Category Archives: #geoblog

Geo-pic of the week: Trilobite Romance

trilobite meeting in pennsylvanian

The photo above shows trace fossils that record the travels of two trilobites.  Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya).   The tracks the animal left are known as the trace fossil, Cruziana.  It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while.  At the center of the photo are resting traces known as Rusophycus.  Perhaps they became friends or maybe they were even more than friends?  It is Valentine’s Day.  Their traces are preserved in the Atoka Formation of west-central Arkansas.  

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Geo-pic of the week: Fracture-Fill at Shine-Eye

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The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.

Geo-pic of the week: Sand and clay of the Wilcox Group

wilcox sand canyon pit enhanced

This rather handsome outcrop of the Wilcox group consists of alternating layers of sand and clay of the Eocene Epoch which lasted from about 56-34 million years ago.   The Wilcox Group is a non-marine unit mostly composed of sand with lesser clay, silt, gravel, and lignite (low-grade coal).  

This geologic unit is part of a larger sequence of loosely-consolidated sedimentary rocks exposed in south central Arkansas, south of Pulaski county.  These rocks are the northern extent of the West Gulf Coastal Plain, a physiographic province that stretches from central Arkansas, south, to the Gulf of Mexico.

Geo-pic of the week: Pegmatite from Magnet Cove

Agerine from Magnet Cove, AR

In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite.  Pegmatites are igneous rocks characterized by extremely large crystals.  Sometimes they also contain unusual mineral species.   This sample was collected from Magnet Cove, Arkansas.  Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface. 

Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock.  Millions of years of erosion eventually unearthed that rock.  Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals.  Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.

Geo-pic of the week: Phantom Quartz

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Phantom quartz refers to quartz crystals that show the outline of smaller quartz crystals inside of them.  They are thought to form when there are at least two periods of growth.  The phantom crystal is visible either because it differs slightly in composition or because it was coated with some material prior to the second round of crystal growth. 

In the crystals above, all of which were collected near Mt. Ida, Arkansas, partial dissolution of the phantom crystals prior to the second round of growth left the phantoms with a ragged appearance.

Geo-pic of the week: Rock Beds

Bluff above Buffalo River edited

Why do rocks have beds?  Are rock beds where geologists sleep?  Sometimes, but that’s not the point of this article.  The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers).  The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.

Here’s an example;  when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes.  Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit.  That weathered surface will eventually differentiate the flood deposits into distinct beds of rock. 

Bedding can also form as a result of flowing water gaining or losing velocity.  The size of sediment that water carries (and eventually deposits) is directly related to flow rate.   A sudden change in flow rate creates bedding distinguished by differences in grain size.

Everyone in the photo above was eventually air-lifted to safety… Just kidding!  They’re still up there clip_image001

Geo-pic of the week: Solution sinkholes

Sinkhole from angela

A sinkhole is an area of ground that has no external surface drainage.  Water that enters a sinkhole exits by draining into the subsurface.  Many people are leery of sinkholes because of the damage they sometimes cause.  Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake. 

Not every kind of sinkhole is the dangerous kind though.  The picture above shows a solution sinkhole.  Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression.  Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground. 

In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone.  These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.