Ancient barrier islands are poorly understood relative to other clastic depositional environments, despite being prominent features along modern coastlines and important for understanding ...transgressive shoreline deposits. A new dataset of ancient barrier island dimensions (n = 83 examples) addresses this knowledge gap with a quantitative analysis of barrier island sand body dimensions including thickness (vertical), length (shore-parallel direction), and width (shore-perpendicular direction). This dataset of barrier island deposits was compared to planform measurements made for modern islands (n = 274), to investigate possible scaling relationships and other aspects of modern to ancient linkages. These measurements are nuanced and challenging to perform, and first-pass comparisons show that modern barrier islands should not be used as direct analogs for ancient systems. Nevertheless, results emphasize key depositional and preservation processes, and the dimensional differences between deposits formed over geologic vs. modern time scales. Using the methods outlined herein, barrier island deposits appear to be 2-5x longer (p50 modern = 10.7 km; p50 ancient = 20.0 km), and 6–15x wider (p50 modern = 1.2 km; p50 ancient = 7.3 km) than modern barrier islands. We interpret the results to indicate that ancient barrier islands are time-transgressive deposits recording vertical amalgamation, and barrier island growth by lateral accretion, and progradation. When comparing single barrier islands, thickness measurements do not vary systemically between modern and ancient examples, suggesting that local accommodation dictates barrier island thickness as a preservation control. Gross length, width, and thickness measurements are too coarse for robust paleomorphodynamic calculations, therefore more detailed sub-environment analysis (e.g., upper shoreface delineation), with improved facies models, is required before rigorous quantifications can be generated. However, these initial comparisons do show scaling trends between length and width which could be leveraged, with caution, in the interim. As sea levels continue to rise, understanding barrier island motion and preservation will be central to predicting coastal change.
The East Gobi Fault Zone (EGFZ) is a dominant structural feature in southeastern Mongolia. The EGFZ's protracted history includes at least five distinct deformation events occurring over the last 250 ...million years. Two of these phases are marked by left-lateral motion, but the total and incremental displacement history is poorly known. A better understanding of the displacement history across the EGFZ is required to place its Mesozoic and Cenozoic evolution within the larger context of central Asian tectonics, including evaluating the EGFZ's possible role as an intracontinental extrusion feature. This study presents analysis of sedimentary and igneous rocks as proposed offset markers on either side of the EGFZ. New geochronologic data, integrated with existing structural and stratigraphic data, are used to characterize and quantify two periods of left-lateral displacement along the EGFZ. The slip history of the EGFZ includes a total of ∼250–300 km of left-lateral displacement. Approximately 150–200 km of slip occurred along a major shear zone during the Late Triassic (∼225–210 Ma), based on restoration of Carboniferous intrusive suites and Permian remnant ocean basin deposits. During this first period of left-lateral displacement, the EGFZ acted as a continental extrusion feature responding to oblique collision between the South and North China Blocks and northern Mongolia and Siberia, an event analogous to the later India–Asia collision during the Cenozoic. A second period of ∼90–100 km of left-lateral slip occurred in the Late Oligocene, based on offset of Lower Cretaceous strata and cross-cutting relationships. This brittle deformation phase was contemporaneous with dynamic boundary conditions along the Pacific margin, and the Indo–Asia collision. Cenozoic movement along the EGFZ may have coincided kinematically with the Altyn Tagh Fault in China via dispersed fault zones in the Alxa region. Major displacement along the EGFZ likely ceased in the early Miocene, coincident with changing relative plate motions at the Pacific margin. Magnitudes of Cenozoic offset across the EGFZ are significantly smaller than estimated for the Altyn Tagh Fault, which likely reflects distributed deformation in heterogeneous crust of the southern Central Asian Orogenic Belt.
