摘要 :
Twenty confirmed impacts over a 7-year time period on Mars were qualitatively and statistically compared to 287 secondary craters believed to originate from Zunil, an ～500 ka, 10-km diameter, primary crater. Our goal was to estab...
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Twenty confirmed impacts over a 7-year time period on Mars were qualitatively and statistically compared to 287 secondary craters believed to originate from Zunil, an ～500 ka, 10-km diameter, primary crater. Our goal was to establish criteria to distinguish secondaries from primaries in the general crater population on the basis of their horizontal planforms. Recent primary impacts have extensive "air blast" zones, distal ray systems (>100 crater radii, R), and ephemeral ejecta. Recent primaries formed clusters of craters from atmospheric fragmentation of the meteoroid body. Secondary craters have ejecta blankets with shorter rays that are consistent with emplacement by low-impact velocities (near 1 km/s). The mean extent of the continuous ejecta blankets was less distal for secondaries (5.38 ± 1.57R) versus primaries (18.07 ± 7.01R), though primary ejecta were less fractal (Fractal Dimension Index (FD_I) < 1.30) and more circular on average (Circularity Ratio (C R) = 0.55 ± 0.25 versus 0.27 ± 0.13 for secondaries). Crater rims were remarkably circular (primaries C_R = 0.97 ± 0.02, secondaries at 0.94 ± 0.05), though secondaries have the lowest values (C_R< 0.9). Secondary crater rims were elongated toward or orthogonal to their primary of origin. Uprange source directions for most secondaries, determined by ejecta planform and crater rim ellipticity, point toward Zunil, although contamination from other primaries is considered in some areas. Ejecta blanket discrepancies between recent primaries and Zunil secondaries are attributable to differences in impact velocity and retention age. After removal of the ejecta blanket, crater rims are generally not diagnostic for determining crater origin. Fragmentation of primaries may play some role in steepening the size-frequency distribution of crater diameters in the 5 m < D < 30 m range.
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The impact breccias encountered in drill hole Yaxcopoil-1 (Yax-1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick...
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The impact breccias encountered in drill hole Yaxcopoil-1 (Yax-1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine-grained top (unit 3; 23 m thick; nuee ardente) and a coarse breccia (unit 4; ～15 m thick) below. As such, they consist of a melange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (～24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic-matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super-heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax-1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary.
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The raised rims of impact craters consist of ejecta deposited onto structurally uplifted target rocks. Although it is commonly accepted that ejecta makes up 50-75% of the total rim height, no previous measurements on well-preserve...
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The raised rims of impact craters consist of ejecta deposited onto structurally uplifted target rocks. Although it is commonly accepted that ejecta makes up 50-75% of the total rim height, no previous measurements on well-preserved, naturally occurring impact craters exist. Relying on data from the Lunar Reconnaissance Orbiter, I report the first direct measurements of the rim-forming constituents from 21 lunar craters ranging in diameter from 2.2 km to 45 km. Results show that the ejecta deposit accounts for no more than ~20% of rim relief, and structural uplift is the primary mechanism of rim development in both simple and complex craters. Thick, quasi-horizontal outcrops of coherent bedrock indicate that this uplift is the result of subsurface injection of debris ormelt into the surrounding target rocks. Crater growth, at least during the latest portion of the excavation stage, therefore, proceeds mainly through injection-rather than ejection-of material. These results suggest that ejecta volumes and excavation depths may be factors of 3 to 4 less than previously considered.
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The lengths of the shadows cast within simple, bowl-shaped impact craters have been used to constrain their depths on a variety of planetary bodies. This technique, however, only yields the "true" crater depth if the shadow transe...
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The lengths of the shadows cast within simple, bowl-shaped impact craters have been used to constrain their depths on a variety of planetary bodies. This technique, however, only yields the "true" crater depth if the shadow transects the crater center where the floor is deepest. In the past, attempts have been made to circumvent this limitation by choosing only craters where the shadow tip lies very near the crater center; but this approach may introduce serious artifacts that adversely affect the slope of the regressed depth vs. diameter data and its variance. Here we introduce an improved method for deriving depth information from shadow measurements that considers three basic shape variations of simple craters: paraboloidal, conical, and flat-floored. We show that the shape of the cast shadow can be used to constrain crater shape and we derive improved equations for finding the depths of these simple craters.
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Evidence of "Upper Cretaceous" sediments above the melt rock/breccia assemblage at Chicxulub has been used to dispute the link between this large impact crater and the Cretaceous-Tertiary (KT) extinction horizon. We have evaluated...
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Evidence of "Upper Cretaceous" sediments above the melt rock/breccia assemblage at Chicxulub has been used to dispute the link between this large impact crater and the Cretaceous-Tertiary (KT) extinction horizon. We have evaluated core samples and well logs from the Petroleos Mexicanos (Pemex) Yucatan No. 6 exploratory well located approx 50 km from ground zero. Despite previous reports to the contrary, the sequence of crystalline rocks and breccias located at depths exceeding 1000 m below sea level are characteristic of the upper lithological sequence observed at other large impact basins such as the 220 km Sudbury structure. Furthermore, the "Upper Cretaceous" sediments overlying the melt rocks and impact brecias at Chicxulub contain abundant glass shards and shocked minerals, demonstrating conclusively that these are reworked debris involved in the impact event, and not normal marine sediments. Core samples straddling the KT boundary indicate that the impact event created a basin several hundred meters deep.
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