slide1 n.
Skip this Video
Loading SlideShow in 5 Seconds..
Introduction PowerPoint Presentation
Download Presentation

Loading in 2 Seconds...

play fullscreen
1 / 19

Introduction - PowerPoint PPT Presentation

  • Uploaded on

Introduction. Turbidites: geological formations that have their origins in turbidity currents deposits, that deposit from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean. .

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
  • Turbidites:geological formations that have their origins in turbidity currents deposits, that deposit from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.
  • Sediments are transported and deposited bydensity flow, not by tractional or frictional flow.
  • Bouma sequence:from conglomerates at the bottom to shales on the top

Idealised sequence of sedimentary textures and structures

in a classical turbidite, or Bouma sequence (Bouma, 1962).

  • Interest of the off-fault paleoseismology
    • GPS → high degree of certainty, in few years, of the crustal strain accumulation.. But just for a portion of a cycle..
    • Earthquake records → not long enough
    • Onshore paleoseismology → erosion, urban area..
    • Off-fault paleoseismology
  • Interest of marine turbidite records
    • Have to prove they are earthquake-triggered
    • Marine records: more continuous, extend further back in time, more precise in time (datable foraminifera)
  • Method used
    • 74 piston, gravity cores from channel/canyon systems draining Northern California
    • Mapping channels with multibeam sonar (bathymetry, channel morphology, sedimentation patterns
    • Sampled all major channel systems between Mendocino and north of Monterey Bay
  • Results
    • Good agreement with shorter land record
    • Opportunity to investigate long tem earthquake behaviour of North San Andreas Fault

Piston core


from corer

Piston corer

Split piston

core being


4 segments of SAF:
    • Santa Cruz Mountains
    • Peninsula
    • North Coast
    • Offshore
  • Several onshore paleoseismic sites:
    • Vedanta: max slip rate in late Holocene 24 +/- 3mm/yr and 210 +/- 40 years
    • Fort Ross: ~230 yr
    • South of the Golden Gate: 17 mm/yr
how to identify earthquake triggered turbidites
How to identify earthquake-triggered turbidites
  • Possible causes of turbidites:
      • Storm or tsunami wave loading
      • Sediment loading
      • Storm discharges
      • Earthquakes
  • Seismically triggered turbidites are different:
      • Wide area extent
      • Multiple coarse fraction pulses
      • Variable provenance
      • Greater depositional volume
  • Use a temporal and spatial pattern of event correlation over 320 km of coastline
synchronous triggering and correlative deposition of turbidites
Synchronous triggering and correlative deposition of turbidites
  • Regional stratigraphic datum missing
  • Correlations depend on stratigraphic correlations of other datums and radiocarbon ages
  • The Confluence Test:
    • If one canyon contains n turbidites and a second canyon also shows n turbidites, and if these n events have been independently triggered, the channel below the confluence should contain at least 2n instead of only n.
  • 8 major confluences
  • 3 heavy minerals
event fingerprinting
Event “fingerprinting”
  • All cores are scanned, collecting P-wave velocity, gamma-ray density, magnetic susceptibility data, imaged with X-radio and grain size analyzed
event fingerprint
Event “fingerprint”
  • First, these data were used to correlate stratigraphy between coresat a single site
  • Found that it was possible to correlate unique physical property signatures ofindividual turbiditesfrom different sites within the same channel
  • Even possible to correlate turbiditesbetween different channels(some of which never met)
  • The turbidite “fingerprint” = basis of long-distance correlations
event fingerprint1
Event “fingerprint”

Evolution of a single event down channel

over a distance of 74 km

radiocarbon analysis
Radiocarbon analysis
  • Extraction ofplanktic foraminiferafrom the hemipelagic sediment below each turbidite
  • Bioturbation and basal erosion do not biase 14C ages
  • Method:
    • Determine hemipelagic thickness
    • Estimate the degree of basal erosion
    • Observe that differential erosion is most likely source of variability at any site
    • Conversion of hemipelagic thickness to time (using average of sedimentation rate)

Both have 22 events

Less dated turbidites

Low foram abundance


Upper section

poorly preserved

results confluence and mineralogy
Results: confluence and mineralogy
  • Good correlationbetween these cores suggests that input mixing at each confluence has little effect on the stratigraphy of the turbidites
  • Synchronous triggering is theonly viable explanation
  • Non-synchronous triggering should produce an amalgamated record that increases in complexity below each confluence, with only partial correlations for the synchronous events
  • Strict test of synchroneity
results stratigraphic correlation
Results: stratigraphic correlation

Regional correlation of turbidite stratigraphy spanning the Holocene

results stratigraphic correlation1
Results: stratigraphic correlation

Noyo canyon is cut by the NSAF and as an epicentral distance of zero → explains thicker turbidite records

time series
Time series
  • -The youngest 15 events have a mean repeat time of ~200 yr +/ 60 yr
  • ~95 yr: minimum interval
  • ~270 yr: maximum value
  • Values consistent with previous paleoseismic data onshore
  • Same total number of events onshore and offshore = land and marine record the same events
  • Good correspondencewith land paleoseismic dates (individual matching, total number of events)
  • Offshore turbidites as paleoseismic indicators for the NSAF
  • Mean recurrence interval coherent with onshore
  • Epicentral distanceis the controlling factor for turbidite size
  • Turbidites correlate across channels where the mineralogy is different, the physiography is different the sediment sources are different and the underlying geology is different too
  • Minimum magnitude and triggering distancefrom the earthquake hypocenter : at least M7.4
  • But observations of turbidites of small events may also be a function of the resolutions of the observations
  • Majority of repeat time intervalsbetween 150 and 250 yr