reduced erosion rates for the next two and a half years. A stable channel developed in the impoundment during the rapid erosion phase. These results were predicted by a two-phased erosion response model developed from observations at sand-filled impoundments, thus expanding its applicability to include impoundments with a sand-over-mud stratigraphy. A similar two-phase erosion response has been reported for sediment releases at other dam removals in the United States, France and Japan across a range of dam and watershed scales, indicating what practitioners and communities should expect in similar settings. Downstream, repeat surveys combined with discharge and sediment gaging showed rapid transport of eroded sediments through a 5-km reach, especially during the first year when discharges were above normal, and little overbank storage.

Maryland↗
USGS2002

Geoarchaeological investigations at the Winger site: A Late Paleoindian bison bonebed in Southwestern Kansas, U.S.A

The Winger site is a deeply buried Late Paleoindian bison bonebed in a playa basin on the High Plains of midcontinental North America. The site is one of few stratified, Late Paleoindian bison kills recorded in the region. The bonebed is exposed in the bank of an intermittent stream that cut into the edge of the playa basin. Avocational archaeologists excavated a small portion of the exposed bonebed in the early 1970s and reported flakes in association with the skeletal remains. Limited reinvestigations of the site were undertaken in 2001, and a monthlong excavation was conducted in 2002 to assess the stratigraphy, geochronology, and archaeology. The bonebed is 35 ni long in a buried soil developed in fine-grained basin fill overlain by early Holocene alluvium (arroyo fill). Recent alluvium overlies a soil developed in the early Holocene alluvium, and modern deposits of eolian sand 2 to to < 35 cm thick mantle the site area. Artifacts found at the site include two Allen points and a flake tool discovered in the bone bed, and a biface and Allen point fragment in disturbed bonebed deposits. Excavation of 9 m2 of the bone bed revealed some fully articulated skeletons, and taphonomic observations suggest some of the bison collapsed while standing in a playa or pond margin setting. The remains of at least six bison are represented in the excavated sample from 2002, but many more animals are represented in the bonebed. A 14C age of ca. 9000 yr B.P. was determined on collagen from bison rib fragments. This age is consistent with the diagnostic artifacts found at Winger. ?? 2003 Wiley Periodicals, Inc.

Geoarchaeology↗
USGS2007

Geoarchaeology of stratified paleoindian deposits at the Big Eddy site, Southwest Missouri, U.S.A

The Big Eddy site (23CE426) in the Sac River valley of southwest Missouri is a rare recorded example of distinctly stratified Early through Late Paleoindian cultural deposits. Early point types recovered from the site include Gainey, Sedgwick, Dalton (fluted and unfluted), San Patrice, Wilson, and Packard. The Paleoindian record at Big Eddy represents only a fraction of the site's prehistoric cultural record; stratified cultural deposits in alluvium above the Paleoindian components span the entire known prehistoric sequence, and terminal Pleistocene alluvium may contain pre-Early Paleoindian cultural deposits. This study focused on the paleogeomorphic setting, stratigraphy, depositional environments, pedology, geochronology, and history of landscape evolution of the late Pleistocene and early Holocene alluvium at the site. The Paleoindian sequence is associated with a complex buried soil 2.85 m below the modern surface (T1a) of the first terrace of the Sac River valley in the site vicinity. This soil formed at the top of the early submember of the Rodgers Shelter Member (underlying the T1c paleogeomorphic surface) and contains at least 70 cm of stratified Paleoindian cultural deposits, all in floodplain and upper point-bar facies. A suite of 36 radiocarbon ages indicates that the alluvium hosting the Paleoindian sequence aggraded between ca. 13,250 and 11,870 cal yr B.P. (11,380 and 10,180 14C yr B.P.). Underlying deposits accumulated between ca. 15,300 and 13,250 cal yr B.P. (12,950 and 11,380 14C yr B.P.). By ca. 11,250 cal yr B.P. (9,840 14C yr B.P.) the T1c paleogeomorphic surface was buried by the earliest increment of a thick sequence of overbank sheetflood facies, ultimately resulting in deep burial and preservation of the Paleoindian record. The landform-sediment assemblage that hosts the Paleoindian and possibly earlier cultural deposits at Big Eddy is both widespread and well preserved in the lower Sac River valley. Moreover, the terminal Pleistocene and early Holocene depositional environments were favorable for the preservation of the archaeological record. ?? 2007 Wiley Periodicals, Inc.

