A method to assess event magnitude and target water depth for marine-target impacts. Part 1: Granulometry of resurge deposits
Authors: Ormö Jens , Gulick Sean S.P. , Whalen Michael T. , King Jr. David T. , Sturkell Erik , Morgan Joanna 
Published: September 2020

&#160;The rim wall of water formed from even a modestly-sized marine impact may be kilometers in height. The portion that collapses inwards may generate a resurge flow with tremendous transport energy. We compare deposits generated by this ocean resurge inside one of the largest marine-target craters on Earth, the 200-km wide Chicxulub crater, with those from five other marine-target craters. Examination of the wide range of target water depths (H) relative to projectile diameters (d) reveals a high correlation between location at the crater, average clast frequency () and d/H from which any of these variables can be estimated if the others are known.Introduction, aim and methods&#160;Here we compare data on the resurge deposits recovered by International Ocean Discovery Program (IODP)- International Continental Drilling Program (ICDP) Expedition 364 Site M0077 located on the Chicxulub peak ring, a ~500-m topographic high, with resurge deposits in eight drill cores from five other marine-target craters. At Site M0077 (21.45&#176;N, 89.95&#176;W), core between 505.70 and 1334.73 mbsf (meters below sea floor) was recovered. The core shows, from top down, a sequence that begins with ~ 110 m of post-impact hemipelagic and pelagic Paleogene sedimentary rocks, which are followed downwards by ~ 130 m of a graded, polymict suevite (impact melt-bearing breccia) of which ~90 m are interpreted as a deposit from a single, forceful resurge [1, this study]. This sequence, in turn, overlies mainly impact melt rock and felsic basement rocks [1]. Here we estimate the effect of the oceanic resurge on deposition of the graded suevite. We apply the drill core line-logging method previously used in studies of resurge deposits at other impact craters formed at various relative water depths [e.g., 2, 3]. We compare the granulometric information of the M0077A core with previously obtained data on Lockne, Tv&#228;ren, Chesapeake Bay Impact Structure (CBIS), Wetumpka, and Flynn Creek [cf., 2, 3, 4, 5].&#160;To allow the transport in a suspension flow capable of generating the thick, single graded sequence observed in M0077A the amount of available water must also have been large. Factors that would influence the amount of water in the transported medium are: i) the relative target-water depth (vs. diameter of impactor), and ii) the type of target. It seems logical that a small crater formed in deep water has a more water-rich resurge than a much larger crater at the same water depth when considering the differences in the amount of debris from ejecta and rip-up. Likewise, it may be that an impact into poorly consolidated target sediments has a greater in-flux of debris than for a hard rock target. To investigate these factors, we looked at the relation between impactor diameter (d) and target water depth (H) as a way to relate the water depth to the magnitude of the event, and the average clast frequency per meter (here ) as well as the average of the mean clast size per meter (here ) for the whole of the resurge deposits in the core. Critically, we also note the sedimentology, interpreted flow character, and location of the relevant cores to place these values in context.&#160;Results and discussion&#160;For essentially the same , a high is considered to indicate a water-rich flow, whereas a low indicates a high matrix content. A high mud content despite the normal grading indicates a deposition from a hyperconcentrated flow [e.g., 2].&#160;The advantage of having applied the same method at each of the investigated craters is that it allows direct comparisons. Intriguingly, when all ratios of d/H are plotted against , two seemingly linear trends are clearly discernable (Fig. 1). The Lockne crater (especially the Lockne-1 core) with its relatively high water content and consolidated seafloor target rocks is at one end of the scale (with suspension deposits), whereas the Chesapeake Bay impact structure, despite having had a target water depth in the order of 200-500 m [6], is affected by its larger size in combination with a great thickness of poorly consolidated target sediments providing a relatively higher amount of material to the flow. A similar target situation is also known for the Wetumpka crater [7]. In the latter two craters, the resurge sediments are in the form of debris and mudflow deposits [3, 4].&#160;Notably, the location of Site M0077 on top of the peak ring here can be compared to the situation of the Lockne-1 drill core, which due to the prolonged &#8216;washing&#8217; of the resurge deposits at the inner flank of the crater rim obtained a relatively high clast frequency [cf. 2, 8].&#160;Thus, one trend line seems to be for cores in deep crater locations such as annular troughs of complex craters or the center of simple craters. The other trend is for cores located proximal to elevated locations (e.g., Lockne-1 and Chicxulub M0077A). The exception is the CBIS Eyreville A core, which was drilled in the basement crater moat [e.g., 9], but plots along the trend line for elevated locations. Possibly, the in Eyreville A encountered 500 m thick granitic slab and other material from the mega-block slumping preceding the resurge created a topography affecting the flow dynamics. In the absence of any other common factor between the cores on the trend lines than the relative positions within the respective craters we suggest that depositional setting is the controlling factor between the two trend lines.&#160;Furthermore, the two seemingly linear relations between mean clast frequencies of the resurge deposits and the d/H ratios for the craters provide an additional utility. The equations for the linear trends allow any of the four variables (i.e., location, clast frequency, projectile diameter, and target water depth) to be estimated if the three others are known. The physics behind these relations are analyzed by Herreros and Orm&#246; [10].&#160;&#160;