Geologic Overview of Southern California Coastal Areas
Continental Margin
Coastal & Offshore Faulting
Geology of Offshore Islands
Orange County Coastal Geology
Sediment & Sedimentary Rock
Long Shore Currents and Coastal Cells
Depositional vs. Erosional Coastlines
Geology of San Onofre
Landslide and tsunami hazards
Geology of Dana Point area
Mark Bordelon Irvine Valley College March 2012
Continental Margins Where land meets the ocean
Margins near plate boundaries are ―active.‖
Margins far from plate boundaries are ―passive.‖
Fig. 3.11
Offshore Islands – N. Channel Islands
Four northern Channel Islands are an extension of the Santa Monica Mts. • San Miguel, Santa Rosa, Santa Cruz &
Anacapa Islands
• Geology consists of Miocene volcanic rocks: pillow basalts, lava flows, tuff
• Islands were isolated from one another 12-10 y.b.p. when sea level rose
Pygmy Mammoth on Santa Rosa Island
Radiocarbon date = 13,000 years old
Lower sea level
created one large
island, 5 miles
from mainland
Isolated when
sea level rose
12,000 y.b.p.
http://www.nps.gov/chis/pygmy.htm
Offshore Islands – S. Channel Islands
• Southern islands include Catalina, San Clemente, Santa Barbara and San Nicolas Islands
• Remnants of up-faulted blocks
• Marine Terraces well-developed on San Clemente Island
Catalina Island
• Catalina Island has extensive outcrops of Catalina Schist on northern portion of island • Represents blueschist metamorphism which is
typical of subduction zones.
Subduction Metamorphism
Subduction creates the unique blueschist facies.
A low geothermal gradient—low T, high P.
Blueschist minerals
Glaucophane
Lawsonite
Sediment vs. Sedimentary Rock Drilling into the bottom of a coastal area, we encounter:
Soft mud and loose sand, silt, pebbles, and shells. Then:
Similar materials that are more solidified. Then:
Fragments of solid rock.
Cemented sand and silt.
Cemented shells.
Transition from loose sediment into cemented sedimentary rock.
This change occurs as burial depth increases.
Classes of Sedimentary Rock
Clastic—loose rock fragments (clasts) cemented together.
Biochemical—cemented shells of organisms.
Chemical—minerals that crystallize directly from water.
Chemical Clasti
c
Biochemica
l
Beach
• Active Zone of sediment transportation
• Beach lies between:
• Erosional area above water level
• Depositional area below water level
Sand Budgets
• Estimates of sand gains (inputs) and losses (outputs) to a beach
• Beach profile studies • Inputs:
• Rivers (Los Angeles, San Gabriel, Santa Ana) • Cliff erosion • Onshore transport of sediment by LSC
• Outputs: • Offshore transport of sediment by LSC • Submarine Canyons
Depositional vs. Erosional Coasts
• Depositional Coasts • Rate of accumulation exceeds erosion
• LSC slowed, sand deposits
• Features: spits, deltas, barrier islands
• Erosional Coasts • Strong wave activity; erosion rates high
• Rate of erosion depends on
• Local geology
• Degree of wave exposure
• Tidal range
• Features: Cliffs, sea stacks, wave-cut platforms
Erosional
Shoreline
Features
(Exposure to wave
energy high; no
slowing of LSC)
Sea Arches
Sea Stacks- isolated rock remnants
Emergent & Submergent Coasts
Emergent Coastlines
Result from uplift and/or sea level drop
Features: marine terraces
Submergent Coastlines
Result from downdrop and/or sea level rise
Features: estuaries
Tsunami Risk at SONGs
Honshu Earthquake 3/2011
Magnitude 9.0 (USGS)
Depth 19.9 mile
http://neic.usgs.gov/neis/eq_depot/2011/eq_110311_c0001xgp/neic_c0001xgp_l.html
Most earthquake damage is due to ground shaking.
Tsunamis also can significant damage in coastal areas.
December 26, 2004—Indian Ocean tsunami.
March 11, 2011—eastern coast of Japan.
Fukushima power plant damage
The Fukushima nuclear power plant was damaged.
Water breached the seawalls placed to protect the plant.
Hydrogen explosions destroyed the reactor buildings.