Geoarchaeology↗
USGS2011

Soils and late-Quaternary landscape evolution in the Cottonwood River basin, east-central Kansas: Implications for archaeological research

Temporal and spatial patterns of landscape evolution strongly influence the temporal and spatial patterns of the archaeological record in drainage systems. In this geoarchaeological investigation we took a basin‐wide approach in assessing the soil stratigraphy, lithostratigraphy, and geochronology of alluvial deposits and associated buried soils in the Cottonwood River basin of east‐central Kansas. Patterns of landscape evolution emerge when stratigraphic sequences and radiocarbon chronologies are compared by stream size and landform type. In the valleys of high‐order streams (≥4th order) the Younger Dryas Chronozone (ca. 11,000–10,000 14 C yr B.P.) was characterized by slow aggradation accompanied by pedogenesis, resulting in the development of organic‐rich cumulic soils. Between ca. 10,000 and 4900 14 C yr B.P., aggradation punctuated by soil formation was the dominant process in those valleys. Alluvial fans formed on the margins of high‐order stream valleys during the early and middle Holocene (ca. 9000–5000 14 C yr B.P.) and continued to develop slowly until ca. 3000–2000 14 C yr B.P. The late‐Holocene record of high‐order streams is characterized by episodes of entrenchment, rapid aggradation, and slow aggradation punctuated by soil development. By contrast, the early and middle Holocene (ca. 10,000–5000 14 C yr B.P.) was a period of net erosion in the valleys of low‐order streams. However, during the late Holocene small valleys became zones of net sediment storage. Consideration of the effects of these patterns of landscape evolution on the archaeological record is crucial for accurately interpreting that record and searching for buried archaeological deposits dating to specific cultural periods.

Kansas↗
USGS2006

The Valley and Ridge Province of eastern Pennsylvania - Stratigraphic and sedimentologic contributions and problems

Many contributions that have led to a better understanding of Appalachian geology have resulted directly from work in the folded Appalachian Mountain and Great Valley sections of the Valley and Ridge physiographic province of eastern Pennsylvania. Disagreements have been common since H.D. Rogers first described the geology of the area in 1858. Many differing opinions still exist regarding the stratigraphy, structural geology, geomorphology, and glacial geology. The rocks in the area, which range from Middle Ordovician to Late Devonian in age, are more than 25000 feet (7620 m) thick. This diversified group of sedimentary rocks was deposited in many different environments, ranging from deep sea, through neritic and tidal, to alluvial. In general, the Middle Ordovician through Lower Devonian strata are a sedimentary cycle related to the waxing and waning of Taconic tectonism. The sequence began with a greywacke-argillite suite (Martinsburg Formation) representing synorogenic basin deepening. This was followed by basin filling and pro-gradation of a sandstone-shale clastic wedge (Shawangunk Formation and Bloomsburg Red Beds) derived from the erosion of the mountains that were uplifted during the Taconic orogeny. The sequence ended with deposition of many thin units of carbonate, sandstone, and shale on a shelf marginal to a land area of low relief. Another tectonic-sedimentary cycle, related to the Acadian orogeny, began with deposition of Middle Devonian rocks. Deep-water shales (Marcellus Shale) preceded shoaling (Mahantango Formation) and turbidite sedimentation (Trimmers Rock Formation) followed by another molasse (Catskill Formation).

Pennsylvania↗
USGS2011

Identification of last interglacial deposits in eastern Beringia: a cautionary note from the Palisades, interior Alaska

Last interglacial sediments in unglaciated Alaska and Yukon (eastern Beringia) are commonly identified by palaeoecological indicators and stratigraphic position ~2-5m above the regionally prominent Old Crow tephra (124 + or - 10ka). We demonstrate that this approach can yield erroneous age assignments using data from a new exposure at the Palisades, a site in interior Alaska with numerous exposures of last interglacial sediments. Tephrochronology, stratigraphy, plant macrofossils, pollen and fossil insects from a prominent wood-rich organic silt unit are all consistent with a last interglacial age assignment. However, six 14C dates on plant and insect macrofossils from the organic silt range from non-finite to 4.0 14C ka BP, indicating that the organic silt instead represents a Holocene deposit with a mixed-age assemblage of organic material. In contrast, wood samples from presumed last interglacial organic-rich sediments elsewhere at the Palisades, in a similar stratigraphic position with respect to Old Crow tephra, yield non-finite 14C ages. Given that local permafrost thaw since the last interglaciation may facilitate reworking of older sediments into new stratigraphic positions, minimum constraining ages based on 14C dating or other methods should supplement age assignments for last interglacial sediments in eastern Beringia that are based on palaeoecology and stratigraphic association with Old Crow tephra.

Alaska;Yukon↗
USGS2022

Reproducibility and variability of earthquake subsidence estimates from saltmarshes of a Cascadia estuary

