1. 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

2. 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

3. 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

4. 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

5. 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

6. click!
An unconformity separates the Santa Cruz Mudstone and Purisima Formation.
Note that the normal fault (red line) predates the unconformity.
(Click)
Carbonate seep structures are highlighted in a bluish color.
Note that the seep structures have two main form:
a) elongated parallel to bedding
b) oriented at high angle to bedding a b
Aiello & Garrison, 2002

7. 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

8. pc = opal-CT porcelanite
The carbonate structures are always associated with porcelanite beds (pc) and never occur in the mudstone layers (ms).
(Click) cb pc ms
cb = carbonate
pc = opal-CT porcelanite
The porcelanite beds (pc) are well fractured as compared to the relatively unfractured mudstone layers (ms).
Aiello & Garrison, 2002

9. cb pc ms
plunge S40ºE
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

10. 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

11. Detailed map of the main seep locality
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

12. click!
Bedding parallel carbonate slabs containing subvertical conduits.
(click)
Aiello & Garrison, 2002

13. The carbonate slabs are outlined in yellow
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

14. cb pc ms
Irregularly shaped, bedding parallel carbonate seep structure (cb) with prominent conduit (red arrow) parallel to bedding. Associated rocks are porcelanite (pc) and mudstone (ms).
click!
click!
Aiello & Garrison, 2002

15. 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).
N40ºW cb
N50ºE
click!
Aiello & Garrison, 2002

16. “Twin” vertically oriented carbonate pipes encased in fractured porcelanite.
Note that the two pipes plunge in different directions:
N50ºW green arrow
N40ºE blue arrow
(click)
click!
N50ºW
N40ºE
The contour plot (lower hemisphere projection) shows the average plunge of carbonate pipes and conduits.
Note that carbonate pipes parallel the fracture sets.
Aiello & Garrison, 2002

17. Microphotograph of an authigenic carbonate (low-Mg calcite).
Although the original biosilica was almost completely replaced by carbonate, the primary texture of the sediment (namely diatom frustules) is still preserved.
XRD data indicate that some frustules are still composed of opal-A, thus indicating that carbonate replacement occurred before the opal-A to opal-CT silica phase change occurred.
Opal-A
Diatom frustule
Aiello & Garrison, 2002

18. CONCLUSIONS Early fracturing and gas-fluid buildup
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.
Aiello & Garrison, 2002
Early fracturing and gas-fluid buildup
CONCLUSIONS

19. Gas-fluid discharge and Seawater downflow
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

20. 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

21. 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
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
Aiello & Garrison, 2002