Sunday, June 3, 2018

Miocene spring-deposited tufa in southern California Pt 1

In 1981-1982, while doing field work on rocks of middle Miocene age in southern California, I and my students came across some interesting tubular and bulbous-shaped geologic features which are sedimentary rock structures, called spring-tufa deposits, made by waters seaping onto the floors of alkaline lakes. Tufa is composed of calcium carbonate (calcite). 

One of the locales is in the Barstow Formation in the Calico Mountains, west of the “ghost town” of Calico, near the town of Barstow, San Bernardino County, Mojave Desert, California.

Google Earth (2018) image
The tufa deposits at the Calico Mtns. locale occur in at least two beds (8 m apart vertically) in a fluvial-deltaic facies. The tufa deposits are laterally persistent but not continuous. About every 15 to 20 m along strike, there are in situ tubes (columns). In between, the tubes are busted apart because of weathering. All the tufa deposits are within the same general area. 

Outcrop of the Barstow Formation, with tufa deposits indicated by the red arrows. There is horizontal development of tubes in the lower right-hand side of photo, scattered horizontal accumulation of tubes on left-side of photo, and a small cluster of fan-shaped tubes on the middle right-hand side of photo. Jacob's staff is 50 cm length; rock hammer is 32 cm length (12.5 in.).

Close-up of same small cluster of fan-shaped tubes shown in the previous photo (hammer is 32 cm length, 12.5 in). Notice the horizontal orientation of the tubes at the base of the cluster, whereas they bend upward with growth. Also, the tube diameter increases upward.

Close-up of some tubes showing corrugated or fluted surfaces. Coin (USA quarter) is 2.5 cm width.

Close-up of some tubes showing their cross section structure (white part of Jacob's staff is 10 cm length). Some of the tubes are hollow, some one-half filled, and others are filled. Tubes vary in diameter, from pencil size to about 10 cm.

Largest cluster, approximately 1.3 m high.

Cross-section of one of the tubes, which shows wavy growth bands. Ruler is in cm (total of 15) on its left side and inches (total of 6) on its right side. The growth bands might have been influenced by microbial and/or algal processes.

Part 2 concerns another Miocene Formation with similar spring-fed tufa deposits. This formation crops out in the Orocopia Mountains, southern California.

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