Cari L. Johnson

Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia

The East Gobi basin of Mongolia is a poorly described Late Jurassic–Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, dominated the Unegt- Zuunbayan landscape for much of latest Jurassic–Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence. The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia.

Occurrence, age, and implications of the Yagan–Onch Hayrhan metamorphic core complex, southern Mongolia

Mylonitic rocks associated with the south-dipping detachment fault of the Yagan–Onch Hayrhan metamorphic core complex in southernmost Mongolia indicate subhorizontal south-southeast–directed extension in the Early Cretaceous; synkinematic biotites give 40 Ar/ 39 Ar ages of 129 to 126 Ma. The Yagan–Onch Hayrhan core complex demonstrates that late Mesozoic localized high-strain extension, recently recognized in other parts of eastern Asia, also occurred in Mongolia. The presence of Mesozoic metamorphism at Onch Hayrhan, previously presumed to be Precambrian, brings into question the existence of the South Gobi microcontinent.

Paleogeographic reconstruction of a late Paleozoic arc collision zone, southern Mongolia

Permian sedimentary sequences exposed in southern Mongolia record final closure of the paleo–Asian Ocean and suturing of the terranes of northern China with the southern margin of a vast accretionary complex (the Altaids) in Mongolia. Detrital zircon U-Pb geochronology results presented here are the first of their kind in southern Mongolia. Geologic mapping, stratigraphic interpretations, and provenance data including U-Pb zircon geochronology suggest that sedimentary strata at two localities in southern Mongolia, Bulgan Uul and Nomgon, were once part of the same closing ocean basin. The Upper Permian sedimentary deposits at Bulgan Uul record an upward-shallowing marine succession that is unconformably overlain by Lower Triassic fluvial and alluvial strata. The Bulgan Uul marine succession is composed of distal turbidite fan deposits in the lowest portion of the section, with interbedded sandstone and limestone interpreted as shallow-marine deposits at the top of the section. Exposures of Permian-aged distal turbidite units at Nomgon are similar in stratigraphic architecture, sandstone provenance, and detrital zircon age distributions to those documented at Bulgan Uul. Paleocurrent measurements, sandstone provenance data, and U-Pb ages from detrital zircons collected from both study locations document southeastern transport directions for sediment derived from extinct Carboniferous and Ordovician–Silurian arcs of the southern Altaids. Results are consistent with depositional models for remnant ocean basins and indicate diachronous west-to-east closure of the paleo–Asian Ocean (a northern segment of Paleotethys) in the Late Permian. Finally, basin reconstructions place the coeval turbidite deposits at Nomgon to the southeast of Bulgan Uul during the Late Permian. These correlative turbidite successions at Nomgon are currently northeast of Bulgan Uul, offset by ∼250 km of left-lateral strike-slip faulting across the East Gobi fault zone.

EARLY ESTABLISHMENT OF SEAFLOOR HYDROTHERMAL SYSTEMS DURING STRUCTURAL EXTENSION : PALEOMAGNETIC EVIDENCE FROM THE TROODOS OPHIOLITE, CYPRUS

Abstract 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.

Sedimentology and Reservoir Architecture of a Synrift Lacustrine Delta, Southeastern Mongolia

Abstract In the East Gobi basin of southeastern Mongolia, non-marine strata fill Jurassic-Cretaceous intracontinental rift basins. Amphitheater-style cliff exposures at Har Hotol on the northern margin of the Unegt subbasin provide more than three kilometers of continuous outcrop, displaying early synrift strata that thicken and coarsen upward from prodelta lacustrine deposits to channelized fluvial sandstone. Outcrop investigations include photopanorama mapping and correlating measured sections, followed by interpretation of lithofacies units and architectural elements. Lithofacies distinctions are based on differences in grain size and bedding style, including massive to planar-bedded mudstone, and lenticular sandstone bodies with erosional bases and common soft-sediment deformation features. Architectural elements are categorized by first-order sedimentary structures (microscale,

Geochemical characteristics and correlation of oil and nonmarine source rocks from Mongolia