•Approximately 150–200 km of slip occurred along the East Gobi Fault Zone in the Late Triassic•Cenozoic offset across the East Gobi Fault Zone is much less than estimated for the Altyn Tagh Fault•Dissected sedimentary basins are viable piercing points for fault zone displacement calculation
The Central Asian Orogenic Belt, or Altaids, is an amalgamation of volcanic arcs and microcontinent blocks that records a complex late Precambrian–Mesozoic accretionary history. Although ...microcontinents cored by Precambrian basement are proposed to play an integral role in the accretion process, a lack of isotopic data hampers volume estimates of newly produced arc-derived versus old-cratonic crust in southeastern Mongolia. This study investigates metamorphic tectonites in southern Mongolia that have been mapped as Precambrian in age, largely on the basis of their high metamorphic grade and high strain. Here we present results from microstructural analyses and U-Pb zircon geochronology on samples from Tavan Har (44.05° N, 109.55° E) and the Yagan-Onch Hayrhan metamorphic core complex (41.89° N, 104.24° E). Our results show no compelling evidence for Precambrian basement in southeastern Mongolia. Rather, the protoliths to all tectonites examined are Paleozoic–Mesozoic age rocks, formed during Devonian–Carboniferous arc magmatism and subsequent Permian–Triassic orogenesis during collision of the South Mongolia arc with the northern margin of China. These results yield important insights into the Paleozoic accretionary history of southern Mongolia, including the genesis of metamorphic and igneous basement during the Paleozoic, as well as implications for subsequent intracontinental reactivation.
The Oligocene–Miocene Maikop Series is a world-class source rock responsible for much of the oil and gas found in the South Caspian Basin. It is composed of up to 3 km of marine mudstone, and ...contains a nearly continuous record of deposition during progressive tectonic closure of the basin as the Arabian Peninsula converged northward into Eurasia. Historically, the stratigraphy of this interval has been difficult to define due to the homogenous nature of the fine-grained, clay-dominated strata. Outcrop exposures in eastern Azerbaijan allow direct observation and detailed sampling of the interval, yielding a more comprehensive stratigraphic context and a more advanced understanding of syndepositional conditions in the eastern Paratethys Sea. Specifically, the present investigation reveals that coupling field-based stratigraphic characterization with geochemical analyses (e.g., bulk elemental geochemistry, Rock-Eval pyrolysis, bulk stable isotope geochemistry) yields a more robust understanding of internal variations within the Maikop Series. Samples from seven sections located within the Shemakha–Gobustan oil province reveal consistent stratigraphic and spatial geochemical trends. It is proposed that the Maikop Series be divided into three members based on these data along with lithostratigraphic and biostratigraphic data reported herein. When comparing Rupelian (Early Oligocene) and Chattian (Late Oligocene) strata, the Rupelian-age strata commonly possess higher TOC values, more negative δ
15N
tot values, more positive δ
13C
org values, and higher radioactivity relative to Chattian-age rocks. The trace metals Mo and V (normalized to Al) are positively correlated with TOC, with maximum values occurring at the Rupelian–Chattian boundary and overall higher average values in the Rupelian. Across the Oligocene–Miocene boundary, a slight drop in V/Al, Mo/Al ratios is observed, along with drops in %S and TOC. These results indicate that geochemical signatures of the Maikop Series are regional in nature, and furthermore that analogous fine-grained sections may be better characterized and subdivided using similar techniques. In general, geochemical indicators suggest that the basin was in limited communication with the Tethys Sea throughout the Oligocene–Early Miocene, with suboxic to anoxic conditions present during the Oligocene and to a lesser extent in the Miocene. This increased isolation was likely due to tectonic uplift to both the south and north of the study area, and greatly enhanced by global sea-level fluctuations. These data serve as the basis for a more detailed understanding of the tectonic evolution of the region, and support a standardized chemostratigraphic division of the important petroleum source interval.
Classification of barrier island morphology stems from the seminal work of M. O. Hayes and others, which linked island shape to tidal range and wave height and defined coastal energy regimes (i.e., ...wave-dominated, mixed energy, tide-dominated). If true, this general relationship represents a process-based framework to link modern and ancient systems, and is key for determining paleomorphodynamic relationships. Here we present a new semi-global database of barrier islands and spits (n=702). Shape parameters (aspect, circularity, and roundness) are used to quantify island boundary shape, and assess potential correlation with coastal energy regime using global wave and tide models. In adopting the original energy classification as originally put forth (i.e., wave dominated, wave-influenced mixed, tide-influenced mixed, tide dominated), results show that wave-dominated islands have statistically different mean shape values from those in the mixed energy fields, but the two mixed energy designations are not distinct from each other. Furthermore, each energy regime field contains a wide range of island shapes, with no clear trends present. Linear regression modeling shows that tidal range and wave height account for <10% of the documented variance in island shape, a strong indication that other controls must be considered. Therefore, while energy regime distinctions can be used descriptively, their utility in predicting and constraining island shape is limited: barrier island shape is not indicative of coastal energy regime, and vice versa. Our analysis also demonstrates empirical scaling relationships among modern barrier islands for the first time, with implications for subsurface prediction.
