SUBSURFACE PLUMBING AND THREEDIMENSIONAL GEOMETRY IN MIOCENE FOSSIL COLD SEEP FIELDS, COASTAL CALIFO

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SUBSURFACE PLUMBING AND THREE-DIMENSIONAL
GEOMETRY IN MIOCENE FOSSIL COLD SEEP FIELDS,
COASTAL CALIFORNIA
Contribution 3, by Ivano W. Aiello and Robert
E. Garrison Ocean Sciences Department, University
of California, Santa Cruz, CA 95064 and Moss
Landing Marine Laboratory, CA 95039
Aiello Garrison, 2002
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ABSTRACT
Miocene low-magnesium calcite concretions
resembling modern carbonate structures that form
at cold seeps occur in fractured opal-CT
porcelanites that are interbedded with mudstones
in coastal cliffs at Santa Cruz, California.
The morphologies of the carbonate structures
differ markedly from conventional concretions and
are spatially aligned with orthogonal joints in
the porcelanites. The structures contain tubular
holes that are identical to fluid and gas
conduits in modern carbonate seep structures the
orientations of these tubes suggest that fluid
and gas flow was both vertical and horizontal,
the latter along extensional joints that formed
preferentially in the brittle, silica-rich layers
that had enhanced bedding-parallel permeability.
Petrographic and isotopic characteristics
(d13C -4 to -9 PDB d18O -1.8 to 1.9 PDB)
of the carbonate structures indicate that calcite
precipitation occurred in a shallow, subseafloor
environment in either the zone of microbial
sulfate reduction or methanogenesis, prior to or
possibly simultaneously with the silica phase
transformation of opal-A in diatom shells to
opal-CT.
Aiello Garrison, 2002
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click!
The fossil seep structures are located in coastal
cliffs at Santa Cruz, California. This tectonic
sketch of the northern Monterey Bay highlights
the occurrence of several NW-SE trending faults
in the area which belong to the overall Santa
Andreas Fault System. In particular, the seep
structures are located next to the trace of the
Ben Lomond Fault, which probably has been active
since Miocene.
Aiello Garrison, 2002
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click!
Geologic sketch of the western cliff of the city
of Santa Cruz. The carbonate seep structures
occur in the uppermost part of the Upper Miocene
Santa Cruz Mudstone Formation, just below the
unconformity with the overlying Mio-Pliocene
Purisima Formation The main locality where the
Seep Structures have been studied is also
highlighted.
Aiello Garrison, 2002
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click!
Outcrop in the coastal cliff showing the vertical
succession of the Upper Miocene Santa Cruz
Mudstone, the Mio-Pliocene Purisima Formation,
and the overlying Pleistocene terrace
deposits. (see next slide)
Aiello Garrison, 2002
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click!
An unconformity separates the Santa Cruz Mudstone
and Purisima Formation. Note that the normal
fault (red line) predates the unconformity. (Clic
k)
Aiello Garrison, 2002
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Bedding parallel carbonate seep structure as they
occur in beds of fractured opal-CT porcelanite in
the uppermost part of the Santa Cruz Mudstone
Formation. Commonly, bedding parallel carbonate
slabs show finger-like projections which may be
connected with carbonate pipes having central
conduits.
Aiello Garrison, 2002
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The carbonate structures are always associated
with porcelanite beds (pc) and never occur in the
mudstone layers (ms). (Click)
Aiello Garrison, 2002
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Carbonate seep structure (cb) that is elongated
at high angle to bedding.The structure is a
carbonate pipe with a central conduit (not
visible). The seep structure and the central
conduit plunge S40ºE. The carbonate pipe occurs
in three amalgamated fractured porcelanite beds
(pc) which occur alternating with mudstone layers
(ms). (Click)
Aiello Garrison, 2002
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2 m
3-D block diagram showing the geometry of the
carbonate seep structures (dark gray), of their
conduits (black), and the relationships with
fractured opal-CT porcelanite beds (light gray)
and mudstones (white). Note the two main
morphologies of the carbonate structures, namely
those that are bedding parallel and those
elongated at high angle to bedding.
Aiello Garrison, 2002
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Detailed map of the main seep locality. Green
color indicates carbonate seep structures. (click
)
Red lines mark the main elongation of the bedding
parallel carbonate structures. These directions
are plotted in the rose diagram. They parallel
the two main directions of the fracture set in
porcelanite beds (indicated by red arrows in the
rose diagram) N40ºW and N50ºE
Aiello Garrison, 2002
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click!
