Abstract-The Lonar crater in Maharashtra state, India, has been completely excavated on the Deccan Traps basalt (approximately 65 Ma) at approximately 570 ± 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] o to the horizon where the total duration of the shock event was approximately 1 s. It is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east-west plane of impact (Misra et al. 2010). The present study shows that some of the rock magnetic properties of these shocked target basalts, e.g., low-field anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM) ⁄ bulk susceptibility (v), and high-coercivity and high-temperature (HC_HT) magnetization component, are also almost symmetrically oriented with reference to the plane of impact. Studies on the relative displacements of K 3 (minimum) AMS axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2-3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact. The biaxial distribution of AMS axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts, although transitional type distribution also exists. The degree of anisotropy (P¢) of AMS ellipsoids of the shocked basalts decreases by approximately 2% when compared with those of the unshocked target (approximately 1.03). The NRM ⁄ v (Am )1 ) values of the shocked basalts on the rim of the Lonar crater do not show much change in the uprange or downrange direction on and close to the east-west plane of impact, and the values are only approximately 1.5 times higher on average over the unshocked basalts around the crater. However, the values become approximately 1.4-16.4 times higher for the shocked basalts on the crater rim, which occur obliquely to the plane of impact. The target basalts at approximately 2-3 km west of the crater center in the downrange also show a significant increase (up to approximately 26 times higher) in NRM ⁄ v. The majority of the shocked basalt samples (approximately 73%) from around the crater rim, in general, show a lowering of REM, except those from approximately 2-3 km west of the crater center in the downrange, where nearly half of the sample population shows a higher REM of approximately 3.63% in average. The shocked target basalts around the Lonar crater also acquired an HC_HT magnetization component due to impact. These HC_HT components are mostly oriented in the uprange direction and are symmetrically disposed about the east-west plane of impact, making an obtuse angle with the direction of impact. The low-coercivity and low-temperature (LC_LT) components of both the unshocked...

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In this study, we integrate environmental magnetic, sedimentological, and geochemical records of sediment core of Hole NGHP-01-10D overlying methane hydrate deposits to decipher the controls on the evolution of fracture-filled gas-hydrate system in the Krishna-Godavari (K-G) basin. Four distinct sedimentary units have been identified, based on the sediment magnetic signatures. An anomalous zone of enhanced magnetic susceptibility (Unit III: 51.9-160.4 mbsf) coinciding with the gas hydrate bearing intervals is due to the presence of magnetite-rich detrital minerals brought-in by the river systems as a result of higher sedimentation events in K-G basin and has no influence over hydrate formation. A strong to moderate correlation between magnetite concentration and chromium reducible sulfur (CRS) content indicates significant influence of sulfidization on the magnetic record and could be further exploited as a proxy to decipher paleo-H 2 S seepage events. Analysis of high-resolution seismic, bathymetry, and subbottom profiler data reveals the existence of a regional fault system in K-G basin. The opening and closing dynamics of the faults facilitated the migration and trapping of required gas concentrations resulting in accumulation of gas hydrates at the studied site. The seismic data provides support to the rock-magnetic interpretations. The observed variations in magnetic and geochemical properties have resulted from the episodic flow of methane and sulfide-enriched fluids through the fracture-filled network formed as a result of shale-tectonism. Our study demonstrated the potential of using an enviro-magnetic approach in combination with other proxies to constrain the evolution of gas-hydrate system in marine environments. Key Points:Magnetic signatures of detrital and diagenetic processes associated with evolution of gas-hydrate system. Changes in magnetic and geochemical properties controlled by underlying gas-hydrates. Magnetic proxy to decipher paleo-H2S seepage events in marine sediments.

We evaluate the environmental magnetic, geochemical, and sedimentological records from three sediment cores from potential methane-hydrate bearing sites to unravel linkages between sedimentation, shale tectonics, magnetite enrichment, diagenesis, and gas hydrate formation in the Krishna-Godavari basin. Based on downcore rock magnetic variations, four sedimentary magnetic property zones (I-IV) are demarcated. A uniform band of enhanced magnetic susceptibility (zone III) appears to reflect a period of high-sedimentation events in the Krishna-Godavari basin. Highly pressurized sedimentary strata developed as a result of increased sedimentation that triggered the development of a fault system that provided conduits for upward methane migration to enter the gas hydrate stability zone, leading to the formation of gas hydrate deposits that potentially seal the fault system. Magnetic susceptibility fluctuations and the presence of iron sulfides in a magnetically enhanced zone suggest that fault system growth facilitated episodic methane venting from deeper sources that led to multiple methane seepage events. Pyrite formation along sediment fractures resulted in diagenetic depletion of magnetic signals and potentially indicates paleo sulfate-methane transition zone positions. We demonstrate that a close correlation between magnetic susceptibility and chromium reducible sulfur concentration can be used as a proxy to constrain paleomethane seepage events. Our findings suggest that the interplay between higher sedimentation events and shale tectonism facilitated fluid/gas migration and trapping and the development of the gas hydrate system in the Krishna-Godavari basin. The proposed magnetic mineralogical approach has wider scope to constrain the understanding of gas hydrate systems in marine sediments.

[1] The Pliocene-Pleistocene Karewa Group sediments of the Kashmir basin, India, provide an important continental archive for past climatic reconstruction. The present study reevaluates the magnetic polarity stratigraphy and the nature of the depositional environment at a 440-m-thick section along Romushi river near Pakharpur (33°48′50″N, 74°45′54″E). Magnetic remanences are predominantly carried by Ti-rich titanomagnetite and magnetite. We identified eight normal and eight reversed-polarity magnetozones in this succession, ranging between 4.40 and 0.77 Ma. The polarity sequence includes the new identification of the Cochiti and the Mammoth and their preceding and succeeding reversed/normal as well as the Jaramillo subchrons. Anisotropy of magnetic susceptibility data suggest the existence of northeast-and northwest-flowing fluvial system before 4.18 Ma, indicating the Pir Panjal range at the southwest as the sediment source area. Following this, the valley was under the influence of fluviolacustrine environment between 4.18 and 0.77 Ma. Our results suggest relatively strong flow velocity toward the northeast during the upper Gilbert, Gauss, and lower and middle Matuyama chrons (4.18-1.07 Ma). In the upper Matuyama chron (<1.07 Ma), the prevailing paleocurrent direction in the basin changed toward the northwest with a reduced flow velocity, indicating the emergence of the ancestral Jhelum river. On the basis of the magnetic polarity chronology, the sediment accumulation rate indicates a very low value of ∼4.6 cm kyr −1 before 1.95 Ma to ∼33 cm kyr −1 between 1.95 and 1.77 Ma and 23 cm kyr −1 after 1.77 Ma. We attribute temporal changes in sedimentation rate to the interplay between climate (predominantly westerlies) and tectonics (pulsating Pir Panjal uplift).