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Sedimentary Rocks & Structures

Crystal-fiber gypsum vein, Rincon Mountains, Tucson, Arizona

Age & Formation: 
Host rock is Miocene fanglomerate of the Pantano Formation, ~26 Ma.
Missing from this specimen is the host rock itself. What remains is just the gypsum vein filling. The lateral edges of this vein mark the contacts with the host rock. The line (surface) down the center of the gypsum vein can be thought of as the line (surface) of original fracturing of the host rock. The gypsum fibers progressively filled the fracture as the walls of the fracture opened. The direction of fiber growth is oblique to the fracture surface, revealing that there was both opening and lateral slip.
Whereabouts: 
I collected this from a clay quarry located along the southern margin of the Rincon Mountains. I like it because it is such a clear ‘inside’ view of crystal fiber veins and how they grow. Gypsum is a lovely, satiny material, made even better when it grows like fibers in response to structural deformation.
Rock Image: 
rock23_R.jpg
QTVR URL: 
gypsum.mov

Quartz vein with crystal fiber lineation in sandstone, Tucson Mountains, Arizona

Age & Formation: 
rock for vein is Jurassic Recreation Red Beds, ~175 Ma. Quartz vein may have been introduced ~70 Ma, at a time of nearby volcanic activity.
The quartz vein incrementally filled a tiny fault surface, which was simultaneously slipping and opening up. The crystal fibers of quartz record the combination of the two movements, in the form of shear and dilation. The lines are mineral lineation, the penetrative presence of which documents that the vein was continuously deforming while it was being precipitated along a fault surface. The black with tinges of red is hematite and manganese, the products of precipitation of silica, iron, and manganese from hydrothermal fluids containing silica and iron.
Whereabouts: 
I collected this while mapping the geology of Brown Mountain with Evans B. Mayo in 1971. There are dozens and dozens of these veins, which stand out sharply as milky lines in contrast to the red host rock. I never identified any fault offsets along these veins, based on looking for offset marker beds. Now, looking back, I think it might be possible to figure out slip direction by analyzing the angles between quartz fibers and the fault surfaces. I like how tightly ‘welded’ this vein is attached to the host rock, and am attracted to the white/black streaking, which accentuates the crystal-fiber growth direction. This rock also reminds me of some unfinished business I have yet to do on Brown Mountain. It is easy to ignore a single isolated exposure of a streaky quartz vein, but not a hillside with hundred and hundreds.
Rock Image: 
quartz_vein_still.jpg
QTVR URL: 
quartz_vein.mov

Calcite crystal fiber vein in limestone, Monument Uplift, Utah

Age & Formation: 
Lithographic limestone from the Jurassic Carmel Formation, ~165 Ma. Time of vein-filling of fracture/fault not known, but probably in the latest Cretaceous/earliest Tertiary (~65 Ma) or mid-Tertiary (~25 Ma).
This is a perfect example of a crystal-fiber vein with crystal-fiber lineation. The source of the calcite comprising the vein is undoubtedly the limestone itself. When clayey limestone is subjected to tectonic stress, it creates an environment where calcite becomes more soluble, even in the absence of elevated temperature. Pressure solution causes the calcite to go into solution. It then re-precipitates nearby in tensional openings. This particular tensional opening, strictly speaking, was not a joint, but rather a fault marked both by shear and dilation.
Whereabouts: 
I collected this in ~1999 while working with Alex Bump along the Comb Ridge monocline on the east flank of the Monument Uplift. It was another beautiful day working on the Colorado Plateau, and we were positioned inside the steeply dipping limb of the giant Comb Ridge monocline. Surrounded by such BIG structure, it was quite something to see the delicacy of small-scale tectonic adjustments within individual outcrops, and to be reminded again of how each lithology may have its own way (its own mechanisms) for accommodating and expressing deformation. In this case we are seeing semi-brittle faulting.
Rock Image: 
rock25_R.jpg
QTVR URL: 
rock_04.mov

Cobble of conglomerate cut by a fault, and healed by a quartz vein, Rillito River, Tucson, Arizona

Age & Formation: 
The granule conglomerate probably came from Miocene fanglomerate, ~25- 20 Ma. Faulting and vein-filling is probably no older than ~18 Ma.
This cobble is full of history. The rock itself reflects sediment deposited in a braid-fan deposit, produced by intermittent streams flowing in a semi-arid environment. Times of higher flow are preserved in the granule conglomerate. The black laminae are heavy sands, including magnetite. If this were not enough, the rock had been faulted, as is evident by the 1 cm offset of the layers. The fault surface was later dilated slightly and filled by quartz, now in the form of the quartz vein. Then, at some stage the rock layer made it to the surface through uplift and erosion.
Whereabouts: 
A former undergraduate student gave me this in the 1990s sometime. I’m 'blanking' on who gave it to me. She found it in the Rillilto River in Tucson while hiking. I like the range of history that is bound up in this rock: so many different chapters are preserved.
Rock Image: 
conglomerate_still.jpg
QTVR URL: 
conglomerate-1.mov

