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Stratigraphic Units (stratigraphic + unit)
Selected AbstractsLagoon,tidal flat sedimentation in an epeiric sea: Proterozoic Bhander Group, Son Valley, IndiaGEOLOGICAL JOURNAL, Issue 2 2001Chandan Chakraborty Abstract The Bhander Group, the uppermost stratigraphic unit of the Proterozoic Vindhyan Supergroup in Son Valley, exhibits in its upper part a 550,m thick, muddy siliciclastic succession characterized by features indicative of deposition in a wave-affected coastal, lagoon,tidal flat environment suffering repeated submergence and emergence. The basic architecture of the deposit is alternation of centimetre- to decimetre-thick sheet-like interbeds of coarser clastics (mainly sandstone) and decimetre-thick mudstones. The coarser interlayers are dominated by a variety of ripple-formed laminations. The preserved ripple forms on bed-top surfaces and their internal lamination style suggest both oscillatory and combined flows for their formation. Interference, superimposed, ladder-back and flat-topped ripples are also common. Synsedimentary cracks, wrinkle marks, features resembling rain prints and adhesion structures occur in profusion on bed-top surfaces. Salt pseudomorphs are also present at the bases of beds. The mudstone intervals represent suspension settlement and show partings with interfaces characterized by synsedimentary cracks. It is inferred that the sediments were deposited on a coastal plain characterized by a peritidal (supratidal,intertidal) flat and evaporative lagoon suffering repeated submergence and emergence due to storm-induced coastal setup and setdown in addition to tidal fluctuations. The 550,m thick coastal flat succession is surprisingly devoid of any barrier bar deposits and also lacks shoreface and shelfal strata. The large areal extent of the coastal flat succession (c. 100,000,km2) and its great thickness indicate an extremely low-gradient epeiric basin characterized by an extensive coastal flat sheltered from the deeper marine domain. It is inferred that the Bhander coastal flat was protected from the open sea by the Bundelkhand basement arch to the north of the Vindhyan basin, instead of barrier bars. Such a setting favoured accumulation of a high proportion of terrigenous mud in the coastal plain, in contrast to many described examples from the Proterozoic. Copyright © 2001 John Wiley & Sons, Ltd. [source] Provenance of sandstones from the Wakino Subgroup of the Lower Cretaceous Kanmon Group, northern Kyushu, JapanISLAND ARC, Issue 1 2000Daniel K. Asiedu Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high-grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote-amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle-derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted. [source] The geoarchaeological and paleoenvironmental context of Paleoindian sites in western Middle Park, Colorado, USAGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 2 2010James H. Mayer Geoarchaeological investigations in western Middle Park provide important information for understanding the soil-stratigraphic context of Paleoindian components, as well as the latest Quaternary environmental change and landscape evolution in a Southern Rocky Mountain intermontane basin. Paleoindian components are associated with the oldest two of four latest Quaternary stratigraphic units (1,4) recognized in co-alluvial mantles (combined slopewash and colluvium) in uplands and in alluvial valley fills. Limited data suggest accumulation of unit 1 as early as ,12,500 14C yr B.P. in alluvial valleys and by at least ,11,000 14C yr B.P. in uplands was followed by brief stability and soil formation. A relatively widespread disconformity marks earliest Holocene erosion and substantial removal of latest Pleistocene deposits in upland and alluvial settings followed by unit 2 deposition ,10,000,9000 14C yr B.P., perhaps signaling the abrupt onset of an intensified summer monsoon. In situ Paleoindian components in uplands are found in a moderately developed buried soil (the Kremmling soil) formed in units 1 and 2 in thin (,1m) hillslope co-alluvial mantles. The Kremmling soil reflects geomorphic stability in upland and alluvial settings ,9000,4500 14C yr BP, and represents a buried landscape with the potential to contain additional Paleoindian components, although elsewhere in western Middle Park Early Archaic components are documented in morphologically similar soils. Kremmling soil morphology, the relative abundance of charcoal in unit 2 relative to younger units, and