Trip Information

The Virtual Field Trip to the Austin Chalk Group, is in central Texas. In fact it is be entirely within Travis county. The map above shows the outcrop belt of the Upper Cretaceous (exclusive of the Taylor and Navarro Groups) in Texas. The position and outline of Travis County is shown. Below is a slightly larger version of Travis County, with the general outcrop position of the Austin shown.

Lithology, Thickness, and Nature of Stratigraphic Contact
of the Formations of the Austin Chalk Group

Formation	Thickness	Dominant Lithology	Upper Contact
Sprinkle Formation	~ 100m	Massive slightly calcareous smectitic claystone	Disconformity of regional importance
Pfugerville Formation	~ 8m	Soft wackestone with chalky and marly horizons (clay <12-15% - volcanic?), some hardgrounds	Gradational
Burditt Marl	5m	Soft, fissile, and clayey (<15%) wackestone, basal reworked pyroclastic fragments - soft marly slopes	Gradational
Dessau Chalk	28m	Overall, white chalk. Sparse to dense wackestone to mudstone. Medial shell layer of Phrygia aucella (5m) may form boundstone, "nontronitic" pyroclasts in upper 1-3m	Locally disconformable
Jonah Formation	14m	Overall, fragmental limestone. Combination of fossiliferous wackestone, with packstone and grainstone at the top and base, clasts of grainier rocks	Widespread "glauconitic" corrosion zone
Vinson Chalk	~ 31m	Overall, white Chalk. Soft or hard, starkly white, sparse wackestones, storm beds, intraclasts of limestone and inoceramids, Marly interbeds present but relatively thick and widely spaced.	Conformable, may be sharper to the southwest
Atco Formation	~ 33m	Overall, and a chalky limestone, characterized by the interbedding of chalky Wackestones (typically 0.3-1.5m) and fissile marly wackestones (typically 0.05-0.3m). Contains a conspicuous bentonite bed in lower more marly portion of formation (between ~ 6-7 meters on composite section). Longshore scoured (?) channels present in lower beds. Perhaps as a result, bed seem less geographically presistant.	Gradational
South Bosque	~ 5m	Calcareous shale, calcite and montmorillonite	Disconformable, "condensed zone"

The thickness, dominant lithology, and nature of stratigraphic contact between the formations of the Austin Group and the underlying uppermost Eagle Ford (the South Bosque) and the overlying lowermost Taylor (the Sprinkle Formation) in the Austin area, central Texas. (some data modified fromYoung, 1977, 1985).

The Austin Chalk Group has been studied in the Austin area for more than a hundred years. The table below summarizes some of the major contributions and changes to stratigraphic understanding through the development of our current concepts. The resulting, evolving, stratigraphic nomenclature is shown diagrammatically in a figure. A brief summary of the core concepts revealed through time follows (the citations given in the table and figure should be consulted for specific references).

The initial recognition that the white chalk, or chalky limestone, of central Texas was lithologically distinct from other strata of the Cretaceous developed into a deeper understanding that allowed for a subdividing of the Austin into distinct units. Two fundamental criteria aided in this division, they are lithofacies and biofacies distinctions. The salient characteristics which are present in the Austin are these: 1) the Austin is lithologically distinct from the shales of the Eagle Ford below, and the calcareous claystone of the Taylor above. 2) An arenaceous, or fragmental limestone, is present in the center of the Austin. 3) Below the arenaceous layer a lower and upper unit can be distinguished. The lower body is characterized by the interbedding of chalky limestones and thin marl seams, and by the presence of great abundances of "Inoceramus subquadratus". The upper body is characterized by comparatively thick and more classically chalky layers interbedded with fewer and thicker marl seams, and by the conspicuous presence of "Inoceramus undulatoplicatus". 4) Above the arenaceous zone a marly chalk or chalk-marl overlies a chalk with interbedded (but relatively thick and widely separated) marl seams. 5) The upper marl – chalk-marl can be divided into a marl and an overlying marly chalk. 6) The layer directly overlying the arenaceous layer (but below the marls) has several distinctive changes throughout. The lower third is more a limestone than a chalk, the lower ~ 2/3 contains abundant Phrygia aucella (Gryphaea wratheri and Gryphaea aucella = Phrygia aucella), and an upper portion containing Exogyra laeviscula, Exogyra tigrina, and Exogyra ponderosa.

It is these preceding characteristics that were, and are, present in the rocks of the Austin Chalk Group, which awaited discovery and classification. The history of the development of its stratigraphic concepts is one of the description and codification of these criteria, leading to a stable nomenclature. The composite section on the main Virtual Field Trip Page follows the most recent synthesis.