Faulted chert nodule, Rincon Mountains, near Tucson, Arizona

Age & Formation: 
Horquilla Formation of Pennyslvanian age, ~310 Ma.
This chert nodule was originally embedded within limestone, but because of its comparative resistance to erosion completely weathered out of the bedrock. It is fascinating to see that the fractures cutting the chert nodule are actually faults with clear offset. The effect of the faulting was to lengthen the nodule in one direction (parallel to the length) and flatten it at right angles (perpendicular to the oblate spheroid).
Whereabouts: 
I collected this in the late 1970s in the Rincon Mountains in the Happy Valley area. It is one thing to find a small rock that is marked by a single fault, and quite another to find one with a system of faults. I like the fact that by eye it is ‘easy’ to ‘unfault’ the rock and get back to the original shape and size of the chert nodule.
Rock Image: 
chert_nodule_still.jpg
QTVR URL: 
chert_nodule.mov

Faulted pebble from conglomerate, Scottish Highlands

Age & Formation: 
Pebble from the Old Red Sandstone, Devonian age, ~380 Ma.
This pebble is a very fine grained volcanic rock (rhyolitic?), which was contained in a mechanically soft matrix of silt, sand, and clay. The pebble responded to tectonic stresses by fracturing and faulting. In particular, this rock is cut by a series of subparallel faults, movement along which lengthened and flattened the pebble. Faint slickenlines are apparent on some of the fault surfaces.
Whereabouts: 
I collected this rock in 2008 in the Scottish Highlands where the Old Red Sandstone is cut and displaced by the Highland Boundary fault. I was on a field trip led by Midland Valley Exploration, Ltd. I like this rock because it is part of the classic “Old Red Sandstone,” which I had never seen in outcrop. I find it intriguing that this pebble does not simply fall apart, given all of the faults that cut it. Instead, there must be ‘cement’ sealing the faults, a cement deposited during deep ground water circulation or even syntectonic precipitation of vein material.
Rock Image: 
rock28_R.jpg
QTVR URL: 
rock_12.mov

Pebble from conglomerate faulted into form of a tiny graben, Scottish Highlands

Age & Formation: 
from the Old Red Sandstone, Devonian age, ~380 Ma.
This pebble is a very fine grained volcanic rock (rhyolitic?), which was contained in a mechanically soft matrix of silty, sandy, and clayey material. The pebble responded to tectonic stress by fracturing and faulting. In particular, this rock is cut by inward-dipping normal fault zones, slip along which has lengthened and flattened the pebble overall. The central part of the pebble is a graben structure. Faint slickenlines are apparent on some of the fault surfaces.
Whereabouts: 
I collected this rock in 2008 in the Scottish Highlands where the Old Red Sandstone is cut and displaced by the Highland Boundary fault. I was on a field trip led by Midland Valley Exploration, Ltd. I like the fact that this pebble is a miniature thumbnail of the kind of faulting that can deform regions. Besides, how often can you carry a graben around with you in your pocket?
Rock Image: 
rock29_R.jpg
QTVR URL: 
rock_13.mov

Faulted, slickenlined petroliferous limestone, Appalachian Mountains, West Virginia

Age & Formation: 
Paleozoic limestone. Formation uncertain.
This wedge-shaped block of limestone is enclosed by fault surfaces marked by slickenlines. The slickenlines are mainly crystal-fiber lineations of calcite. The origin of the calcite vein material was undoubtedly pressure solution. The limestone itself is very dark gray in appearance. Hardened oil locally coats the fault surfaces.
Whereabouts: 
I collected this near Seneca Rocks, West Virginia, back in ~1966. My wife and I were touring part of "West by god Virginia" (proud name applied by proud people) with my parents, and I just ‘happened’ to see a fine fault exposure in one of the road cuts, and managed to bring this home with me. I like the fact that in this one small rock it is possible to see the intersecting of faults, and with a coordination of movement that is clear in the slickenline orientations. Moreover, this rock is a great example of zones of fault intersections becoming places where fluids, in this case hydrocarbons, migrate because of the porosity and permeability of such zones.
Rock Image: 
rock30_R.jpg
QTVR URL: 
sparkly_rock.mov

Faulted limestone

This faulted limestone layer displays, on both upper and lower bedding surfaces, fault scarps (steps) with heights equal to the fault offset. I think of this specimen as a miniature example of a large, actively faulted region, where a plateau (upper bedding surface in this case) is dropped down by fault action. This fault has been “healed” by calcite veining, introduced in a ‘crack-and-seal’ process. Note that the scarp along the upper surface is expressed in the white calcite. Lineation in the calcite provide the slip direction for the faulting.

Whereabouts: 
I collected this on Mt. Lykaion in the Peloponessos of Greece during the summer of 2010. Following complete 3D photography of this specimen, I will then have it ‘thin’ sectioned along closely spaced cuts that cross the specimen at right angles to the fault trend. Microscopic analysis of the thin sections should reveal at the microscopic scale the history of fracturing, faulting, and sealing, …over and over again.
Rock Image: 
sed.jpg
QTVR URL: 
sedimentary_rock.mov

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