charcoal morphology indicate the expansion of forest cover, including Pinus, and grass cover during the early and middle Holocene, suggesting conditions moister than present. © 2010 Wiley Periodicals, Inc. [source] Inverted clast stratigraphy in an eolian archaeological environmentGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 7 2002Brenda J. Buck Understanding the geomorphic history of eolian basins is important in interpreting the archaeological record and human responses to past environments. One hundred forty soil profiles were excavated and described in southern New Mexico and West Texas. Seven major late Quaternary stratigraphic units were found: La Mesa, eolian Jornada (I, II), eolian Isaacks' Ranch, eolian Organ (I, II, and III), Historical Blowsand, and the playa deposits of Petts Tank and Lake Tank. Each unit represents a period of landscape instability, eolian erosion, and concurrent deposition, followed by landscape stability and soil formation. Eolian erosion can form local surficial lag deposits if materials larger than the competence of the wind are present. However, erosional processes alone cannot explain the presence of older clasts at the surface with intact, younger deposits underneath. We propose a combination of processes: deflation in eolian windows, followed by lateral movement of clasts over areas that have not been deflated. The effects of these processes on artifact stratigraphy and archaeological interpretations could be significant. © 2002 Wiley Periodicals, Inc. [source] Triassic metasedimentary successions across the boundary between the southern Apennines and the Calabrian Arc (northern Calabria, Italy)GEOLOGICAL JOURNAL, Issue 2 2005A. Iannace Abstract The boundary area between the Apenninic fold-and-thrust belt and the crystalline Calabrian Arc, located around Sangineto in northern Calabria, has been investigated. New geological mapping in the Sant'Agata area has been performed on the Triassic successions traditionally attributed to the metasedimentary San Donato Unit. This, coupled with a reappraisal of the stratigraphy and tectonics of coeval successions present more to the south in the Cetraro Unit, results in a new reconstruction of the Triassic evolution of all the metasedimentary successions found in the region. Four informal stratigraphic units have been distinguished in the S. Agata area. The lowest one (Unit A) consists of well-bedded metalimestones and bioturbated marly limestones that correlate with Ladinian,Carnian carbonates in nearby areas. A second unit (Unit B), never recognized before, contains a complex alternation of dolomites, phyllites and some meta-arenites containing several beds of Cavernoso facies, attributed to the Carnian. They grade upward to platform and platform-margin dolomites of Norian,Rhaetian age (Unit C) that in turn are replaced upward and laterally by a fourth unit (Unit D) consisting of well-bedded, dark dolomites and metalimestones with marly interlayers locally found as resedimented large blocks in slope conglomerates. Unit D correlates with Rhaetian,Liassic beds in nearby areas. Several pieces of evidence of post-metamorphic contractional tectonics, with 140°N and 30°N trends, are found together with evidence of SW-directed extension. The siliciclastic Carnian beds of Unit B are correlated with the phyllites of Cetraro, formerly believed to be Middle Triassic; moreover, it is suggested that in the Cetraro area Unit C is almost totally replaced by Unit D. This demonstrates that the former distinction between the two tectonic units in the whole area has to be discarded. We have made a general palaeoenvironmental reconstruction which progresses laterally, during Ladinian,Carnian times, from (i) a coastal, mixed siliciclastic,carbonate,evaporitic area at Cetraro to (ii) a transitional carbonate shelf where siliciclastic input was only episodic, and finally to (iii) a bioconstructed margin which was later replaced by a steepened margin created by tectonic instability. Starting from the Norian, subsidence shifted toward the former coastal area where an intraplatform, restricted basin developed. The proposed stratigraphy corresponds closely to the Alpujarride units of the Betic Cordillera, Spain. Moreover, it is shown that strong affinities also exist, in terms of the structural framework, with the metamorphic units of Tuscany and Liguria. Copyright © 2005 John Wiley & Sons, Ltd. [source] Stratigraphic Control of Flow and Transport CharacteristicsGROUND WATER, Issue 6 2006Dwaine Edington Ground water flow and travel time are dependent on stratigraphic