The Atco Formation

Introduction
Overall the Atco Formation is a chalky limestone, characterized by the interbedding of chalky wackestones (typically 0.3-1.5m) and fissile marly wackestones (typically 0.05-0.3m).  The lower 20 meters, of its approximate total thickness of 30 meters, is noticeably marly (clay mixed with chalk).  At least one conspicuous bentonite bed is seen in this lower more marly portion of formation (between ~ 6-7 meters above the base of the formation). Longshore scoured (?) channels are present in the lower parts of the formation as well. Perhaps as a result, beds of the Atco seem less geographically presistant then those of other formations of the Austin Group.  The base of the Atco is marked by a "lumpy" reworked zone, containing elements of the lower Austin, and upper Eagle Ford.  This can be seen below.

The Atco - Eagle Ford Contact

Lowermost Atco Formation

Moving higher in the Section

The distinctive marl seam near the top of the outcrop pictured above, is near the base of the following section (located along a bend in Walnut Creek).

Inoceramids are common here.

   Continuing up Section

The three closely spaced beds, located at the approximate center of the outcrop pictured above, are located near creek level at this location.

Near the top of this outcrop is a distinctively indented marl seam.  It may be traced to our next outcrop, as we...

Continue up Section

Here again, we may use a distinctive bed near the top of the previous outcrop to tie to the next exposure.  Stratigraphically intervening exposures make this tie sound.  The picture to the right shows the gently dipping beds rising in the distance.  This exposes the layers below - and allows for correlation with the previous outcrop.

Another method of correlation is simple visual matching across breaks in exposure.   The picture on the left below is a distant view (looking upstream) of the outcrop pictured above.  The right picture shows the view 180 degrees from the same vantage point.  The same distinctive bed may be seen near creek level in each.  It is possible to correlate bed to bed between these outcrops.  Nearly identical strata are exposed.

Another view of the outcrop to the right above.

Further downstream, the same distinctive bed can be seen approximately 2 meters above creek level.  A vertical rock face of nearly 10 meters is exposed here, moving us higher still in the section.

Inoceramids are common here as well.

Storm layers of inoceramid hash may also be observed.