New bulk and molecular organic geochemical analyses of source rock and oil samples from Mongolia indicate the presence of lacustrine-sourced petroleum systems in this frontier region. Samples of potential source rocks include carbonate, coal, and lacustrine-mudstone lithologies that range from Paleozoic to Mesozoic in age, and represent a variety of tectonic settings and depositional environments. Rock-Eval and total organic carbon data from these samples reflect generally high-quality source rocks, including both oil- and gas-prone kerogen types, mainly in the early stages of generation. Bulk geochemical and biomarker data indicate that Lower Cretaceous lacustrine mudstone found in core from the Zuunbayan field is the most likely source facies for the East Gobi basin of southeastern Mongolia. Oil and selected source rock samples from the Zuunbayan and Tsagan Els fields (both in the East Gobi basin) demonstrate geochemical characteristics consistent with nonmarine source environments and indicate strong evidence for algal input into fresh- to brackish-water source facies, including elevated concentrations of unusual hexacyclic and heptacyclic polyprenoids. Despite similarities between Zuunbayan and Tsagan Els oil samples, biomarker parameters suggest higher algal input in facies sourcing Zuunbayan oil compared to Tsagan Els oil. Tsagan Els oil samples are also generated by distinctly more mature source rocks than oil from the Zuunbayan field, based on sterane and hopane isomerization ratios.

Interactions between axial and transverse drainage systems in the Late Cretaceous Cordilleran foreland basin: Evidence from detrital zircons in the Straight Cliffs Formation, southern Utah, USA

New detrital zircon geochronologic data from the Straight Cliffs Formation of southern Utah provide insight into the controls on stratigraphic architecture of the Western Interior Basin during Turonian–early Campanian time. Detrital zircon ages ( N = 40, n = 3650) derived from linked fluvial and shallow-marine depositional systems of the Kaiparowits Plateau indicate the majority of zircons in fluvial strata were derived from the Mogollon Highlands (1.25–1.90 Ga, 67% of fluvial zircons), with subordinate contributions delivered from the Sevier fold-and-thrust belt (265–1250 Ma, 17%) and Cordilleran magmatic sources (81–265 Ma, 16%). Integration of these data with fluvial facies distributions, petrography, clast counts, and evidence of magmatic arc sources from the Mohave region of California implies the presence of a northeast-flowing, axial fluvial system. This system was fed by rivers draining the Mogollon Highlands to the south and by transverse drainages from the Sevier fold-and-thrust belt to the west. Compared to the fluvial deposits, shallow-marine sandstones have a greater proportion of Sevier fold-and-thrust belt–derived zircons (42%), which were delivered via longshore currents from the north. Shallow-marine samples also contain less Mogollon input (44%) compared to contemporaneous fluvial systems, and similar input from the magmatic arc (14%). Although Proterozoic zircons associated with the Mogollon Highlands are also present in the Sevier fold-and-thrust belt, several lines of evidence argue for a distinct southerly source for the Straight Cliffs Formation. These include (1) moderate proportions of feldspar and angular quartz grains in fluvial sandstones, which favor a felsic intrusive source, and (2) prominent 1.4 and 1.7 Ga zircon populations. The 1.4 and 1.7 Ga peaks are the only dominant Proterozoic peaks in samples from the Straight Cliffs Formation, whereas samples derived more directly from the Sevier fold-and-thrust belt tend to have a broader distribution of Proterozoic age peaks. Up-section architectural trends in the Straight Cliffs Formation are linked to trends in detrital zircon geochronologic data, underscoring the likelihood of common drivers and controls. The axial system depositing Straight Cliffs fluvial strata was primarily fed by drainages originating in the Mogollon Highlands during a pulse of tectonic activity in the Maria fold-and-thrust belt and generally high subsidence rates in the foreland basin (Turonian–Santonian). Over time, activation of the Paxton duplex in the Sevier fold-and-thrust belt (early Campanian) exhumed proximal foreland basin strata and enabled drainage systems from the Sevier fold-and-thrust belt to feed into the basin more prominently. The results presented here underscore the potential significance of axial fluvial systems and their complex interplay with transverse drainage networks in foreland basins.

