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Jack B. Epstein 《Geological Journal》1986,21(3):283-306
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). 相似文献
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Jack Wisdom 《Meteoritics & planetary science》2020,55(4):766-770
In Wisdom (2017), I presented new simulations of meteorite transport from the chaotic zones associated with major resonances in the asteroid belt: the ν6 secular resonance, the 3:1 mean motion resonance with Jupiter, and the 5:2 mean motion resonance with Jupiter. I found that the observed afternoon excess (the fact that approximately twice as many meteorites fall in the afternoon as in the morning) of the ordinary chondrites is consistent with chaotic transport from the 3:1 resonance, contradicting prior reports. Here I report an additional study of the transport of meteorites from ν6 secular resonance and the 3:1 mean motion resonance. I use an improved integration algorithm, and study the evolution of more particles. I confirm that the afternoon excess of the ordinary chondrites is consistent with transport from the 3:1 resonance. 相似文献
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James Flocks Michael D. Miner David C. Twichell Dawn L. Lavoie Jack Kindinger 《Geo-Marine Letters》2009,29(6):359-378
The barrier-island systems of the Mississippi River Delta plain are currently undergoing some of the highest rates of shoreline
retreat in North America (~20 m/year). Effective management of this coastal area requires an understanding of the processes
involved in shoreline erosion and measures that can be enacted to reduce loss. The dominant stratigraphy of the delta plain
is fluvial mud (silts and clays), delivered in suspension via a series of shallow-water delta lobes that prograded across
the shelf throughout the Holocene. Abandonment of a delta lobe through avulsion leads to rapid land subsidence through compaction
within the muddy framework. As the deltaic headland subsides below sea level, the marine environment transgresses the bays
and wetlands, reworking the available sands into transgressive barrier shorelines. This natural process is further complicated
by numerous factors: (1) global sea-level rise; (2) reduced sediment load within the Mississippi River; (3) diversion of the
sediment load away from the barrier shorelines to the deep shelf; (4) storm-induced erosion; and (5) human alteration of the
littoral process through the construction of hardened shorelines, canals, and other activities. This suite of factors has
led to the deterioration of the barrier-island systems that protect interior wetlands and human infrastructure from normal
wave activity and periodic storm impact. Interior wetland loss results in an increased tidal prism and inlet cross-sectional
areas, and expanding ebb-tidal deltas, which removes sand from the littoral processes through diversion and sequestration.
Shoreface erosion of the deltaic headlands does not provide sufficient sand to balance the loss, resulting in thinning and
dislocation of the islands. Abatement measures include replenishing lost sediment with similar material, excavated from discrete
sandy deposits within the muddy delta plain. These sand bodies were deposited by the same cyclical processes that formed the
barrier islands, and understanding these processes is necessary to characterize their location, extent, and resource potential.
In this paper we demonstrate the dominant fluvial and marine-transgressive depositional processes that occur on the inner
shelf, and identify the preservation and resource potential of fluvio-deltaic deposits for coastal management in Louisiana. 相似文献
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Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation? 下载免费PDF全文
Dominik C. Hezel Graeme M. Poole Jack Hoyes Barry J. Coles Catherine Unsworth Nina Albrecht Caroline Smith Mark Rehkämper Andreas Pack Matthew Genge Sara S. Russell 《Meteoritics & planetary science》2015,50(2):229-242
Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ56Fe values up to +0.35‰ relative to the solar system mean. On average, the δ56Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ56Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ56Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ18O (0.9–1.5‰) than their host chondrites, and the same is true for Δ17O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an 16O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts. 相似文献
18.
Jack A. C. Kaiser 《地球物理与天体物理流体动力学》2013,107(1):357-379
AbstractThe heat transfer by a rotating, differentially-heated annulus of fluid is measured throughout the high amplitude wave regime. Only Δrw T was varied (although v(T15 ).K(T15 ) varied by 46%), and it is found that Nu = C1(λ)Ra? away from the symmetry and low amplitude to wave transition curves and this is independent of ω. (λ is the wavelength.) On the wave side of these transition curves a region exists in which Nu (symmetry) λ Nu λ C1(λ)Ra?. The local heat transfer rate also varies strongly with wave phase.Using a selection of measured internal thermal fields in the steady, high amplitude wave regime, the side-wall thermal boundary layer structure is examined. It is found that Nu, = C2·Gr2 A2; both C 2 and A 2 are independent of ω and λ to first order. For the time mean profiles, A 2 ≈ 0.25; in the high heat transfer portion of the wave A 2 < ¼ and in the low heat transfer portion of the wave A 2 > ?. These relations hold over most of the vertical extent of the side walls. The deviations of the boundary layers from the above behavior which occur on the remainder of the walls is illustrated. The average thicknesses of the wall boundary layers ∞ Ra?¼ except in that phase of the wave in which the wall to mid-gap temperature difference is the largest. 相似文献
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