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Figure 3. Pre-impact target conditions affected the formation of the peak
ring in the ~320-km-diameter Schrödinger basin. In this view oriented
with the south pole toward the top of the image, the peak ring collapsed
below the level of the impact melt and breccia fill in the southeast quad-
rant. Also, differences in crustal thickness affected the morphology of the
peak ring, producing bilateral asymmetry.

topography and thickening of the post- Figure 4. Representative core segments. (A) Suevite from ~645 mbsf con-
impact section in the northeast quadrant tains fragments of impact melt, sedimentary target rocks, and igneous
of the structure producing an anomalous target rocks. (B) Melt rock from ~745 mbsf contains igneous clasts, domi-
gravity low (Fig. 1, inset). The offshore nated by granite. (C) Granite from ~814 mbsf with a gray-colored cataclas­
asymmetry in gravity data may be due to tic vein. (D) Melt rock from ~1268 mbsf contains metamorphic and igneous
lateral variations in the continental shelf clasts. The top of each core segment is located on the left.
of the target which, in the northeast direc-
tion, is argued to have been thicker, topo- Because the platform was so small, only the peak ring, (2) that the uppermost peak
graphically lower, and covered with limited measurements and core descrip- ring is formed from ~150 m of low-velocity
deeper water at the time of impact (Gulick tions could be undertaken before the core impact breccia, and (3) that the peak-ring
et al., 2008). Based in part on that assess- was placed into refrigerated storage and rocks comprised heavily fractured base-
ment, Expedition 364 was designed to shipped to the Bremen Core Repository, ment. The total thickness of the granite-
drill into the northwest quadrant (Morgan Center for Marine Environmental Sciences dominated basement interval is 588 m and,
et al., 2016; Gulick et al., 2017), where (MARUM), University of Bremen, based on seismic reflection data (Morgan
seismic reflection data clearly image a Germany. The science party convened in et al., 2000; Gulick et al., 2013), is pre-
high-relief peak ring that is relatively Bremen in September and October 2016 to dicted to continue to depths of ~3 km at the
close to the surface (Morgan et al., 2011). split the core into two halves, perform a site of the borehole.
suite of standard IODP measurements, and
The borehole was drilled at Site M0077 log and sample the core. The recovery of uplifted granitic rocks
(21.45° N, 89.95° W) a few kilometers from the peak ring (Fig. 5) favors a dynamic
north of the coastline in ~20 m water depth Impactites consisting of melt-bearing collapse model for an over-heightened
(Morgan et al., 2016). Those shallow water breccias (suevites) and impact melt rock central uplift (Morgan et al., 2016), which
depths required the use of a leased jack-up were encountered at 617.33 mbsf and is consistent with observations at other
platform or lift boat rather than one of the formed an ~130-m-thick unit over granite terrestrial craters (Grieve et al., 1981) and
larger IODP ships (Gulick et al., 2017). The and related basement rock types within the with an earlier set of numerical models of
L/B Myrtle was anchored 5 April 2016 and uplifted peak ring (Fig. 4). Thin, the Chicxulub-forming event (Collins et
drilling occurred from 7 April to 26 May <1-m-thick melt horizons were logged al., 2002; Ivanov, 2005; Collins et al.,
2016, followed by downhole logging and within the granite. Also, ~4 m of melt and 2008). It is also similar to the processes
jack down of the platform on 30 May. Core melt-bearing breccias were encountered at inferred from geologic mapping and
recovery began at 505.7 mbsf and contin- ~1000 mbsf, and ~58 m of melt and melt- numerical modeling of the Schrödinger
ued to 1334.7 mbsf with >99% recovery. bearing breccias occur in the lowest 100 m peak ring on the Moon (Kring et al., 2016).
The operator was able to maintain PQ3 of core. The rock sequence encountered Shock metamorphism in the recovered
core barrels from 701 m to the base of the within the borehole is in accordance with Chicxulub peak-ring core samples indicates
borehole, producing ~83-mm-diameter prior interpretations of seismic reflection pressures of ~10–35 GPa, which are also
core, which is wide for ocean drilling. and refraction data (Morgan et al., 2000, consistent with a new, higher-fidelity
2011; Gulick et al., 2013), including cor- numerical model of dynamic collapse at
rectly predicting (1) the depth to the top of Chicxulub (Morgan et al., 2016).

6 GSA Today | October 2017