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•The tectonic evolution and history of intraplate deformation in Asia is presented.•Timing and nature of intracontinental deformation along the East Gobi Fault Zone is ...documented.•Subsurface observations were made based on geophysical data from southeastern Mongolia.•A detailed field investigation for areas within the region of Tavan Har was carried out.
Structural interpretation of 2-D seismic reflection data and subsurface-outcrop correlations reveal six distinct phases of deformation recorded in the Paleozoic basement rocks and Mesozoic-Cenozoic basin fill of the East Gobi Basin (EGB), southeastern Mongolia. These phases include arc accretion and arc-continent collision in late Paleozoic time, Late Triassic sinistral shear-zone development, Early Jurassic fold and thrust belt style shortening, Middle Jurassic-Lower Cretaceous extension and rift basin development, middle Cretaceous shortening with basin inversion and regional unconformity development, and Late Cretaceous-Oligocene thermal subsidence with renewed Paleogene left-lateral strike slip faulting across the fault zone. The five post-amalgamation deformation phases are localized along the East Gobi Fault Zone, suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the EGB. Subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the EGB contain major differences in basement metamorphic and structural fabrics, basin fill patterns, and distinct Mesozoic-Cenozoic fault generations. Potential causes related to far-field boundary conditions and other driving factors are suggested for each of the major deformation phases.
The scale of features shown on outcrop photographs can be critical to geoscience interpretations, yet little is known about how well individuals estimate scale in images. This study utilizes a ...visualization test in which participants were asked to estimate the absolute size of several boxes shown in outcrop images using high resolution, stitched photopanoramas (Gigapans). Participants viewed two different outcrops that highlight different kinds of photographic distortion, first using static images and then with "interactive" Gigapans that permitted zooming and panning. A test group was given basic scaling cues in the form of distance to and height of the outcrops, whereas a control group completed the test without any scaling cues. Other population comparisons were investigated (e.g., gender, age, experience level, and major) but no other statistically significant population difference was observed. Therefore, scaling cues seem to invoke a primary effect at least in the first part of the exercise. Results show that scaling cues increase accuracy overall, but with wider spread and a tendency to cause overestimation of size. The control group, which was not given any scaling information, was less accurate overall and tended to underestimate the size of features. Both groups gave more accurate scale estimates with smaller standard deviations for the extension-distorted photopanorama than the compression-distorted image. Participants also generally showed improved accuracy in the second part of the test, which probably reflects the impact of interactivity, although a training effect cannot be discounted. These results suggest that nonembedded scaling cues (as opposed to physical objects denoting scale in photographs) can be useful for some individuals to estimate the size of features shown in outcrop images. Results also underscore the importance of interactivity and multiple exposures in classroom applications.
Paleomagnetic data from the Troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of ...large-tonnage sulfide deposits at oceanic ridges. We have sampled dikes from the Troodos sheeted complex that have been subjected to variable hydrothermal alteration, from greenschist alteration typical of the low water/rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks (epidosites) altered within high water/rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits. These dikes are moderately to highly tilted from their initial near-vertical orientations due to rotations in the hangingwalls of approximately dike-parallel, oceanic normal faults. Comparison of characteristic remanence directions from these dikes with the Late Cretaceous Troodos reference direction, therefore, allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting. Remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the Late Cretaceous Troodos reference direction upon restoration of dikes to near-vertical positions about a NNW-trending, horizontal axis. These data, along with field observations of focused alteration along normal faults, suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation. This sequence of events is similar to that observed for creation of large-tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension. We suggest that the dike-parallel normal faults were initiated as extensional fractures during this early stage of crustal extension, thus providing the necessary permeability for focused fluid flow, and that later slip along these structures during rotational-planar normal faulting caused reduction in permeability due to gouge formation.
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