We examine fossil foraminiferal assemblages from 20 sediment cores to assess sudden relative sea-level (RSL) changes across three mud-over-peat contacts at three salt marshes in northern Humboldt Bay, California (~44.8°N, -124.2°W). We use a validated foraminiferal-based Bayesian transfer function to evaluate the variability of subsidence stratigraphy at a range of 30-6000 m across an estuary. We use the consistency in RSL reconstructions to support estimates of coseismic subsidence from megathrust earthquakes. To assess the variability of subsidence estimates, we analyzed: nine examples of the 1700 CE earthquake (average of 0.64 ±0.14 m subsidence; range of 0.24 ±0.27 to 1.00 ±0.44 m), five examples of the ca. 875 cal a BP earthquake (average of 0.43 ±0.16 m; range of 0.41 ±0.36 to 0.48 ±0.39 m), and six examples of the ca. 1120 cal a BP earthquake (average of 0.70±0.18 m; range of 0.47 ±0.36 to 0.80 ±0.49 m). Our subsidence estimate results suggest ~±0.3 m of within-site (intra-site) variability, which is consistent with previous research. We also identify inconsistencies between sites (inter-site) at northern Humboldt Bay greater than one-sigma uncertainties, driven by variable foraminiferal assemblages in the mud overlying the 1700 CE subsidence contact. Therefore, we recommend at least two quantitative microfossil reconstructions across the same stratigraphic sequence from different marsh sites within an estuary to account for estimate variability and provide increased confidence in vertical coseismic deformation estimates. Our results have broad implications for quantitative, microfossil-based reconstructions of coseismic subsidence at temperate coastlines globally.

California↗
USGS2023

Synthesis of larval lamprey responses to dewatering: State of the science, critical uncertainties, and management implications

Objective Dewatering of fine sediments in rivers and streams can kill many thousands of larval lampreys (order Petromyzontiformes) that are burrowed in these habitats. The larval life stage for lampreys lasts 3–10 years, and because larvae often aggregate in large numbers, negative impacts from dewatering could potentially deplete local populations and affect multiple year-classes. Larval lampreys have not traditionally been considered during instream projects, but recent efforts to increase awareness of lamprey habitats have resulted in guidance on dewatering approaches to limit impacts to lampreys. Salvage efforts to rescue and relocate lampreys aim to mitigate losses, but a lack of understanding of lamprey responses limits the optimization of dewatering and salvage procedures. Methods We summarize the state of the science for nine factors that influence larval lamprey ( Entosphenus and Lampetra spp.) responses to dewatering, including burrowing depth, the prevalence and timing of emergence, movements, survival, shoreline slope, dewatering rate, light, and lamprey size. Result Research suggests that (1) shoreline slope influences movement capability, (2) hot and sunny conditions increase the risk of mortality, (3) salvage activities cause minimal direct mortality, and (4) smaller larvae are especially vulnerable to negative impacts from dewatering because they are more likely to emerge and are less capable of movement. Critical uncertainties associated with dewatering include cues that drive emergence, the influence of sediment composition and stratigraphy, vertical distribution of larvae in natural settings, use of the hyporheic zone, the scale of predation losses, and the effectiveness and impacts of salvage activities. Conclusion Balancing investments in salvage operations and lamprey exclusion efforts (e.g., screening) and developing field survey approaches to evaluate lamprey use of the hyporheic zone are identified management implications and research needs. Addressing the critical uncertainties discussed here and providing updated, science-based guidance on dewatering and salvage practices are suggested management actions to support lamprey conservation.

North American Journal of Fisheries Management↗
USGS1996

Integrated biostratigraphic and sequence stratigraphic framework for Upper Cretaceous strata of the eastern Gulf Coastal Plain, USA

Upper Cretaceous (Santonian-Maastrichtian stages) strata of the eastern US Gulf Coastal Plain represent a relatively complete section of marine to nonmarine mixed siliciclastic and carbonate sediments. This section includes three depositional sequences which display characteristic systems tracts and distinct physical defining surfaces. The marine lithofacies are rich in calcareous nannoplankton and planktonic foraminifera which can be used for biostratigraphic zonation. Integration of this zonation with the lithostratigraphy and sequence stratigraphy of these strata results in a framework that can be used for local and regional intrabasin correlation and potentially for global interbasin correlation. Only the synchronous maximum flooding surfaces of these depositional sequences, however, have chronostratigraphic significance. The sequence boundaries and initial flooding surfaces are diachronous, and their use for correlation can produce conflicting results. The availability of high resolution biostratigraphy is critical for global correlation of depositional sequences. ?? 1996 Academic Press Limited.

Cretaceous Research↗
USGS2000

Surface ages and resurfacing rates of the Polar Layered Deposits on Mars

Interpretation of the polar stratigraphy of Mars in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics. We have reassessed the cratering record in both polar regions using Viking Orbiter and Mariner 9 images. No craters have been found in the north polar layered terrain, but the surface of most of the south polar layered deposits appears to have been stable for many of the orbital/axial cycles that are thought to have induced global climate changes on Mars. The inferred surface age of the south polar layered deposits (about 10 Ma) is two orders of magnitude greater than the surface age of the north polar layered deposits and residual cap (at most 100 ka). Similarly, modeled resurfacing rates are at least 20 times greater in the north than in the south. These results are consistent with the hypotheses that polar layered deposit resurfacing rates are highest in areas covered by perennial ice and that the differences in polar resurfacing rates result from the 6.4 km difference in elevation between the polar regions. Deposition on the portion of the south polar layered deposits that is not covered by the perennial ice cap may have ceased about 5 million years ago when the obliquity of Mars no longer exceeded 40°.

Icarus↗