Fossil Friday


Ichnology is a discipline of Paleontology that is the scientific study of tracks and trace fossils.  Knowing what kinds of animals occupy certain depositional environments, the traces and tracks they leave behind, and an understanding of geochemistry help us to translate ancient geologic outcrops.  This beautiful example was collected in Lauderdale County by Leslie Potter and sent to Office of Geology staff this week for identification through our “Ask a Geologist” online public outreach program. These excellently-preserved fossil decapod burrows are part of the Lower Eocene age Tallahatta Formation.  This ichnofossils is named Thalassinoides and were made by numerous marine burrowing shrimp that once occupied a tropical sandy shallow sea floor that once occupied the east-central Mississippi area some 50 million years ago. They are preserved as stone casts from being naturally cemented with silica minerals that have now become harder than the surrounding sandstone so that they weather in relief.  Because the chemistry of the sediments didn’t preserve any other fossils of marine life that once certainly occupied the area, these ichnofossils prove important to helping us understand the ancient depositional environment of the sandstones of the Tallahatta Formation.  Every so often some rare fossil sea shell impressions have been found there but are extremely rare, whereas these Thalassinoides burrows are quite abundant.  This one in particular is a pretty special find.  The gassy-fill substance in the burrow is called Tallahatta Agate.  It is a natural mineral formation of nearly pure chalcedony and opaline silica.  Thank you again Leslie Potter for sharing your discovery with our geology research staff and allowing us to share it with others.

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Ancient marine fossils, much older than the days of the dinosaurs can be found in chert gravels right here in Mississippi. This crinoidal chert (a type of rock that was once the sea floor dominated with the skeletal remains of fossil crinoids) from Jefferson County was photographed this week by Office of Geology staff.  Complete fossils of crinoids are rarely preserved intact, though there individual segments and partial stems and calyx are some of the most common chert gravel fossil found in Mississippi. Crinoids are echinoderms, relatives of starfish and sand dollars and are still alive today in the worlds oceans.  Paleozoic era chert gravel fossils from Mississippi can be found naturally though much of the state. They were once eroded from ancient limestone bedrock sources north of here, up in the mid-continent, and were brought down and deposited by by ancient rivers flowing to the Gulf of Mexico. Collecting gravel fossils are often the first exposure folks have to our rich paleontology resources in Mississippi. Crinoids are one of the many treasures to be found in our Mississippi chert gravels. Chert gravels is an important economic resource commonly used in many civil applications throughout Mississippi. So, gravel fossils can be found almost anywhere from playground, aggregate, to gravel roads and driveways, even in cement gravels along downtown sidewalks.

 

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Calcite septarian nodules (commonly called dragon stones) are natural concretions of calcium carbonate that exhibit secondary mineral growth of yellow “dog-tooth” calcite along an intricate network of desiccation-like  cracks. They make excellently beautiful mineral specimens, especially when they are carved, cut, and polished or as interesting curiosities if left natural. They typically form in impure, sandy, and fossil shell-rich limestones (called marls) from the geochemistry and movement of shallow groundwater. Groundwater moving through the marl formation derives carbonate minerals from dissolving and leaching from fossil sea-shells in the rock and concentrates it into a nodule. As the nodule solidifies to a hard rock it begins to shrink and a network of cracks form throughout the nodule. The voids created by the desiccation-like cracks then begin growing “dog-tooth” yellow calcite mineral precipitated from the carbonate-rich groundwater water.  These nodules are much harder and more erosionally-resistant than the surrounding geologic formation they formed in. Therefore, they tend to weather in relief from the outcrop or completely weather out and can concentrate as unusually large bolder-gravels along stream beds that wind their way through the formation.  Only a few geologic formations in Mississippi exhibit this phenomenon. Most notably is the early Eocene age Bashi Formation of east-central Mississippi in the vicinity of Meridian. Another example (photo below) of exceptional quality was recently documented by MDEQ, Office of Geology staff from the Late Eocene in age Moody’s Branch Formation while describing geologic outcrops in the field with in Yazoo County, Mississippi.

Click here for more on Mississippi’s Geologic past.


Ichnology, a discipline of Paleontology, is the scientific study of tracks and trace fossils.  Knowing what kinds of animals occupy certain depositional environments, the traces and tracks they leave behind, and an understanding of geochemistry help us to translate ancient geologic outcrops. These excellently-preserved fossil decapod burrows where photographed in the field last week by MDEQ, Office of Geology staff. The fossil burrows were discovered eroding from an Eocene (Claiborne age) outcrop of the Creola Member of the upper Cockfield Formation exposed in the creek floor. They are preserved as stone casts from being naturally cemented with siderite, a distinctive iron carbonate mineral. Siderite commonly forms in shallow marine, brackish water, near-shore environments. With these clues found in the outcrop in the floor of the creek along with numerous seashell fossils it contains, it’s not hard to imagine a place very much like the Mississippi Sound today…shallow muddy water filled with burrowing crab and shrimp, as well as snails, oysters, and clams while drum fish, sea trout and flounder tail along the grassy shallows…but this is almost 40 million years ago, in a world filled with much more stranger beasts…and in Yazoo County, Mississippi. #fossilfriday


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