Author	Summary of Important Relevant Contributions
Ferdinand Röemer, 1849, 1852	First crystallization of central Texas geography and geology. Recognized outcrop bands of Tertiary, Mesozoic, Paleozoic, and "Azoic" in area. Described Paleozoic and Mesozoic fossils. Collected Austin age rocks, considered them Turonian in age.
Shumard, 1860	Named "Austin Limestone," considered them to underlie the Comanche Peak (Lower Cretaceous, he did not recognize the Balconies Fault Zone) and included the "bluish" clay of the Taylor and Navarro.
Marcou, 1862	Placed Austin Limestone (with included Taylor and Navarro) at top of Cretaceous section.
Hill, 1887, 1889a, 1890	Inconsistent use of Austin Limestone, Dallas Limestone, Austin-Dallas Limestone, and White Rock Division to describe then uncertain stratigraphic placement of rocks now considered Austin.
Taff, 1892	Post recognition (Cope, 1880) and naming of Balcones Fault (Hill, 1889b), divided the Austin into four units: an unnamed basal unit, an arenaceous horizon (Jonah Formation), an aucella bed (Dessau Chalk with included Phrygia aucella bed, and an upper marly lime zone. This was correctly given as overlain by the chalky marls (of the lower Taylor).
Hill, 1901	Described the Austin in Williamson County
Stephenson, 1918	Cited the occurrence o f 13m of Austin in Travis County
Adkins, 1933	Described the strata and paleontology of the Austin in central Texas and named Taff’s marly lime zone the Burditt. Divided the remainder of the Austin into upper, middle and lower – which roughly correlate to Taff’s divisions in central Texas, but not to the terminology used in Dallas and the Rio Grande Embayment.
Stephenson, 1937	Separated the lower portion (unnamed) of Taff’s lower Austin from an upper Inoceramus undulatoplicatus zone. Included the arenaceous layer in a Gryphaea wratheri zone which included much of Taff’s aucella horizon, the remainder of which was divided into an Exogyra tigrina layer. Divided the Burditt of Adkins into an Ostrea centerensis and an Ostrea travisana zone.
Young and Marks, 1952	Divided the unnamed lower Austin (of Taff, 1892) into a lower portion characterized by the occurrence of Inoceramus subquadratus, and an upper part similarly characterized by Inoceramus undulatoplicatus. The arenaceous zone was named the Texanites internodosus Zone. The aucella horizon was divided into a Gryphea aucella Zone and Exogyra laeviscula Zone.
Durham, 1955, 1957	Informally named the lower formations of the Austin in unpublished dissertation, included a regional correlation of Austin beds. The lower three units (Atco, Vinson, Jonah) are essentially the three lowest Zones of Young and Marks (1952), and the Dessau, the auccella bed of Taff (1982). Divided the Burditt of Adkins (1933) into the Burditt and the Big House.
Murray, 1961	Published cross-sections and discussions using Durham’s terminology.
Young, 1977	Formally named remaining formations of the Austin Group, codifying Durham’s names, but using Pflugerville in place of (the geographically unnamed) Big House.
Adkins, 1933; Stephenson et al., 1942; Young and Marks, 1952; Young, 1963, Pessagno, 1969; Young, 1982; Hazel and Brouwers, 1982; Young, 1985	Correlated the age of Austin formations to those of the European stage names using various macro and microfauna. Table 3 summarizes the relationship of these correlations.

Figure diagramatically illustrating the changing stratigraphic nomenclature through time.

The most significant control on the nature and extent of the outcrop belt of the Austin Chalk Group in Travis County, is its position relative to the Balcones Fault Zone. Both in map view (below)and in cross-section (further down) the bracketing of the Austin by this Zone can clearly be seen. To the northwest, Lower Cretaceous rocks, dominated by the Glen Rose and Edwards, are exposed, while it is the Upper Cretaceous Taylor Group that constrains the southeastern extent of the Austin. The dipping beds of the Austin (~ 1-2° ) do not rise to surficial expression east of the Zone, and appreciable exposures of the soft easily weathered rocks of the Upper Cretaceous, are long since eroded away to the west. Thus, any investigation of sections of the Austin must take into account the significant effect of these faults. The degree to which field relationships such as dip and topographic extrapolations can often be misleading even in correlating short distances has probably been under appreciated at times.

Most outcrops of the Austin do not make up a significant proportion of its total thickness. While it is true that a detailed knowledge of the lithofacies of the Group do typically allow for the recognition of its component formations, often 3-4 meters of white chalk looks quite a lot like any other 3-4 meter section of white chalk. This is the motivation to understand meter scale stratigraphy within the group. A fundamental characteristic of the Austin Chalk (and indeed most chalks) is the regular interbedding of chalk beds with marls and marly chalks. The thickness of marl seams, and of the chalks between them, varies considerably throughout the Group. The stacking pattern of these beds, while far from non-unique and often open to multiple interpretations, can be used to correlate from exposure to exposure. Associating chalk-marl patterns with distinctive fossil occurrences and lithologic characteristics can strengthen these correlations. Although I am typically reticent to extend such correlations over long distances, I am convinced of there utility on a local scale. Use of this technique within the boarder historically developed lithostratigraphic and biostratigraphic framework for the Austin Group, is the key to work in these rocks.

A generalized cross section through the Austin area, Central Texas (modified after Garner and Young, 1976)