Lubomir F. Jansa

Sedimentary signatures and processes during marine bolide impacts: a review

Abstract Studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts. This results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters. In subaerial impacts, the targets are commonly hard rocks, frequently of igneous and/or metamorphic origin, whereas in submarine impacts, the targets are mostly unconsolidated or poorly lithified sediments, or sedimentary rocks, with high volumes of pore water. Such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area. Impacts in shallow-water marine (neritic) environments produced craters with low or absent rims and wide and shallow brims, as characterize by both the Montagnais (on the Scotian shelf), the Mjolnir (in the Barents Sea), 45 and 40 km in diameter, respectively, and the Chesapeake Bay (90 km in diameter). Lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity, as the water in the crater is vaporized. During this process, resurge gullies can be cut across the rim, while mass- and debris-flows, turbidites, and other gravity deposits are produced as results of tsunami and crater-wall and central high collapse, during and after the crater excavation stage. Such deposits are found both within and outside the crater structure. The only difference between gravity deposits triggered by an impact or other rare events, such as earthquakes, is the admixture of various melt particles and possible enrichments in iridium in the former. Impacts near the shelf edge may cause partial collapse of the continental margin as shown by the Montagnais and Chicxulub impacts. Some of the gravity and debris flows generated by margin collapse may be channelized, with final deposits up to several hundred meters thick, extending for hundreds of kilometers from the impact site. Other impact features such as shatter cones, tektites, spherules and Ni-spinels, shocked quartz, isotropication, and partial melts, are common to both submarine and subaerial impacts. Theoretical calculations of the destructive forces of mega-tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence. A dearth of gigantic tsunami evidence for the Chicxulub impact outside of the Gulf of Mexico, where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep North Atlantic basin should have been >1 m/s, could be the result of the mitigating effect of the bathymetry of surrounding area, causing wave diffraction and interference. Calculated maximum horizontal orbital velocity near the seafloor at the shelf for the Montagnais impact is 22 m/s for a 200-m-high wave, 5.5 m/s for a 50-m-high wave at 500 km, decreasing to 0.5 m/s at 1000 km, strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record. However, lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows.

Characteristics of the shale gas reservoir rocks in the Lower Silurian Longmaxi Formation, East Sichuan Basin, China

The Sichuan basin is an oil-bearing and gas-rich basin with extensive development of the Lower Silurian Longmaxi Formation shale in southwestern China. The gas shows in the shale were identified in exploration wells mainly located between southeastern Sichuan basin and Western Hubei-Eastern Chongqing. The thickness of the Lower Silurian Longmaxi Formation shale ranges from 65 to 516 m. The base of the Longmaxi Formation shale is graptolite-rich transgressive black shale. Its thickness increases eastward in the study area, similarly as the sand content in the formation, with the latter also increasing stratigraphically upward. The Longmaxi Formation is comprised by eight lithofacies, including laminated and nonlaminated mudstone/shale, dolomitic siltstone, laminated lime mudstone/shale, argillaceous siltstone, laminated and nonlaminated silty mudstone/shale, fine grained silty sandstone, calcareous concretions and nonlaminated shale enriched organic matter. The biota in the formation is dominated by graptolites, ostracods, echinoderms, brachiopods, trilobites and radiolarian. Longmaxi Formation contains 0.2% to 6.7% of organic carbon (TOC). The organic matter is overmature, with Ro 2.4%−3.6% and dominated by Type II-kerogen. Quartz silt, which is the second important component of the shale gas reservoir quality, occurs as laminae and/or disseminated and varies from 2% – 93% in the shale. The size of quartz silt ranges from 0.03 to 0.05mm, with terrigenous origin. Porosity measured on the core samples of the shale from the Longmaxi Formation in exploratory wells ranges from 0.58% to 0.67%. The microporosity observed in the thin sections of the shale is about 2%, and dominated by the intercrystal and intragranular pores, with the pore size ranging from 100nm to 50μm. The other pore types are related to fractures, with the width of ranging from 2 to 5μm. The formation mechanism of the shale reservoir rocks includes favorable mineral composition, diagenesis and thermal cracking of organic component. There are some differences between Longmaxi Formation shale and Barnett shale in USA. The former is buried deeper, higher degree of thermal evolution, lower gas content, denser, more quartz of terrigenous origin. The prevailing low content of organic matter and highly variable quartz content in the Longmaxi Formation shale suggests there are only marginal conditions for exploration of shale gas resource. However, the high variability in both the content of TOC and quartz in the shale indicates that locally, particularly in the southeastern part of the basin, favorable conditions for shale gas may have developed. More detailed paleogeographic, burial history, gas content and quartz origin studies are needed to better access shale-gas potential of the Lower Silurian Longmaxi Formation shale.