architecture, which is governed by competing processes that control the spatial and temporal distribution of accommodation and sediment supply. Accommodation is the amount of space in which sediment may accumulate as defined by the difference between the energy gradient and the topographic surface. The temporal and spatial distribution of accommodation is affected by processes that change the distribution of energy (e.g., sea level or subsidence). Fluvial stratigraphic units, generated by FLUVSIM (a stratigraphic simulator based on accommodation and sediment supply), with varying magnitudes and causes of accommodation, were incorporated into a hydraulic regime using MODFLOW (a ground water flow simulator), and particles were tracked using MODPATH (a particle-tracking algorithm). These experiments illustrate that the dominant type of accommodation process influences the degree of continuity of stratigraphic units and thus affects ground water flow and transport. When the hydraulic gradient is parallel to the axis of the fluvial system in the depositional environment, shorter travel times occur in low,total accommodation environments and longer travel times in high,total accommodation environments. Given the same total accommodation, travel times are longer when sea-level change is the dominant process than those in systems dominated by subsidence. [source] Water Resource Implications of 18O and 2H Distributions in a Basalt Aquifer SystemGROUND WATER, Issue 6 2000Kathryn R. Larson Ongoing decline of water levels in the confined basalt aquifers of the Pullman-Moscow Basin of Washington and Idaho has prompted study of the timing, amount and distribution of recharge to the system. Previous radiocarbon ages indicate residence times on the order of 103 years and greater and suggest a low rate of recharge to the lower basalt aquifer since the end of Pleistocene time. By contrast, more recent hydrodynamic flow modeling studies invoke a larger Holocene recharge rate through the unconfined loess unit to the upper and lower basalt aquifers, which implies relatively short residence times (102 years). Stable isotopes were used to independently assess contrasting recharge models by comparing 18O/16O and D/H ratios of late-Holocene shallow ground water and deep ground water. Linear regression of local precipitation ratios yields ,D = 6.9 ,18O ,18.5. There is no evidence of fractionation of ground water ratios by recharge processes or water-rock interactions. Deep basalt ground water ,18O values are depleted by 0.4 to 4.9 per mil relative to shallow, recently recharged ground waters and have ,18O values statistically distinct from waters sampled from other stratigraphic units. These findings suggest that the deep waters in the basin were not precipitated under current climate conditions and that aquifer recharge rates to the deep basalt aquifer are substantially lower than have been recently estimated. This in turn suggests that a sustainable ground water exploitation scheme must reduce reliance on the deep ground water resource. [source] Tectonic vs. climate forcing in the Cenozoic sedimentary evolution of a foreland basin (Eastern Southalpine system, Italy)BASIN RESEARCH, Issue 6 2009N. Mancin ABSTRACT This paper discusses the Cenozoic interaction of regional tectonics and climate changes. These processes were responsible for mass flux from mountain belts to depositional basins in the eastern Alpine retro-foreland basin (Venetian,Friulian Basin). Our discussion is based on the depositional architecture and basin-scale depositional rate curves obtained from the decompacted thicknesses of stratigraphic units. We compare these data with the timing of tectonic deformation in the surrounding mountain ranges and the chronology of both long-term trends and short-term high-magnitude (,aberrant') episodes of climate change. Our results confirm that climate forcing (and especially aberrant episodes) impacted the depositional evolution of the basin, but that tectonics was the main factor driving sediment flux in the basin up to the Late Miocene. The depositional rate remained below 0.1 mm year,1 on average from the Eocene to the Miocene, peaking at around 0.36 mm year,1, during periods of maximum tectonic activity in the eastern Southern Alps. This dynamic strongly changed during the Pliocene,Pleistocene, when the basin-scale depositional rate increased to an average of 0.26 mm year,1 (Pliocene) and 0.73 mm year,1 (Pleistocene). This result fits nicely with the long-term global cooling trend recorded during this time interval. Nevertheless, we