Monday, April 23, 2018

Mystery Sandstone Spheres Follow Up (Part 2)


This is a continuation of my immediately preceding post.



Where the mantle is absent, such as bare spots on ridge tops and gullies, especially on their sloping slides, the spheres commonly cover much of the exposed surface. The above view is of the side of deeply incised canyon with abundant exposed spheres, most of which are about 10 mm in diameter. Staff is 2 m in length.



Close-up of spheres from previous photo. Geological Society of America scale, 10 cm intervals on left side of scale.


Freshly broken layer shows no spheres on fresh surface (along right side slab) compared to the older surface with many spheres attached. This photo is very important because it shows that the  spheres form on exposed surfaces and are apparently the result of weathering (i.e., formed after the sediments became rock).



Spheres of various sizes on vertical surface. Within a given layer, the spheres are fairly uniform in size. In incised washes, the spheres occur in multiple layers, which can be interspersed and/or bounded by sphere-free layers. The beds that contain them can be over a meter in thickness and can be completely covered by the spheres on exposed surfaces.


                                              

The spheres are not armored-mud balls (interior is mud and the exterior is coated by an assortment of angular particles of many sizes), which form by rolling along the floor of a desert stream. Armored-mud balls do occur in the same area as the spheres but are comparatively uncommon; one is shown on the right. On the left, is an unusually large and liberated sand sphere with its characteristic smooth surface.

ADDITIONAL OBSERVATIONS:

The spheres are not concretions. Unlike concretions, the spheres have an uniform composition all the way through (see the original post), without concentric shells, and they do not leave concave depressions behind where they were formed. Concretions are hard solid masses that form slowly via chemical changes induced by groundwater percolating through the sediments before they become a sedimentary rock.

The presence of spheres in several road cuts along Interstate 15 (see index map in preceding post) all indicate that the spheres do not require geological time to form. Interstate 15 was completed in 1964, so the road cuts containing the spheres were made sometime just prior to that construction phase. Because the spheres require surface exposure to form, this puts an upper limit on the formation time at these locations to less than 55 or 60 years. They might well require a much shorter time to form than that.  





Tuesday, April 10, 2018

Mystery Sandstone Spheres Follow Up (Part 1)


This is a follow-up of my post on “Mystery Sand Spheres,” issued February 11, 2018. 

David Liggett, who provided the initial reconnaissance information used in the February post, recently made another field trip to the Mojave Desert, California and made in-depth, very revealing observations about these mysterious structures.  The comments given below are from him, and the photos were taken by him.

AREAL DISTRIBUTION: (as currently known)
The green line in this Google Earth image shows the approximate boundary of the area that I covered.  (This was a reconnaissance trip, and I didn’t thoroughly cover all of this area.)  The area west of the red line contains abundant lake beds, and I did not recognize any spheres in association with the lake deposits.  I would expect that the spheres continue to the northeast and northwest beyond what I have shown here.  Because the spheres are in an I-15 road cut I would expect them to continue to the southeast as well.   (I also found an isolated, small occurrence of the spheres in Dunn Wash so they could expand across Dunn Wash to the west also.)  I did not look anywhere south of I-15.



The spheres were found at an elevation range of 510 to 585 meters.  There doesn’t appear to be any genetic relationship to the late Pleistocene Lake Manix shorelines.  Both Miller et al. (2014) and Reheis et al. (2014) (see below) have mapped the green-enclosed area, but there appears to be no mention of anything like the sand spheres in their papers.  However, this is a marginal area, away from their focus of interest: Miller et al. was mainly interested in Fort Irwin to the north, and Reheis et al. was focused on Lake Manix to the south.



Overview of the environment where the spheres occur. The foreground in mostly devoid of the overlying fanglomerate mantle, and the "bare areas" are nearly covered with thousands and thousands of small spheres.




Abundant spheres on vertical surface overlain by beds containing fewer spheres which are, in turn, overlain by rock mantle. Some of the layers contain no spheres.




Sphere-bearing layers commonly form where where the rocky mantle of fanglomerate (pebbles and cobbles of angular igneous an metamorphic rocks) cover is thin, usually on a sloping surface. 



The spheres shown here are mostly ones before they become detached from the host bed. Car key for scale. The spheres are composed of the same material as their host beds; mainly a sandstone with some clay fraction. They are not made of grus per se; probably an arkose. They are weakly to moderately cemented between your fingers. The degree of cementation seems to an important factor in the sphere formation. Not enough cement and the spheres will not form, or if they do forms they won't be able to withstand further weathering; that is to say they will not last long after formation. Too much cementation and the spheres will not separate from the surrounding sandstone.



Spheres in various stages of development on a smooth surface (bedding plane?).

