Caddisfly larvae construct underwater protective cases using surrounding materials, thus providing information on environmental conditions in both modern and ancient systems. Microbial bioherms ...associated with caddisfly cases are found in the Berriassian-Hauterivian (~140-130 Ma) Shinekhudag Formation of Mongolia, and yield new insights into aspects of lacustrine paleoecosystems and paleoenvironments. This formation contains the earliest record of plant-armored caddisfly cases and a rare occurrence of microbial-caddisfly association from the Mesozoic. The bioherms are investigated within the context of stratigraphic correlations, depositional environment interpretations, and basin-evolution models of the sedimentary fill. The bioherms form 0.5-2.0 m diameter mound-shaped bodies and are concentrated within a single, oil shale-bound stratigraphic interval. Each bioherm is composed of up to 40% caddisfly cases along with stromatolites of millimeter-scale, micritic laminations. Petrographic analyses reveal these bioherms are composed of non-systematic associations of columnar and oncoidal microbialites, constructed around colonies of caddisfly cases. The cases are straight to curved, slightly tapered, and tube-shaped, with a progressively increasing length and width trend (7-21 mm by 1.5-2.5 mm). Despite these variations, the case architectures reveal similar construction materials; the particles used for cases are dominated by plant fragments, ostracod valves, carbonate rocks, and rare mica and feldspar grains. Allochems within the bioherms include ooids, ostracods, plant fragments, rare gastropods, feldspar grains bound in micritic matrices, and are consolidated by carbonate dominated cements. The combination of microbial-caddisfly association, plant fragment case particles, and ooids/oncoids are indicative of a shallow, littoral lake setting. Stratigraphic juxtaposition of nearshore bioherms and the bounding distal oil-shale facies suggests that the bioherms developed in an underfilled lake basin, resulting from an abrupt and short-lived lake desiccation event. Lake chemistry is believed to have been relatively alkaline, saline to hypersaline, and rich in Ca, Mg, and HCO3 ions. Through analyzing bioherm characteristics, caddisfly case architecture, carbonate microfacies, and stratigraphic variability, we infer larger-scale processes that controlled basin development during their formation.
•Evolution of the Kura Basin during closure of the South Caspian Basin was complex.•Spatial trends in oxicity and organic type are shown through geochemical results.•A small portion of the Maikop ...source interval produces most of the hydrocarbons.•Chemostratigraphy can be used to effectively subdivide fine-grained Maikop strata.
Fine-grained Paleocene–Miocene strata of eastern Azerbaijan record the evolution of the Kura Basin from an open marine system connecting the South Caspian Basin to the Paratethys of eastern Europe to an isolated epicontinental sea. Tectonic uplift associated with the Greater Caucasus mountains in Azerbaijan resulted in exposure of these strata over much of the northeastern part of the basin, allowing detailed observation and sampling over a broad area. Four hundred Paleocene–Miocene samples collected in eastern Azerbaijan demonstrate spatial and temporal geochemical heterogeneity within the predominately mudstone succession, indicating a complex evolution of this elongate basin. Through detailed organic geochemical analysis of samples, trends can be seen in the distribution of total organic carbon (TOC), pyrolysis characteristics, and biomarker results from a subset of twenty-four samples. In addition to confirming the well-documented regional basin restriction during the Oligocene to early Miocene deposition of the Maikop Series, heterogeneity of these geochemical indicators across the study area shows spatial variability in oxicity and terrestrial organic input within the basin on a sub-regional scale.
Observed geochemical variability suggests a complex evolutionary history of the Kura Basin during the Paleocene–Miocene, and this has important implications for hydrocarbon prospectivity in the region. Within the 400 collected samples, total organic carbon (TOC) values range from 0.3 to 6.3%, with Oligocene–Miocene samples showing average values of 1.4%, compared to lower TOC values found in Paleocene–Eocene strata (average=0.3%). Rock-Eval pyrolysis shows that the majority of strata are organic lean, immature, and mixed oil/gas to gas prone (Type II/III to Type III), with a smaller group of latest Eocene–Early Miocene samples having better oil prone source potential. Gas chromatography and biomarker analysis of 24 samples reinforces the immaturity of most samples, shows varying levels of terrestrial input in all samples, suggests well-oxygenated waters prevailed with periodic suboxic to dysoxic/anoxic events, highlights gross differences between the Paleocene–Eocene and Oligocene–Miocene stratigraphic intervals, and shows generally good lateral correlation between samples. These interpretations reinforce what is seen through inorganic geochemical evaluation of trace metals, bulk composition, and stable isotopic ratios, and offer more detail as to the evolution of this basin and the implications for oil and gas prospectivity of the region.
