The Vinson Chalk

Introduction:  The Vinson Chalk in the Austin area is in general, white chalk, interbedded with relatively thick marl seams.  The chalk is typically a soft, starkly white, sparse wackestones.  Like the Dessau Chalk, it thus more closely resembles a  European concept of The Chalk, then other units of the Austin Group.  Throughout the Vinson storm beds or lags with intraclasts of limestone and inoceramids can be seen.  The marly interbeds are relatively thick and widely spaced, compared to the underlying Atco.  The contact with the Atco is not exposed in Travis County, but has been observed in cores taken in the North East portion of the County.  The contact is gradational, and in fact a field boundary strata-type would be difficult to define in any case.  Regardless, no significant section of the lower Vinson is available for study in the Austin area.  The upper contact ranges from gradational, in the north, to disconformable in southern Travis County.  The type section for the Vinson, shown below, records this disconformable contact.

Type Section - upper Vinson

Outcrops at overpass of I35 with Onion Creek