Sedimentary basins of the late Mesozoic extensional domain of China and Mongolia

A major rift province, comparable in extent to the Cenozoic Basin and Range province of North America, developed in east Asia during late Mesozoic time. Like the Basin and Range, the east Asian province features numerous extensional basins that developed in both low-strain and high-strain settings within heterogeneous crust that had experienced prior shortening. The low-strain basins comprise isolated or linked half-grabens that are characterized by fluvial-lacustrine facies assemblages and internal drainage. In northeastern China, normal faults that bound numerous Lower Cretaceous half-graben basins sole into Jurassic thrust faults at depth, demonstrating that the geometry of low-strain extension was strongly influenced by the pre-rift basement structure. In southeastern Mongolia, the petroliferous East Gobi basin coalesced from several Upper Jurassic-Lower Cretaceous synrift depocenters as a consequence of fault linkage. An Upper Cretaceous post-rift megasequence was deposited following an episode of mid−Cretaceous transpression that inverted the basin. The high-strain basins associated with the Hohhot metamorphic core complex in northern China constitute a supradetachment basin system that was filled with thin (<1.5 km) successions of dominantly coarse-grained alluvial strata derived from the footwalls of successive detachment splays. It is likely that the late Mesozoic extension in east Asia was related in some manner to the collapse of overthickened crust that developed during the numerous late Paleozoic−early Mesozoic orogenic events responsible for the early assembly of Asia.

Facies Analysis, Reservoir Characterization, and LIDAR Modeling of an Eocene Lacustrine Delta, Green River Formation, Southwest Uinta Basin, Utah

This study presents facies descriptions and interpretations of the Eocene Sunnyside delta interval of the Green River Formation in Nine Mile Canyon, Utah, to document and constrain reservoir architecture and heterogeneity in lacustrine strata. Detailed measurement of vertical outcrop sections, paleocurrent directions, lithologic descriptions, and facies mapping of photopanoramas and digital Light Detection and Ranging (LIDAR) data were completed to define and interpret stratigraphic architecture at centimeter to meter scales. Results favor a depositional setting that fluctuated among delta plain (overbank, distributary channel, and mouth bar), marginal lacustrine, shoal water, and offshore lacustrine deposits. These strata were deposited at a distributary-dominated shoal-water deltaic margin and form four prograding parasequences that coarsen upward over the approximately 150 m (492 ft) thick study package. Measured channel dimensions and calculated sinuosities indicate a meandering fluvial system, and channel deposits include isolated lenticular, amalgamated lenticular, and amalgamated undulatory sandstones. Along with distributary mouth bar deposits, these channelized sandstone bodies represent the most promising reservoir units based on their lateral continuity and stacking geometries (ranging from 40- to 115-m [130- to 377-ft] wide 4- to 6.5-m [13- to 20-ft] thick), as well as measured porosity and permeability relationships. Quantified dimensions of sandstone bodies and characterization of their lateral and vertical connectivity are significantly aided by a digital outcrop model. This model was generated by ground-based LIDAR, and results were exported as spatial and geologic constraints for geocellular modeling. Such quantitative outcrop-based modeling provides guidelines for predicting subsurface reservoir quality and dimensions, and may be extrapolated to analogous fluviodeltaic and lacustrine hydrocarbon prospects.

Time–space variability of paralic strata deposited in a high accommodation, high sediment supply setting: example from the Cretaceous of Utah

Abstract A previously unstudied section of the John Henry Member (Upper Cretaceous, Straight Cliffs Formation) preserves four stacked regressive–transgressive cycles of paralic strata from the Kaiparowits Plateau in south-central Utah. Meso-scale (10–100 s m thick) shoreface, wave-dominated delta and estuarine depositional environments stack vertically and show the complexity of paralic facies in a single location through time. Correlations with nearby exposures show the palaeogeographical variability updip and along-strike over c. 6.5 myr. Such variability highlights the importance of high accommodation settings in preserving transgressive deposits, including landwards-stepping barrier island and lagoon systems. The Buck Hollow section is expanded two to three times compared with correlative successions only 15–40 km away. Tectonics, eustasy and climate contributed to relative shifts in base level, but these regional controls do not explain the dramatic local thickening observed. Local controls on accommodation were quantified through decompaction analysis. The results showed that the expanded thickness of the John Henry Member in Buck Hollow can be explained by differences in decompaction (c. 9%), local erosion by fluvial incision (c. 5%), early compaction (c. 30%) and local structures such as faults (c. 100–150%). This outcrop-based study illustrates facies variability within a thick paralic succession and investigates accommodation controls on the preservation of these strata with the goal of improving predictive models for analogous deposits. Supplementary material: 40Ar/39Ar age data methods and sample location available at https://doi.org/10.6084/m9.figshare.c.3277436.v1