The Jonah Formation

Introduction
The Jonah Formation is essentially a fragmental limestone, and has the appearance of a pelletal packstone in hand sample.  It is this arenaceous aspect of the Jonah that distinguishes it from the chalks above and below.  However, the upper contact, a regional unconformity, does not separate distinctly different lithologies (chalks are found higher still in the section - see The Dessau Chalk for more information).  The grainy appearance of the Jonah has a noticeable effect on the weathered aspect of it's outcrops.  Note the somewhat irregular surface of the outcrop pictured below.  The fissile interbeds of the Jonah tend to be thinner and less marly then is the case in other portions of the Austin.   Numerous macrofossils are present in the Jonah.

The lower Jonah

Creek level is approximately two to three meters above the Vinson Chalk

From Creek elevation up through eight meters of section.

The upper portion of the Jonah

This section stratigraphically overlaps the outcrop above.  The upper portion of the photograph shows the white rocks of the overlying lower Dessau Chalk.   Approximately one meter of section (directly overlying the section in the photograph above) is not exposed at this location.

The transition into the Dessau (as well as the covered central region) can be better seen in the expanded section below.

The covered region is present, because of the weathering characteristics (easily weathered) of a distinctive meter scale marl 2-3 meters below the Jonah - Dessau contact.  This is visible along a sharply curving tributary to Walnut Creek, nearby the outcrop pictured above.

Fossils

An uncharacteristically (for the Austin Group) diverse macrofossil assemblage is found in the Jonah Formation.  Some of these are pictured below.

    Texanites americanus                                   Hemiaster texanus                     Cladiceramus undulatoplicatus

The fossils above are among the many which are found in the lower portion of the Jonah Formation.  Interestingly Cladiceramus (Inoceramus) undulatoplicatus has been used as a marker fossil for the Vinson Chalk.  However, it does occur in the lowermost Jonah.  Another interesting paleontological aspect of the Jonah Formation, is the presence of great numbers of gastropods.  This is not true for the rest of the Austin.