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Paleoshorelines Offshore of the San Luis Range, San Luis Obispo County, Central Coastal CaliforniaPresented by Hans AbramsonWard
Authors
Hans AbramsonWardBrian GrayKathryn Hanson
Study Location
William D. Page
Basic data provided by:
William Lettis
H.Gary Greene(Professor Emeritus)
Bedrock is chiefly Miocene Monterey Fm. and Obispo Fm. overlying basement of Cretaceous Sandstone and Franciscan Complex
Broad, gently-sloping inner shelf, triangular region bounded on west by Hosgri fault
Overview of geology
Localized mobile sand sheets
Location of next slide
Estero Bay
Point Buchon
Point San Luis
Hosgri Fault
Offshore Data
Multibeam bathymetry (from Cal State Monterey)
Shallow seismic reflection data (from USGS)
Geologic map developed from PG&E Dataset:
Diver geologic samples
Borings (not shown here)
Interpretation of seafloor texture
Overview of paleoshorelinesEXPLANATION
Paleoshorelines evident in multibeam bathymetry in areas of rocky seafloor,Paleoshorelines evident in seismic reflection profiles where bedrock is buriedLocation of data examples on subsequent slidesSeismic reflection profile
Paleoshorelines are:• discontinuous• preserved in all rock
types, and in places can be traced across inferred geologic contacts
N
Paleoshorelines west of Pt. San Luis
VE = 7.9
C
D
A B
A’ B’
A
B
A’
A’
B’
Paleoshorelines west of Pt. San Luis
VE = 7.9
C
D
A B
A’ B’
C
D
A’
C’
D’
Monterey Formation -21 m Platform Profile
Distance (m)
1,2001,1001,0009008007006005004003002001000
-15-16-17-18-19-20-21-22-23-24-25-26-27-28-29-30-31-32-33
Paleoshoreline at -21 m +1/-2
Paleoshoreline northwest of Pt. Buchon
VE = 17.6
A A’
A
A’
Dissected wave-cut platform
-38 m SLA Platform
1,1001,0009008007006005004003002001000
-20
-25
-30
-35
-40
-45
-50
-38 m SLA platform
1,2001,1001,0009008007006005004003002001000
-20
-25
-30
-35
-40
-45
-50
Paleoshoreline south of Pt. Buchon
VE = 9.1
Obispo Fm. resistant tuff
Monterey and Obispo Fm.
SLA at -38 +/-1 mSLA at -38 +/-1 m
A BA’ B’
A
B
A’
B’
Hosgri Fault
Wave-cut platform buried by young sand
Representative bedding in Tertiary rock
Bedrock at seafloor
Paleoshoreline imaged on S6-08-SC SPARKER profile PBS-021
~80 m
~1,250 m
SW NE
Seafloor multiples
Shoreline Angles
Comparison to emergent marine terraces
Shoreline angles from MIS 5a, 5e marine terraces, and remnants of older terraces near mouth of Crowbar Canyon. Mapping from Hanson et al. (1994)
MIS 5eMIS 5a
Excerpt of longitudinal profile along coastline showing preliminary shoreline correlations
Regional correlations are being developed to evaluate potential constraints on tectonic deformation
Correlation of terraces is complicated by lack of continuity, possibly due to erosion by fluvial incision, marine planation during transgressions and regressions, and potentially tectonic deformation.
DISTANCE (kilometers)
Slopes and widths of submerged platforms
Bradley and Griggs (1976) indicate slopes of wave-cut platforms near Santa Cruz are commonly about 2-4% for the inshore segment (near the seacliff) and 0.7 to 1.7% for the offshore segment
Slope of active wave-cut platform in study area ranges from about 1.5 to 3.4%.
Platform Width (m)
Slo
pe %
0.000
1.000
2.000
3.000
4.000
5.000
6.000
0.0 200.0 400.0 600.0 800.0 1000.0 1200.0
Sketch profile of generalized wave-cut Platform
Inshore segmentOffshore segment
Comparison of submerged platforms to ranges presented by Bradley and Griggs (1976)
Seacliff
Shoreline angle
Parameters for modern (mid to late Holocene) wave-cut platform
Geologic Unit (no. measurements)
Mean Width (m)
Mean Depth (m) at platform outer edge
Mean Slope (%)
Franciscan metavolcanics (2) 189.0 5.5 2.9
Ophiolite (3) 225.0 7.7 3.4Cretaceous sandstone (14) 282.2 7.1 2.5Obispo Fm., resistant tuff (2) 284.5 6.0 2.1
Obispo Fm., (5) 353.3 5.6 1.6
Monterey Fm. (5) 401.0 8.8 2.2Obispo Fm., diabase (2) 528.0 12.5 2.4
Total (33) 308 7.3 2.4
Outer edge of modern wave-cut platform may be used to estimate the depth of wave scour into bedrock
VE = 9.5
Holocene wave-cut platform in Monterey Formation
Latest Pleistocene to Holocene sea level curves
ICE-5G (VM2) model
Coral-based estimates from Barbados, indicating sample depth and growth range (Peltier and Fairbanks, 2006)
Coral-based estimates by Lambeck and Chappell (2001)
Cyan – BarbadosOrange – Bonaparte GulfBlack – Huon PeninsulaGray – TahitiPurple – Sunda Shelf
Walbroek et al. (2002)
Rates of platform development
Rates of shoreline retreat (platform growth) are inferred based on estimated 6,000 years of erosion
Mean rate of 50 mm/yr indicates approximately 4,000 years of relative sea level stability is required to carve a 200-m wide platform
Average rates of mid to late Holocene shoreline retreat
Oph
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Cre
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Mon
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Obi
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Form
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Obi
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Fm. d
iaba
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Obi
spo
Fm.
resi
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Fran
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etav
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Late Quaternary sea level curves
Note that there is very little agreement among researchers regarding the exact timing and elevations of highstands and lowstands between ~30 and 80 ka.
Inferred ages of paleoshorelines
Due to the uncertainty in elevation of paleo sea levels between ~30 and 80ka, shoreline ages inferred from assumed uplift rates are unreliable.
However; considering the length of time required to develop the wave-cut platforms in bedrock, it is likely that the broad paleoshorelines were scoured during intermediate highstands and/or lowstands prior to 30ka.
Conclusions
Several paleoshorelines are eroded into bedrock offshore of the San Luis Range. We identified a sequence four well-developed shorelines and additional five shorelines that are less well-developed
We are in the process of correlating paleoshorelines to help understand tectonic deformation of this part of the coast
Our analysis of the Holocene rise in sea level and the amount oftime required to develop broad wave-cut platforms indicates that he broad paleoshorelines between 20 and 70 meters depth are older than 30ka
The mid to late Holocene rate of development of wave-cut platforms averages about 50 mm/yr and ranges from about 30 to 80 mm/yr.
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