Existing barrier island facies models are largely based on modern observations. This approach highlights the heterogeneous and dynamic nature of barrier island systems, but it overlooks processes ...tied to geologic time scales, such as multi-directional motion, erosion, and reworking, and their expressions as preserved strata. Accordingly, this study uses characteristic outcrop expressions from paralic strata of the Upper Cretaceous Straight Cliffs Formation in southern Utah to update models for barrier island motion and preservation to include geologic time-scale processes. Results indicate that the key distinguishing facies and architectural elements of preserved barrier island systems have very little to do with “island” morphology as observed in modern systems. Four facies associations are used to describe and characterize these barrier island architectural elements. Barrier islands occur in association with backbarrier fill (FA1) and internally contain lower and upper shoreface (FA2), proximal upper shoreface (FA3), and tidal channel facies (FA4). Three main architectural elements (barrier island shorefaces, shoreface-dominated inlet fill, and channel-dominated inlet fill) occur independently or in combination to create stacked barrier island deposits. Barrier island shorefaces record progradation, while shoreface-dominated inlet fill records lateral migration, and channel-dominated inlet fill records aggradation within the tidal inlet. Barrier islands are bound by lagoons or estuaries and are distinguished from other shoreface deposits by their internal facies and outcrop geometry, association with backbarrier facies, and position within transgressive successions. Tidal processes, in particular, tidal inlet migration and reworking of the upper shoreface, also distinguish barrier island successions. In sum, this study expands barrier island facies models and provides new recognition criteria to account for the complex geometries of time-transgressive, preserved barrier island deposits.
Tectonic studies of the East Gobi Fault Zone in southeastern Mongolia reveal multiple, distinct intracontinental deformation events postdating late Paleozoic arc accretion and continental ...amalgamation. Metamorphic tectonites of the Tsagan Subarga and Tavan Har blocks, previously mapped as Precambrian basement, comprise a sinistral shear zone dominated by steeply-dipping, northeast-striking foliations. Field observations and petrographic analyses indicate that the protoliths of the metamorphic tectonites are Paleozoic arc volcanic and sedimentary sequences.
40Ar/
39Ar step-heating analyses of minerals from pre-, syn-, and late- to post-kinematic lithologies bracket the timing of ductile sinistral shear as Late Triassic. The main phase of distributed deformation associated with cooling through amphibolite–upper greenschist-facies conditions occurred ca. 225
Ma and shear zone activity waned ca. 210
Ma. Cooling rates inferred from the
40Ar/
39Ar data are on the order of 40–20
°C Myr
−
1
; apparent differences for the two basement blocks may reflect subsequent differential uplift during Late Jurassic–Cretaceous rifting. Relatively rapid Late Triassic cooling suggests a transtensional component to the deformation and is coincident with core complex formation in northern China. Late Triassic intraplate deformation in southeastern Mongolia is likely the result of far field forces associated with collision between Mongolian arcs and the Siberian craton (i.e. closure of the Mongol-Okhotsk ocean) and/or collisions associated with closure of the Paleotethys. The ductile shear zone has been documented over 250
km along strike and has been modified by subsequent brittle deformation events in the Mesozoic and Cenozoic.
►Regional Middle to Late Triassic sinistral shear zone in southeastern Mongolia. ►Rapid cooling through amphibolite–upper greenschist-facies conditions. ►Intraplate deformation focused in region of juvenile crust.
Marginal marine deposits of the John Henry Member, Upper Cretaceous Straight Cliffs Formation, were deposited within a moderately high accommodation and high sediment supply setting that facilitated ...preservation of both transgressive and regressive marginal marine deposits. Complete transgressive–regressive cycles, comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. Two main types of boundaries occur between the transgressive and regressive portions of each cycle: (i) surfaces that record the maximum regression and onset of transgression (bounding surface A); and (ii) surfaces that place deeper facies on top of shallower facies (bounding surface B). The base of a transgressive facies (bounding surface A) is defined by a process change from wave‐dominated to tide‐dominated facies, or a coaly/shelly interval indicating a shift from a regressive to a transgressive regime. The surface recording such a process change can be erosional or non‐erosive and conformable. A shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces (bounding surface B). These two main bounding surfaces and their subtypes generate three distinct transgressive–regressive cycle architectures: (i) tabular, shoaling‐upward marine parasequences that are bounded by flooding surfaces; (ii) transgressive and regressive unit wedges that thin basinward and landward, respectively; and (iii) tabular, transgressive lagoonal shales with intervening regressive coaly intervals. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive–regressive cycles. Acknowledging variation in transgressive–regressive cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of high‐frequency cycles.
