Introduction: IODP/ICDP Expedition 364 recovered
core from 505.7-1334.7 m below the seafloor
(mbsf) at Site M0077A (21.45° N, 89.95° W) atop the
peak ring in the Chicxulub impact structure. The core
penetrated Paleogene sedimentary rocks, impactrelated
suevite, melt rock, and granitic basement [1].
Approximately 110 m of post-impact, hemipelagic and
pelagic sedimentary rocks were recovered, ranging
from middle Eocene (Ypresian) to basal Paleocene
(Danian) in age [1]. The transition between suevite and
basal Paleocene sedimentary rocks is a remarkable
succession of fining upward gravel to sand-sized suevite
(Unit 2A) overlain by laminated carbonate-rich
siltstone (Unit 1G, “impact boundary cocktail” [2])
that records the settling of fine-grained material postimpact
[1]. This study concentrates on the carbonaterich
Paleocene sedimentary rocks of overlying Unit 1F
[1]. The degree of bioturbation, or ichnofabric index
(II) [3, 4], provides a semiquantitative estimate of the
density of burrowing within sedminentary facies. Collection
of II data within the context of facies analysis
thus yields insight into the initial and then continued
disturbance of sediment by burrowing organisms recording
the return of life to the crater (Fig. 1).
Unit 1G: The unit extends from 616.58-617.33
mbsf (Fig. 1) and consists mainly of dark brown to
dark grayish brown calcareous siltstone but is complex
with several different lithologies and post-depositional
pyrite nodules that disrupt bedding. The base of the
unit is a sharp, stylolitized contact overlain by two ~1
cm thick, normally graded beds. Overlying, up to
617.17 mbsf, the siltstone contains internally finely
laminated cm-scale beds that alternate between dark
brown and grayish brown. Above, up to 616.97 mbsf is
a package with mm bedded couplets of dark brown and
grayish brown calcareous siltstone that grade upward
into similarly colored cm bedded couplets that then
thin upward into mm bedded couplets again. Above
this interval bedding is indistinct and appears to be
obscured by soft sediment deformation from 616.66-
616.97 mbsf. The upper part of the unit is slightly deformed
with greenish marlstone and interbedded lighter
gray siltstone displaying a distinct downwarp from
616.58-616.66 mbsf. Rare oval structures, that are potential
individual burrows, occur down to 616.65 mbsf.
Unit 1F: The unit records the remainder of the
Paleocene and extends from 607.27-616.58 mbsf (Fig.
1). The base of the unit is a sharp contact at the base of
a greenish claystone (II 2) that overlies Unit 1G [1]. It
consists dominantly of interbedded light gray to light
bluish gray wackestone and packstone (II 3-5) and
light to dark bluish gray marlstone (II 2) at cm-dmscale.
All lithologies contain wispy stylolites. The
lower portion of the unit (616.58 and 607.74) is cyclic
with cm-dm-scale bedding and light greenish-blue to
bluish marlstone bases (II 2-3) that grade upward into
light gray or light bluish gray wackestone and packstone
(II 3-5). Contacts between lithologies are usually
gradational due to burrowing. The upper portion of the
unit from 610.25 to 607.74 mbsf is a light yellowish
brown burrowed packstone (II 4) intercalated with gray
marlstone (II 2). The uppermost 7.5 cm is calcite cemented
with 1 cm wide burrows (II 3-4). Clasts are
fine to coarse sand size and include foraminifera. The
upper surface of this unit is a hardground and minor
unconformity overlain by Eocene rocks [1].
Ichnofabric Index: II data provides a window onto
the return of life post-impact (Fig. 1). Rare structures
in the upper most sandy suevite (Unit 2A) and in
Unit 1G (Core 40R-1) resemble bioturbation structures
but may also represent fluid escape [1]. The first welldefined
oval structures that appear to be burrows occur
in the upper part of Unit 1G (Fig. 1, 616.58-616.65
mbsf). Unequivocal burrows (II 2) that disturb sedimentary
facies occur just above, at 616.56 mbsf in Unit
1F (Fig. 1). II of 3-4 are reached 5-6 cm above indicating
significant disruption of original sedimentary strutures.
