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OUTCROP STUDIES
The Injected Sands Group is currently studying 6 sets of outcrops showing characteristic soft-sediment deformations and injections:
A reservoir scale injection complex occur in the Panoche and Tumey Hills along the western margin of the San Joaquin Valley. The Panoche Giant Injectite Complex (PGIC) formed during the Lower Paleocene and intrudes into strata of Late Cretaceous and Lower Paleocene age. Confirmation of the timing of sand injection is possible because the PGIC extrudes onto a palaeo-seafloor. The PGIC is remarkable as it is exposed intermittently over an area greater than 100 km2 and the relationships between the depositional parent-beds from which sand was remobilised, and the top of the injectite complex where sand extruded onto a palaeo-seafloor are seen. Generally the PGIC has a superficially dyke-dominated, orthogonal geometry, is not sand-rich (<10% sand) but is locally higher (>50%), particularly in the lower part of the Moreno Formation.
| Click on the images to see a high resolution version |
Panoramic view (toward E) of the PGIC along the margin of the Marca Canyon. A complete transect from the parent beds units (bottom left of the photo) to the extrudites (up right of the photo). |
Panoramic view (toward E) of the PGIC south of Capita Canyon. It is possible to see a sill-dominate portion superimposed by a dyke-dominated portion. |
20 m thick parent beds unit with caotic and homogenous deposits that record huge remobilisation of sand. |
Cross section of a major dike. The dyke is 12 m wide and slightly sinuous in plan view (the person for scale is just 1.6m tall). |
Low angle dyke with a meter scale step. The step is located in correspondance of a crosscutting vertical dyke and it contains big clasts of host rock (Jeep for scale on the left of the photo). |
Panoramic view of the West Tumey area showing conical injections, high angle dykes and the extrudites at the top. |
A second injectite complex occurs within the Eocene Kreyenhagen Shale, Tumey Hills, Central Coast Ranges, California. Unlike the laterally extensive PGIC (see above), the Tumey sand injectite has crudely-circular plan-view geometry (d=2km). Relative to the PGIC the Tumey sand injectite is more sand rich and lacks orthogonal geometry. Parent beds crops out to the West while sand extrusions were not identified.
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View to the E of the Tumey Hill Injection Complex. Note the overall irregular geometry of the sand units (light grey rocks) intruded into the diatomaceous shales (host rock, pale pink colour). |
A large stepped and branching sill departing from an irregular dome-like injection. |
Santa Cruz, in the Monterey
Bay, located a short drive south of San Francisco, has world-class, reservoir-scale
exposures of sand injection features that are easily accessed and are located
within an area of active oil migration close to producing oil and gas fields.
The area is active tectonically and consequently provides many features
that are directly analogous to the period when sand injection and oil migration
into the Paleogene deep-water clastic bodies of the North Sea occurred.
Sand intrusions occur in Upper Miocene to Pliocene
strata, mainly encased by the diatomaceous S. Cruz Mudstone (Thompson,
1995). Injectites form reservoir-scale injection
complexes which are commonly tar-saturated. The area aso include some spectacular exposures of parent beds and sand extrusions and provides hints on the fluidodymanics of the injections, mechanisms of fluid expulsion and sand venting at the sea-foor and on hydrocarbons migration patterns.
| Click on the images to see a high resolution version |
Large-scale (30 by 60m) injected sand body with, sharp vertical margins, in the S. Cruz Mudstone. |
Sandstone injection complex in the foreground. The sand body has a complex, folded and faulted margin. |
Fault zone affecting the S. Cruz Mudstone and the remobilised sands included.. |
Injection complex with two main generations of sand intrusions |
Bubble-like intrusions of bituminous sand in yellow, limonite- cemented sandstones. |
Thin sandstone dykes and sills with ptygmatic folds included in the S. Cruz Mudstones |
A sill-dominated, reservoir scale, injection complex occur in Tabarka, Northern Tunisia. Injectites intruded at least 160 m of clay and shales. Injectites consist of medium to pebble-rich very coarse sand. Sills up to 4m thick are connected by dykes and\or pipes.The overall net\gross ratio of injectites is about 35-40%. The injectites are sourced by multiple deep-water sand units belonging to the Numidian Flysch (Upper Oligocene-Lower Miocene). Parent beds at different stratigraphic heights are connected by multiple dykes and show an impressive array of deformation and liquefaction structures.
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Panoramic view of the injection complex exposed in the Fort Genois area, Tabarka - Tunisia. Parent beds are the thick sand units on the right. All the other sand units (grey) intercalated to mud (dark brown) are injectites. |
Orthogonal set of dykes feeding a sill. |
Multiple pipes cutting through a 3m thick muddy interval. |
Several localities along the North-eastern coast of Scotland display
soft-sediment deformation and sand-injection features. The most interesting
localities are probably Eathie, in the Black Isle, and the area of Brora-Helmsdale.
Jurassic shale-prone sequences host the sand injections which can form
an intricate network of dykes and sills, as at Eathie. Different genesis
of injections, early post-depositional and late post-depositional, with
differnt styles can be recognised. Characteristics petrographical textures
differentiate the remobilised sands with respect of the "in situ" sandstones,
which have not been affected by these processes.
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andstone dyke cross-cutting the Upp. Jurassic sequence at Kintradwell (Brora). |
Thin section of the sandstone dyke showing low porosity and numerous fractured grains. |
Thin section of "in situ" sandstone cross-cut by the dyke, showing unfractured grain and high porosity. |
In several localities (Rosans, Bevons, Pierre Avon) sandstone injections are intruded in the Aptian-Albian slope deposits of the Vacontian Basin. The injections show a spectrum of geometries ranging from irregular stepped and abruptly terminating sills, to high sinuous "ptygmatic-like folded" dykes. They are associated with 3D metric scale bedforms which could be isolated in the slope shale deposits or could constitute the filling of decametric scale channels. The channels are supposed to be, by several Authors, as the feeder of the injections.
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Highly sinuous dyke. |
Portion of a centimetric thick dyke cutting the shale slope deposits for sevral meters. |
Channel bodies filled by 3D metric scale bedforms. The bedforms show a clear cross lamination. |
