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.
I was working near Lake Fort Smith State Park this last week when I came across a peculiar mineral deposit resembling Frankenstein’s Scars (Fig. 2). It was just in time for Halloween! The resemblance is uncanny. Despite the horror, there is a lot of geology illustrated in this rock.
The mineral that forms the “scars” seen in the photo is called limonite, and it was deposited within a cavity in a stylolite. A stylolite is a surface, typically a bedding plane, that has recrystallized due to pressure from the weight of overlying rock material. Stylolites can be recognized by their rough, jagged appearance (it’s difficult to see in this photo, but trust me – it’s there). The limonite “scars” formed in a pattern called boxwork and, surrounding the boxwork, limonite is also present in botryoidal form: a crystal shape resembling small round globs (the orange goosebumps around the scars).
At this time, rocks are not thought to celebrate Halloween, although more work needs to be done to verify that.
The above picture shows rocks that were deposited in a delta. A delta is a place where a river or stream empties into a standing body of water, typically the ocean or a lake. As it enters the larger body of water, the stream loses velocity and starts dumping the sediment it’s carrying. Coarse sediment settles out first and finer sediment is carried further from the shore before being deposited.
As the delta develops, it builds a lobe of sediment out into the ocean. The stream advances into the sea atop its own lengthening delta, and new sediment is carried ever further seaward. Eventually, today’s coarse sediment is being deposited on yesterdays fine sediment. Coarse-grained rock at the top, and fine-grained rock at the bottom, is characteristic of delta deposits.
The picture above shows two lobes of deltaic sediment, each with the characteristic coarsening upward architecture. The coarsest, top-most bed of each lobe juts out, creating a waterfall. This picture is of the Atoka Formation and was taken near Lake Fort Smith, Crawford County, Arkansas.
Below is a land satellite image of recent Mississippi River deltas in the Atchafalaya Bay.