Bedding parallel carbonate slabs containing
subvertical conduits. (click)
Aiello Garrison, 2002
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The carbonate slabs are outlined in yellow. The
elongations of these structures are indicated by
dashed red lines. Note parallelism of these
elongations with the directions of the two main
fracture sets, shown in blue.
Aiello Garrison, 2002
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click!
Aiello Garrison, 2002
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Alignment of vertically-oriented seep structures
(green arrows) and parallelism with the N40ºW
fracture set (red dashed line) in the
porcelanite. Seep structure in the lower left
corner is elongated parallel to the N50ºE
fracture set (blue dashed line).
click!
Aiello Garrison, 2002
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click!
Aiello Garrison, 2002
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Aiello Garrison, 2002
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Alternating deposition of diatom-poor and
diatom-rich (abundant opal-A) muds produced
interbedded layers with relatively high contents
of organic matter but contrasting rheologies.
Gases and formation fluids, generated in
organic-rich sediments, ascended from a
thickened depocenter to the northwest toward a
developing Miocene paleohigh owing to a
combination of burial and tectonic compaction.
Tectonic fracturing of layers containing abundant
opal-A diatom tests provided permeability
channels for lateral movement of fluids and
gases. Microbial methanogenesis and possibly also
sulfate reduction yielded relatively heavy
carbon, bicarbonate-rich pore waters with an
increase in alkalinity. Vertical movement of
these fluids and gases, however, was impeded by
the relatively unfractured and impermeable
interbedded mudstone layers, leading to local gas
and fluid buildups. Incipient cementation by
opal-CT may have also begun at this stage thus
increasing the brittleness of the diatom-rich
layers, but the main stage of opal-CT formation
occurred later. The fractured silica-rich layers
thus acted as aquifers, while the interbedded
muds served initially as aquicludes or partial
aquicludes as well as local fluid sources.
CONCLUSIONS
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Continued gas and fluid buildup paired with
episodes of extensional deformation caused the
sealing mechanism of the shallowly buried
mudstones to fail, with consequent
buoyancy-driven upward expulsion of pore fluids
and gases, along with fluidization and
deformation of the overlying sediments.
Gas-fluid discharge and Seawater downflow
Aiello Garrison, 2002
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Precipitation of carbonate structures and modern
outcrop
Precipitation of calcite occurred when
bicarbonate-rich, high alkalinity pore fluids
that were concentrated in fractured siliceous
layers became sufficiently enriched in calcium
ions supplied mainly from seawater. The different
morphologies of the carbonate structures reflect
different flow patterns through the fractured
silica-rich layers. Precipitation of calcite from
laterally moving fluids led eventually to the
formation of type A and type B seep structures,
many of which contain bedding-parallel tubular
conduits. Crosscutting structures, which indicate
mainly vertically directed flow, developed mostly
toward the top of the section where the
silica-rich layers there were more shallowly
buried as well as much more closely spaced or
amalgamated than lower in the section closer
spacing would have allowed vertical connections
between fractures in successive silica-rich beds,
thus facilitating vertically rather than
horizontally directed flow. During subsequent
burial of this stratigraphic interval, the
diatom-rich layers were fully converted to
opal-CT porcelanites, and these layers became
pervasively fractured by recurrent
extension. Continuing deformation caused uplift
and tilting and probable eventual subaerial
emergence and erosional stripping of the
sediments above the carbonate-bearing layers, as
recorded in the angular unconformity at the top
of the Santa Cruz Mudstone.
Aiello Garrison, 2002
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REFERENCES
Aiello, I.W., Stakes, D.S., Kastner, M., and
Garrison, R.E., 1999, Vent structures in the
upper Miocene Santa Cruz Mudstone at Santa Cruz,
California, in Garrison, R.E., et al., eds., Late
Cenozoic fluid seeps and tectonics along the San
Gregorio fault zone in the Monterey Bay region,
California Santa Fe Springs, California, Pacific
Section American Association of Petroleum
Geologists, Volume and Guidebook GB-76, p.
35-51. Aiello, I.W., Garrison, R.E., Moore,
J.C., Kastner, M., and Stakes, D.S.,2001, Anatomy
and origin of carbonate structures in a Miocene
cold-seep field. Geology, 29, p. 1111-1114.
Aiello Garrison, 2002