August 26. Castile Formation

The last stage of Permian Time is called the Ochoan Epoch. At that time, as the Delaware Basin portion of the Permian Basin filled with sediments, the fringing reefs grew higher and higher – but ultimately the basin was cut off from open ocean circulation. This may have been caused in part by the rising reefs, blocking channels, but there must have been some sea-level fall as well, probably due to an advance of glaciers in the southern continent. When the basin was essentially completely landlocked, like the modern Caspian Sea, concentrations of dissolved salts increased, finally reaching the point where they precipitated out. The Castile Formation represents the period when these evaporites formed.  

I just said “essentially completely landlocked,” but there must have been ways for sea water to episodically enter the basin, since it had to come in in order to evaporate. So alternating influx of sea water with periods of arid evaporation are the more likely scenarios, rather than simply a big lake that evaporated down to nothing.

The beginning of evaporite deposition coincided closely with the end of reef growth, but we aren’t really sure if one development caused the other – did the salty conditions kill the reef?  Did the reef constrain the basin so evaporites could form? Or if the two events are more or less coincidental. But it was a dramatic change in the environment no matter what the cause.

Thin alternating bands in Castile Formation. US quarter for scale. Photo by Richard Gibson.

Much of the Castile is thin alternating couplets of anhydrite, calcium sulfate, and calcite, calcium carbonate. Anhydrite, whose name means “without water” is chemically the same as gypsum, but gypsum’s crystal structure has two water molecules bonded to the calcium sulfate. The alternating anhydrite-calcite layers are typically only one to two millimeters thick, and they are pretty evident because the anhydrite layers are white and the calcite layers include enough organic matter to make them darker. They extend vertically through the Castile Formation, which has a maximum thickness of 2,000 feet (600 meters), and individual laminations can be correlated as far as 70 miles laterally.

Believe it or not, the tiny laminations have been counted – and there are at least 260,000 anhydrite-calcite cycles. They are thought to be annual cycles, with the anhydrite deposited during hotter, dry summer seasons, and the calcite with organic material representing more humid annual periods. They could represent annual freshening of the water and associated algal blooms (see Peter Scholle's online article).

While the Castile Formation is mostly within the Delaware Basin, the next formation up, the Salado Formation, extends beyond the marginal reef and far onto the shallow shelf. It’s got lots of evaporites in it too, and the Salado is mined in places for potash for fertilizer. The potash occurs as the mineral sylvite – potassium chloride, the potassium variety of sodium chloride, which is common salt.

The last of the Permian deposits in this area, above the Salado, are pretty much terrestrial deposits of river-borne red sands and silts.

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Today’s birthday is Laurence Louis Sloss, born August 26, 1913. Larry Sloss was a professor at Northwestern University, and was a pioneer in the field of sequence stratigraphy, recognizing packages of sediments from small to large and their implications for earth history. He was co-author, with William Krumbein, of one of the most-used college textbooks, Stratigraphy and Sedimentation, used from 1951 when it was published, into the 1970s.

—Richard I. Gibson

Castile Formation

Photo by Richard Gibson