The spheres in the studied area range from 5 to 40 mm in diameter, but most are about 10 mm in diameter. The sphere size is generally uniform within any given layer, but their size can vary widely from one layer to another.

The references cited above are:

Miller, D.M. et al. 2014. Generalized surficial geologic map of the Fort Irwin area, San Bernardo County, California, chap. B of Beusch, D.C., ed., Geology and geophysics applied to groundwater hydrology at Fort Irwin, California. U.S. Geological Survey Open-File Report 2013–2014, 11 pp., scale 1:100,000, http://dx.doi.org/10.3133/ofr20131024B
ISSN 2331-1258 (online)

Reheis, M.C., et al. 2015. Surficial geology and stratigraphy of Pleistocene Lake Manix, San Bernardino County, California: U.S. Geological Survey Scientific Investigations Map 3312, 46 pp., 2 sheets, scale 1:24,000, https://dx.doi.org/1.3133/sim3312.
ISSN 2329-132X (online)

Wednesday, March 28, 2018

Ship Rock in northwest New Mexico

Ship Rock (also spelled, incorrectly, by some authors as "Shiprock") is an erosional remnant (plug) of the throat of an explosive volcanic eruption, which occurred 27 million years ago during the late Oligocene.


Ground view of Ship Rock, which stands approximately 500 m above the surrounding land. The two main wall-like dikes can also be seen in this view.


The peak elevation of Ship Rock is 7,178 feet (2,188 m) above sea level. The main part of the landform rises 1,583 feet (483 m) above the surrounding high desert in northwest New Mexico, a few miles southwest of the town of Shiprock (one word), New Mexico.

All the following images are via Google Earth (2018).


Overview satellite image

Medium-closeup image.

The landform consists of fractured volcanic tuff breccia with subsidiary minette (lamprophyre) dikes. In addition, several wall-like sheets radiate (various distances) away from the central formation. Ship rock is a classic example of a diatreme volcano, or one that formed explosively from gas-charged magma escaping at great velocity.

Very closeup image. Scale is 1 mile.

Ship Rock is one of more than 80 Oligocene to Miocene (about 28 to 19 million years old) volcanoes and intrusive features in the Four Corners (Utah, Colorado, New Mexico, and Arizona) region of the Colorado Plateau.

For a concise, yet detailed summary on the geology of this iconic landform, Google the phrase: "black rocks protruding up." You will be able to obtain a free pdf by Semken (2003).

The landform is sacred to the Navajo and is located on the Navajo Indian Reservation, which is private property.   



Monday, March 12, 2018

Andalusite, a mineral with an unusual feature

Andalusite is a metamorphic rock mineral (aluminum-neosilicate mineral = Al2Si05). Because its formation involves contact metamorphism (i.e., heating of rocks near the intrusion of igneous magma), andalusite is resistant to high heat and can be used in making spark plugs, furnaces, and kilns.


Cross-section of a crystal of andalusite (variety chiastolite);
 33 mm width
.
The variety chiastolite contains dark inclusions of graphite (carbon), which form a very distinctive and sharply delineated cruciform pattern, when shown in cross section. The graphite is pushed aside by crystal growth during metamorphism. Specimens can be of gemstone quality. Chiastolite is also used to make amulets and charms.



Oblique view of same specimen as above; 30 mm height.

Notice how the black band of graphite on the side of the crystal is coincident with one of the rays of the cruciform pattern. Each  of the rays is coincident with a black band. This crystal occurs in a mica schist, which is the most common occurrence of andalusite. 

Sunday, February 25, 2018

The marine gastropod Fusitriton oregonensis: An interesting species today and in the past

Fusitriton oregonensis (Redfield, 1848) belongs to family Ranellidae (the so-called "tritons"). This species lives today most commonly in cool and relatively deep waters from the Bering Sea to northern California. It has been reported in Japan, and it has been reported (mostly as a fossil) in southern California. It is the state seashell of Oregon. The floating larvae of F. oregonensis can last for an extraordinarily long time (up to 4.5 years), and this would explain why it can be found in Japan today. 

This species has a medium-size shell (4 to 5 inches in length) with an overall fusiform shape. Its six convex whorls have 16 to 18 axial ribs nodulated by the crossing of weaker spiral ribs. There is a single parietal tooth near the top of the aperture.


The picture above is an apertural view of a modern specimen 87.7 mm height (3.5 inches) from beach drift at Friday Harbor, San Juan Islands, Washington. If you have ever visited the area, you will known that the ocean water there is cold enough to discourage a normal person, without a thick wetsuit, from swimming in it. The holes you see in the shell are the result of exposure to erosion while the shell was on the beach. At Friday Harbor, this species is intertidal. Southward, it lives in deeper waters (up to several hundred meters).
Abapertural view of same modern specimen.

