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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

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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.