Pictured above is a little piece of geologic history known as a basal conglomerate. that’s a rock formed after a period of erosion that marks the boundary between two geologic time periods: in this case, the Mississippian (359-318 million years ago) and the Pennsylvanian (318-299 million years ago).
318 million years ago sea level subsided, bedrock was exposed, and the Mississippian Period came to an end. When exposed to erosion at the earth’s surface, pieces break off from bedrock. Flowing water in rivers, streams and oceans wears the edges of those rock fragments till they’re rounded. Once ocean level rises and deposition resumes, the rounded gravel gets mixed with newly accumulating sediment and forms a rock which is made partly of fragments of the older bedrock. Geologists call this type of rock a basal (at the base) conglomerate (containing round gravel) because it is the first bedrock signaling the beginning of a new period of geologic time.
The grooved surface pictured above is a slickenside. Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).
Slickensides form when fault blocks move against each other. The natural irregularities on each scratches grooves into the other. The grooves are parallel to movement; for instance in this example, movement was either to the right or the left. To tell whether it was right or left, you can rub your hand along the slickensides. They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head. Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.
The shale above was photographed in Big Rock Quarry, North Little Rock, AR. It’s a part of the Jackfork Formation (Pennsylvanian).
Pictured above is one of many faults, closely spaced together, in an outcrop of the Atoka Formation, near Lake Fort Smith, Arkansas. The fault pictured extends from the upper right to the lower left and is highlighted. This type of faulting is called syn-depositional faulting, meaning it occurred at about the same time the rock was being deposited. It results in disturbed-looking outcrops like this one.
Around 300 million years ago, plate tectonic forces were deforming the Ouachita Mountains in south central Arkansas. Those forces also caused faulting in the southern Ozark Plateaus, as the sediment that composes this rock outcrop was being deposited. The freshly deposited sediment wasn’t fully consolidated when the faulting took place and the rock surrounding the fault got contorted by the stress.
Some of the deformed features of the outcrop are labeled above. The Zone of Soft-Sediment Deformation is the area surrounding the fault where the rock has been deformed by shearing: there is no recognizable bedding in that zone. The soft clay-rich Deformed Shale was squeezed plastically between the fault blocks in that soft sediment deformation zone. The bedding orientations surrounding the deformation zone (indicated by magenta lines) vary greatly, because the soft bedrock was broken and heaved around by the fault.
Continuing with our previous theme “Sharkansas”, this week’s geo-pic is on Arkansas corals. Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.
The picture above is of a tabulate coral: a now-extinct variety of colonial coral. Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water. The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock. It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.
Other varieties of coral are found in the rocks of Arkansas. For more views of Arkansas corals click here
You wouldn’t know it to look around now, but Arkansas, at times in the distant past, was teaming with sharks (and other marine fish). Indeed, Arkansas was in part or wholly covered by ocean many times in the past. One such time was 250 million years ago, during the Carboniferous Period. The fossilized Cladodus tooth pictured above belonged to a primitive shark that had sharp teeth with multiple points of varying size that it used to gig fish before gulping them down. The long point at the middle of the tooth is broken off and displaced to the right in this picture.
This particular specimen was found near West Fork, Arkansas. It was collected from the Prairie Grove Member of the Hale Formation, a limey sandstone. However, shark teeth can be found locally, throughout other parts of the state, in marine rock layers spanning hundreds of millions of years.
Pictured above is Dardanelle Rock located on the south side of the Arkansas River between the towns of Dardanelle and Russellville. The white truck in the lower right corner shows the scale of this outcrop. It was designated a Natural Area by the Arkansas Natural Heritage Commission in 1976.
The Arkansas River Valley is north of the Ouachita Mountains and is characterized by gently folded sedimentary rock that was subject, to a lesser extent, to the stress that folded the Ouachita Mountains. The rock pictured here is the south limb of a broad syncline, or down-warped fold. The north limb is about two miles to the northeast. The bedrock dips to the north (toward the white truck), goes sub-surface beneath the Arkansas River, then reverses dip direction and rises back to the surface just southwest of Russellville. If you could see a cross-section of the folded rock, it would look like giant a smiley face with the middle of the smile underground and the corners sticking up in opposite directions, two miles apart.
This picture gives perspective to the colossal size of geologic features geologists study. Folds like this one, which can trap upward-migrating fluid, are sometimes rich oil and gas reservoirs.