An II of 5 is first documented at 616.16 mbsf
(Fig. 1). Above this level through the Paleocene succession
II largely varies between 2 and 5 with rare
laminated intervals (II 1). Bioturbation intensity correlates
well with facies changes and more marly facies
display lower levels of bioturbation than more carbonate-
rich facies. This correlation implies a depth
and/or paleoredox control on the distribution of bioturbating
organisms.
Discussion: II and the return of life: The II data
indicate that burrowing organisms were likely reestablished
in the crater before the end of deposition of
Unit 1G. Biostratigraphic analyses document a mix of
Late Cretaceous and earliest Danian taxa within Unit
Lunar and Planetary Science XLVIII (2017) 1348.pdf
1G and lowermost Danian zone Pα documented in the lowermost
part of Unit 1F down to 616.58 mbsf [1]. P1a taxa occur
down to 616.29 mbsf with P1b-P4 recorded upward through
607.27 m [1]. Burrowing organisims were thus active by earliest
Danian indicating a rapid return of life to the crater. Hydrocode
modeling implies that much of the deformation and
peak ring formation was completed within minutes of the impact
[5]. Deposition and reworking of impact breccia by tsunami
and seiches likely extended for several days [6]. More
refined estimates for the return of life to the crater may be
possible with more detailed analysis of the deposition of laminae
within Unit 1G that records marine settling of fine-grained
material that may have taken days to months.
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Evidence for a mid-Holocene tsunami deposit along the Andaman coast of Thailand preserved in a mangrove environment
Klong Thap Lamu, a large mangrove-fringed tidal channel along the northern Andaman Coast of Thailand,
provides an ideal location to test the hypothesis that a paleotsunami record can be preserved in the sediments
of a mangrove forest. The 2004 Indian Ocean tsunami destroyed local swaths of mangrove forest with highly
variable widths — up to 300 m. Left in the wake of the tsunami is a thin mantling of laterally discontinuous
sand, macerated shells, and localized coral rubble that is being mixed rapidly into the underlying mangrove
peat. Transects across the channel's tsunami-modified shore show that the sand layer thins abruptly at the
border of the undisturbed mangroves, suggesting that the energy of the wave dissipated quickly as it entered
the forest. The distribution and sedimentology of the 2004 tsunami deposit (Unit tI) suggest that any
paleotsunami deposit within this mangrove environment should be spatially restricted and thoroughly
bioturbated. Sediment cores collected from within the 2004 tsunami zone penetrate a buried coral-shell peat
unit (Unit tIII) that tapers inland. Unit tIII is strikingly similar to Unit tI, except for Unit tIII's diffuse
sedimentology, which we attribute to extensive bioturbation. Unit tIII also cross-cuts an identified facies
boundary that is traceable across the width of the 2004 tsunami zone. Rather than a facies boundary
associated with the regional early-to-late Holocene sea level regression, stratigraphic correlations suggest
that Unit tIII represents an event horizon (i.e. tsunami). AMS 14C dates on material from within Unit tIII
combined with an upper bracketing age suggest that the tsunami event occurred sometime between 2720 and
4290 cy BP. If correct, this tsunami predates the 3–4 tsunami events recognized to the north at Koh Phra
Thong. Unit tIII is, however, a potential far-field equivalent of a recently recognized paleotsunami deposit on
the southwestern Indian coast ca. 3,710 years before present (Nair et al., 2010).
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- Award ID(s):
- 0605328
- NSF-PAR ID:
- 10469667
- Publisher / Repository:
- Elsevier B.V.
- Date Published:
- Journal Name:
- Marine Geology
- Volume:
- 282
- Issue:
- 3-4
- ISSN:
- 0025-3227
- Page Range / eLocation ID:
- 255 to 267
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.margeo.2011.03.003