The fossil record of F. oregonensis is from approximately middle Pliocene (approximately four million years ago) to recent.
The two pictures shown below are of a fossil specimen 55.5 mm height (2.2 inches) of late Pleistocene age (30,000 to 50,000 years old) from a marine terrace at a beach cliff near Santa Barbara. The shells in this marine-terrace deposit lived during the Wisconsin Glacial Stage, which was the fourth and last stage of the great Pleistocene Ice Age. Based on a comparison with modern bathymetric, temperature, and geographic ranges, the shells indicate a maximum water depth of 10 m and a temperature range from 11 to 20 degrees Celsius (cool temperate). This would have been cooler than the sea temperature off Santa Barbara today but similar to that off the northern California coast today. 

Apertural view of a fossil specimen missing its upper part.

Abapertural view of fossil specimen.


Sunday, February 11, 2018

Mystery sand spheres


A fellow geologist recently sent me the following pictures and information about "sand spheres" he found in the general area of the shoreline region of the late Pleistocene Lake Manix, in the Mojave Desert, between Barstow and Baker, southern California. Lake Manix formed by overflow of the Mojave River between 500,000 and 25,000 years ago.


The spheres are all of small size and range from 0.51 to 1.5 cm in diameter (e.g., they are about the size of a U.S. dime). They consist of friable (= fragments come loose from the spheres when rubbed) and angular, coarse-grained material called grus, which results from the granular disintegration (weathering) of granite in an arid climate. The material making up the spheres is slightly cemented by calcium carbonate. 



The above picture is a cross-section of a sliced sphere. Compared to armored mud balls, the "sand spheres" do not have a mud core and are too uniformly of small size. Armored mud balls form when a clump of gooey mud begins to roll around under a flow of water and fragments of rock adhere to the mud surface.



The "sand spheres" shown above, in the background, might have formed in place. The "loose" ones in the foreground are derived from this more concentrated mass of them.




If you "lean in" on this far-away shot, you can make out how numerous the small spheres are (thousands and thousands) among the much larger fragments of angular rock. The "sand spheres" litter the ground sort of like rabbit or deer droppings. 

The "sand spheres" are at an elevation of 563 m, which is slightly above the Pleistocene shoreline, thus they were most likely not formed by a shoreline process. There has not been any lake water at this elevation in 18,000 years. The "sand spheres" look fairly fresh. They are on the surface and although they are not very delicate, thus it is doubtful that they have been around since the Pleistocene. My colleague believes that they might be a product of a local (recent?) downpour. 

Although the nonmarine "sand spheres" of Lake Manix resemble "sand balls" created by small "bubbler" crabs on modern beaches in the tropical ocean waters of the Indo-Pacific, they cannot share the same origin. By the way, if you have the interest you will be amazed and amused by a BBC Blue Planet video (online) that shows how these crabs form the "sand balls."

In closing, we do not know the origin of these Lake Manix spheres and could not find anything in the literature about them. Determining how they formed would be a worthy project. 

Saturday, January 27, 2018

Bursa californica: a fossil and modern-day gastropod

Bursa californica (Hinds, 1843) is a modern-day, shallow-marine gastropod (ocean snail), whose fossil record goes back to the early Pliocene (about 3 million years ago).

Genus Bursa Röding, 1798 belongs to the small family of large sea shells called Bursidae. Their common name is "frog shells" because the intersection of spiral and transverse ribs can result in a strong nodulose pattern of many knobs, producing a "warty" or "frog skin" appearance.

Bursa californica, also known as the "Californian Frog Shell," is characterized by having mostly two protruding ridges (varices) along the left and right margins of the shell. Nevertheless, the warty appearance is not evident on this particular species. 

Bursa californica is found from Monterey, along the central coast of California, to the Gulf of California, mainly in offshore waters. The animal lives mostly on silty-sand bottoms in depths of about 60 to 350 feet. They are active predators and feed on bristle worms (polychaetes), which they anesthetize with acidic saliva. After a storm, some of the shells can wash up on an adjacent beach. The shell is tan-cream in color with a whitish aperture (opening).


Apertural view (front) of a modern specimen
(9 cm height) from Mexico.
Abapertural view (back) of same specimen
Apertural view of a fossil specimen
(8 cm height) from Palos Verdes Sand,
late Pleistocene, Playa del Rey,
 southern California.
Abapertural view of same specimen.




Top view of spire of same specimen.



A growth series (juvenile to adult) of fossi specimens
of Bursa californica from Palos Verdes Peninsula, southern California.
Specimens 2.7, 5 cm, and 8 cm high.