Cometary impacts into ocean: their recognition and the threshold constraint for biological extinctions

The Montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied. The impact crater is 45 km in diameter and was formed at 50.8 Ma by a fall of probably an old cometary. nucleus 3.4 km in diameter, into shallow (<600 m) ocean. Comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim. Instead, the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent, return water flow into the excavating cavity. By this process most of the fall-out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land. At a microscopic scale, the shock metamorphism features are about the same as those for land impacts. n nGeochemically, impacts of comet nucleii, may not leave a recognizable signature at the impact horizon except for a minor increase in iridium. Thus stratigraphic horizons associated with extinctions and/or major changes in biota have to be closely examined for other impact indicators, like the presence of tectites, glass spherules, and quartz grains with shock features. Occurrence of megatsunami wave deposits, extensive erosion on continental margins, margin failures and faunal mixing above erosional unconformities are other potential impact indicators. There is no single indicator that can provide sufficient proof of an impact event. Such interpretations have to be based on multiparameter studies of global extent, since many of the impact indicators are only of regional extent. n nThe lack of extinction of any marine plankton genera, or of bottom dwellers at the Montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus >4 km in diameter. The calculated frequency for a cometary impact which could result in a 10% extinction of marine genera is about 6 × 10−7 yr−1 and for the K/T boundary type of extinctions about 2 × 10−8 yr−1. Even allowing for a large degree of uncertainty in these estimates, it is unlikely that the biological extinction events for the last 250 Ma identified by Sepkoski (1990) could have been all caused by meteorite impacts.

Transformation of oil pools into gas pools as results of multiple tectonic events in Upper Sinian (Upper Neoproterozoic), deep part of Sichuan Basin, China

A center in the present paper is referred to as an area or region which may include one or more hydrocarbon accumulations. A hydrocarbon generation center is referred to as an area containing high quality source rock which was subjected to thermal maturation. A gas generation center is an area in which an oil pool or accumulation was present, and oil was thermally cracked to generate gas. A gas accumulation center is referred to as an area in which natural gas generated from cracked oil accumulated. A gas preservation center is referred to as an area or region where the present natural gas pool/pools is/are located. As one of the oldest petroleum reservoir rocks in the world, the upper Sinian Dengying Formation (Upper Proterozoic) in the Sichuan basin was deeply buried, and its paleo-oil pools (gas generation centers) underwent complex transformation into paleo-gas pools (gas accumulation centers) and the present gas pools (gas preservation centers) as a result of multiphase tectonic activities. The paleo oil pools (gas generation centers) were the main hydrocarbon sources of the paleo gas pools (gas accumulation centers), which were in turn the main sources of hydrocarbons for todays (remaining) gas pools (gas preservation centers). The key factor in the oil accumulation was the presence of rich hydrocarbon source rocks (hydrocarbon generation centers) in the Early Cambrian strata and a good seal development. Being controlled by the early tectonics and sedimentary development of the basin, the hydrocarbon generation centers appeared to have been stationary in space, while in time the other three centers (gas generation centers, gas accumulation centers and gas preservation centers) migrated as result of tectonic events in the basin. Therefore, the time-spatial relationships between these “three centers” (gas generation centers, gas accumulation centers and gas preservation centers) decides the final distribution of natural gas in the Sichuan basin. Relationship between generation, accumulation and preservation of hydrocarbons in the marine carbonates buried deeper than 4500 m in the Sichuan basin, can be separated into: (1) an accumulation mode with the “three centers” being superimposed; (2) an accumulation mode with “the preservation center” disintegrated; (3) an accumulation mode with the “three centers” migrated for a short distance; (4) a destruction mode with the preservation center lost. The natural gas exploration of the upper Sinian carbonate rocks in the Sichuan basin can be most successful where the “three centers” overlap, such as at the front area of the Micang Mountains, which could be the most promising area for the future gas exploration.