note that the timing of the observed increase may be connected with the presumed onset of major glaciations in the southern flank of the Alps (0.7,0.9 Ma), the acceleration of the global cooling trend (since 3,4 Ma) and climate variability (in terms of magnitude and frequency). All these factors suggest that combined high-frequency and high-magnitude cooling,warming cycles are particularly powerful in promoting erosion in mid-latitude mountain belts and therefore in increasing the sediment flux in foreland basins. [source] Fault architecture, basin structure and evolution of the Gulf of Corinth Rift, central GreeceBASIN RESEARCH, Issue 6 2009R. E. Bell ABSTRACT The style of extension and strain distribution during the early stages of intra-continental rifting is important for understanding rift-margin development and can provide constraints for lithospheric deformation mechanisms. The Corinth rift in central Greece is one of the few rifts to have experienced a short extensional history without subsequent overprinting. We synthesise existing seismic reflection data throughout the active offshore Gulf of Corinth Basin to investigate fault activity history and the spatio-temporal evolution of the basin, producing for the first time basement depth and syn-rift sediment isopachs throughout the offshore rift. A major basin-wide unconformity surface with an age estimated from sea-level cycles at ca. 0.4 Ma separates distinct seismic stratigraphic units. Assuming that sedimentation rates are on average consistent, the present rift formed at 1,2 Ma, with no clear evidence for along-strike propagation of the rift axis. The rift has undergone major changes in relative fault activity and basin geometry during its short history. The basement depth is greatest in the central rift (maximum ,3 km) and decreases to the east and west. In detail however, two separated depocentres 20,50 km long were created controlled by N- and S-dipping faults before 0.4 Ma, while since ca. 0.4 Ma a single depocentre (80 km long) has been controlled by several connected N-dipping faults, with maximum subsidence focused between the two older depocentres. Thus isolated but nearby faults can persist for timescales ca. 1 Ma and form major basins before becoming linked. There is a general evolution towards a dominance of N-dipping faults; however, in the western Gulf strain is distributed across several active N- and S-dipping faults throughout rift history, producing a more complex basin geometry. [source] Controls on early post-rift physiography and stratigraphy, lower to mid-Cretaceous, North Viking Graben, Norwegian North SeaBASIN RESEARCH, Issue 2 2009A.-J. Zachariah ABSTRACT The transition from syn- to post-rift is often poorly constrained and in contrast to syn-rift systems, the controls on the development of post-rift systems are poorly understood. This paper documents the timing of the post-rift onset and discusses the controls that affected the subsequent development of the post-rift infill of the North Viking Graben using an integration of seismic and well data. The study enhances our understanding of post-rift system development in general and provides an analogue for other post-rift systems. Within the early post-rift infill of the North Viking Graben five key seismic surfaces were mapped [Base Cretaceous Unconformity (BCU), Intra-Aptian, Top Albian, Top Cenomanian and Top Turonian], which divide the post-rift interval into four key seismic stratigraphic units (K1,K4). The BCU has an intra-Volgian age on the basin slopes and shelfal and terrace areas and is interpreted to mark the end of rifting in the study area. On the footwall crests adjacent to the graben the BCU represents a complex unconformity from the syn- and post-rift combined, and in the graben it forms a conformable contact. Therefore, the BCU could not be used to date the onset of the post-rift in these locations. The thickness variations and age relationships between the syn-rift stratigraphy and the K-units reveal that the early post-rift infill of the North Viking Graben was dominantly controlled by the significant local syn-rift topography, especially in the K1 and K2 stages. The Cretaceous post-rift stratigraphy was also influenced by relative base level, which controlled the sediment source areas, the development of the basin geometry itself and subsequently the style of sediment deposition in the study area. Regional variations are also recognised in the post-rift stratigraphy although these variances are strongly influenced by the local basin physiography. [source] | ||||||||||