Erosion of topmost varves by turbidite deposition limits
varve-count accuracy in Santa Barbara Basin, California
1
Arndt Schimmelmann , Ingrid
2
Hendy ,
Dorothy
3
Pak ,
and Aaron
1
Zayin
(1) Department of Geological Sciences, Indiana University, Bloomington, IN 47405-1405 (aschimme@indiana.edu)
(2) Geological Sciences, University of Michigan, MI 48109 (ihendy@umich.edu)
(3) Dorothy K. Pak, University of California Santa Barbara, Santa Barbara, CA 93106 (pak@geol.ucsb.edu)
(1) Annually laminated (i.e. varved) sediment in the central
Santa Barbara Basin (SBB) off California has been dated by
consecutive varve-counting of the last ca. 2,000 varves and
by radiometric methods (e.g., radiocarbon ages of foraminifera).
Age differences between dating methods are in part caused by
errors in varve counting that cumulatively reduce dating accuracy
down core. For example, some varves can be missing in the
sedimentary record if the youngest and least con-solidated
sediment is eroded by occasional strong bottom currents. The
silled nature of the SBB usually precludes strong bottom currents,
but the shear of fast-flowing and dense turbidite or hyperpycnic
flood currents from up-slope may erode and suspend topmost
bacterial mat and underlying soft sediment. Sediment from the
20th century in the central SBB had not been reported to contain
massive turbidites or flood deposits. Here we report the first
occurrence.
Santa Barbara
N
Santa Barbara Basin
Bathymetry map from Monterey Bay
Aquarium Research Institute (MBARI)
core #4 core #10
deep correlation
among box cores
(2) 2009 SBB box core #10 (585.8 m water depth; 34° 16.847’ N,
120° 02.268’ W; see Figures above and left) represents continuously
varved sediment from 1931 to 2009 AD. However, below the 1931
varve, a ca. 6 cm thick olive turbidite was found resting on the ca.
1923 varve (see color pictures and scans of epoxy-embedded and
polished sections in Figure on the left), as determined by crosscorrelation of the pre-1924 varve pattern with records from other box
cores featuring continuous varves across the 20th century, including
box core #4 that was retrieved in 2009 at a distance of about 1 km
to the west (see Figures above and left). The turbidite in box core
#10 was likely triggered seismically on 5 August 1930 when a strong
earthquake shook Santa Barbara (Hamilton et al., 1969). The local
1930 turbidite eroded annual varves from 1924 to 1930. Sectioning
of a cylindrical sub-core from the same box core showed ca. 1-cm
large rip-up clasts of varved sediment with various angles of
lamination embedded in the lower portion of the 1930 turbidite. The
presence of occasional thick turbidites in SBB sediments, especially
prior to 1850 AD, likely causes under-counting of varves. A survey
of dated turbidites around the periphery of the central SBB may be
useful for constraining the paleoseismic record in the SBB area.
(3) Our interpretation of the ‘missing varves’ below the 1930 AD
turbidite in box core #10 is corroborated by earlier, overlooked
evidence for basal erosion underneath massive gray layers in the
SBB. X-radiographic surveys of piston and kasten cores had been
provided by Andrew Soutar and Tim Baumgartner (at Scripps
Institution of Oceanography) and others (e.g., Schimmelmann et al.,
2006). The Figure below compares five x-radiographic records
from the central SBB
ODP
representing the late
piston core
893A
piston core
piston core kasten core
6P
14th and early 15th
6P
214
centuries below a ca.
1418 AD
1418 AD flood layer
1418 AD
(Schimmelmann et
missing varves
al. 2003). The two
cores on the right
side of the Figure show the thickest flood layers and evidence of missing varves relative to
coeval records on the left where flood layers are less massive. Hyperpycnal flows of dense flood
slurry can therefore exert sufficient shear to erode topmost varves.
(4) Acknowledgement: This research was funded by NSF grant #OCE-0752068 to A.S.
(5) References:
Hamilton et al. (1969) U.S. Geological Survey Professional Paper #679D.
Schimmelmann et al. (2003) The Holocene 13 (5), 763-778. http://dx.doi.org/10.1191/0959683603hl661rp
Schimmelmann et al. (2006) Journal of Sedimentary Research 76, 74-80. http://dx.doi.org/10.2110/jsr.2006.04