The Upper Cretaceous John Henry Member of the Straight Cliffs Formation preserves regressive shoreface and channel facies, and transgressive lagoonal tidal inlet facies that suggest distinct modal ...sandstone compositions. Detrital modes from six sandstone facies (upper shoreface, lower shoreface, deflected mouth bars, fluvial channels, tidal inlets and washover fans), and their spatial and temporal variations, provide additional data regarding the depositional environments and setting of the John Henry Member. Three statistical methods were utilized: univariate standard deviation and confidence interval and multivariate logratio transformations to assess the compositional difference between facies using estimation of means. Both univariate statistical methods have some inherent problems. Standard deviation does not account for sample size, while the confidence interval method incorporates a t-test to account for only some of the uncertainty of small sample size. The results of the univariate methods differ slightly and both indicate statistically distinct sandstone compositions based on facies. Multivariate statistics are much more robust and suggest similar trends, yet display different compositional means and have inconclusive confidence fields as they incorporate all of the uncertainty associated with small sampling sizes. In general, sandstone compositions become more quartz-rich and compositionally mature as sediment is transported from proximal fluvial environments to distal upper shoreface environments. This trend reflects the degree of preferential reworking or winnowing of unstable grains prior to lithification. More complicated relationships are observed as facies distributions shift through space and time. Sandstone compositions support a deflected wave-dominated deltaic interpretation for the John Henry Member in this area. Sandstone compositions become more lithic-rich from north to south, corresponding to the facies distribution of upper shoreface and deflected mouth bars, respectively. Mouth bars occur at the intersection of the fluvial channel and the coastline where sand is deflected via longshore drift to the southern portion of the field area. Upper shoreface facies located further away from mouth bar facies are heavily reworked by wave processes and contain more mature quartz-rich modal compositions. Additionally, high feldspathic and lithic concentrations in tidal inlet facies suggest that this facies was sourced closer to mouth bars rather than updrift upper shoreface sediments. A shift in sandstone composition occurs at the end of the second transgressive–regressive cycle of the John Henry Member throughout the Rogers Canyon area. This compositional change is concurrent with a relatively large basinward shift in facies, which suggests that it reflects the transition from regressive to transgressive facies associated with the relative sea level fall rather than a change in the main sediment source area.
Mapping and correlation of 2D seismic reflection data define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic–Cretaceous intracontinental rift evolution through ...deposition of at least five distinct stratigraphic sequences. Three major northeast–southwest‐trending fault zones divide the basin, including the North Zuunbayan (NZB) fault zone, a major strike‐slip fault separating the Unegt and Zuunbayan subbasins. The left‐lateral NZB fault cuts and deforms post‐rift strata, implying some post‐middle‐Cretaceous movement. This fault likely also had an earlier history, based on its apparent role as a basin‐bounding normal or transtensional fault controlling deposition of the Jurassic–Cretaceous synrift sequence, in addition to radiometric data suggesting a Late Triassic (206–209 Ma) age of deformation at the Tavan Har locality. Deposits of the Unegt subbasin record an early history of basin subsidence beginning ∼155 Ma, with deposition of the Upper Jurassic Sharilyn and Lower Cretaceous Tsagantsav Formations (synrift sequences 1–3). Continued Lower Cretaceous synrift deposition is best recorded by thick deposits of the Zuunbayan Formation in the Zuunbayan subbasin, including newly defined synrift sequences 4–5. Geohistory modelling supports an extensional origin for the EGB, and preliminary thermal maturation studies suggest that a history of variable, moderately high heat flow characterized the Jurassic–Cretaceous rift period. These models predict early to peak oil window conditions for Type 1 or Type 2 kerogen source units in the Upper Tsagantsav/Lower Zuunbayan Formations (Synrift Sequences 3–4). Higher levels of maturity could be generated from distal depocentres with greater overburden accumulation, and this could also account for the observed difference in maturity between oil samples from the Tsagan Els and Zuunbayan fields.
PDF