Preliminary Results from DSDP Leg 79 Seaward of the Mazagan Plateau off Central Morocco

The region of the Mazagan Plateau (Figure 1) offers unique access for Glomar Challenger to document the Jurassic environment of rifting and the early subsidence history of a segment of passive margin bordering the proto-Atlantic. The edge of the Upper Jurassic carbonate platform bulges seaward here from its normal position beneath the present Moroccan continental shelf, and crops out along a spectacular escarpment that falls about 1 km to a broad slope leading to the abyssal sea floor (Figures 2 and 7). The escarpment reveals a section of Mesozoic strata, near the base of which Oxfordian algal limestone has been dredged. With a series of holes drilled near the escarpment, we planned to piece together a composite section that would record the sequence of depo- sitional environments and subsidence history of this thick carbonate platform, including the timing of platform drowing and the installation of pelagic conditions. The results of drilling here bear strongly on the early evolution of the deeply buried conjugate margin on the North American side.

Processes affecting paleogeography, with examples from the Tethys

Abstract As global change becomes a greater concern to society, a more realistic procedure for interpretation of paleogeography is needed for a balanced view of Earth history. Paleogeography is extrinsically affected by the changes in heat regime of the Earths interior, manifested by a chain reaction of tectonic processes on the Earths surface. Tectonics largely controls oceanography, climate and hence glacio-eustasy, which are the other major forcing factors of paleogeographic change. Since the heat budget of the Earth has steadily declined for the last 4.5 Ga, rates of tectonic processes must have also changed and as a result, paleogeography of the geological past cannot be a mirror image of the present. Thus actualism and uniformitarianism are not applicable to paleogeographic interpretation of geological record when time scale exceeds tens or hundreds of millions years. Major paleogeographic changes can be correlated to tectonic changes of the Wilson cycle, as documented for the late Phanerozoic dispersion of Pangea. Seaway opening in response to shifts in plate position has left variable signals in deposited sediments, such as the termination of evaporite deposition and changes in ocean circulation, bottom water chemistry, sediment composition, and faunal proviciality. Other processes related to plate tectonics, like erratic latitudinal shift in the position of continental plates can lead to erroneous interpretations of climatic or oceanographic responses derived from sediments. Plate rotation under special conditions may result in crustal attenuation along one margin and compression along the other, giving rise to synchronous sea-level changes of opposite polarity. First-order eustatic sea-level cycles operating on a scale of 45–90 Ma in the Atlantic have a long-term controlling effect on paleogeography. These cycles are probably the result of cyclic changes in rate of mantle upwelling and conductive heat transmission through the lithosphere. Because of this dependancy on the Earths heat budget, the length of such cycles should have progressively increased through time. If much more rapid paleogeographic changes resulting from third order Exxon sea-level cycles are caused by rapid stress-induced vertical motions of the lithosphere, then the influence of crustal and mantle inhomogeneities could explain different local and regional sea-level responses to plate tectonic. Application of the generalized, Mesozoic-Cenozoic, third order eustatic sea-level cycles of Vail and his colleagues on solutions of global geological problems can only be considered valid for divergent continental margins in areas under similar intraplate stress regime. Decreasing rates of fluctuation in intraplate stress on a cooling Earth have direct consequences for paleogeographic trends in the gelogical past.