Monday, June 18, 2018

Miocene spring-deposited tufa in southern California Pt 2

Part 1 concerned an occurrence of Miocene spring-deposited tufa in the Barstow Formation, Calico Mountains, southern California. Part 2 is about another Miocene occurrence of spring-deposited tufa, 118 miles to the southeast of the Barstow locality, in the Diligencia Formation in the Orocopia Mountains, northeast of Salton Sea, Riverside County, California.

Google Earth (2018) image,

The Diligencia Formation spring-tufa deposits resemble fossil "logs," as shown above. These "logs" occur mainly as branching-horizontal tubes, in contrast to the mostly vertical buildups of the Barstow Formation spring-tufa deposits. The Diligencia tufa deposits are confined to a single thin bed that crops out for a least 6 km. This bed represents a lake-shoreline environment, which interfingers with basalt flows and river-delta deposits. The hammer, which is the same one shown in all the subsequent images, is 27 cm in length.

The Diligencia tufa tubes, are commonly parallel to bedding, can be up to 65 cm in length and 25 cm in diameter.  Hammer is 27 cm length.

This is a horizontal tube, which has been extensively weathered.

In some places, the Diligencia tufa deposits are transported accumulations of closely packed tubules concentrated into a thin bed of limestone.

In other places, the Diligencia tufa deposits occur as scattered, weathered remains.



All the Diligencia tufa deposits consist predominantly of wavy microcrystalline calcite laminations surrounding by small core that is filled with darker sediment and/or partly with coarser grained calcite. The "dime" coin is 18 mm in diameter.

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.



Sunday, May 20, 2018

Coastal-sabkha strand line near San Felipe, Baja California Pt 2

This post is the second part of the types of organisms found (Aug. 1976) on the extreme western limit of a coastal sabkha along the west side of the northern Gulf of California, in Baja California, Mexico. Part 1 concerned the location of the sabkha, as well as images of mostly terrestrial lifeforms and swimming crabs. Part 2 concerns the mollusks found in a 20-m wide band of drift material restricted to the strand line.



Bivalves were extremely common and consisted only of single valves, many of which were unbroken. Some of the valves occurred in localized concentrations, whereas others were broken and scattered across the mud-cracked surface. Isolated large fragments of wood (as shown above) were also present.




Broken bivalves adjacent to mud cracks.




The infaunal (burrowing) bivalve Mulinia modesta Carpenter,  shown above, was formerly referred to as Mulinia coloradoensis Dall. Before the dams across the Colorado River were built, this endemic (found only in this region) bivalve was  the most abundant mollusk living on the Colorado Delta. Valves of this species occur in huge numbers in about 10 to 13 km to the east, where they have been subsequently washed out of the tidal flats and now form shelly beaches, beach ridges (= cheniers), and shoals of the delta. At the strand line, there are twice as many right-hand valves and there are left-hand valves. This is evidence of selective sorting by waves. The valves found at the strand line were missing also the outermost perisostracum layer (a horny protective layer) and were bleached.


Chionista californiensis? (Broderip) is a common infaunal clam living intertidally on mudflats, as well as offshore.



              
Nassarius howardae? Chace was a moderately common gastropod in the strand-line assemblage. Nassarius gastropods are active scavengers, which burrow horizontally just below the surface of the bottom. Nassarius gastropods are commonly tidal-flat dwellers. The specimens found at the strand line were slightly worn, and the tips of the shells were commonly missing.


Phalium (Semicassis) centiquadrata (Valenciennes) is a gastropod known to live in the sand of very shallow-marine waters. This species was rare (only three found) in the strand-line assemblage. The shells were the following features: protoconch (earliest part of the tip), nodes on the shoulder of the last whorl, outer lip, and some of the more delicate features along the inner side of the aperture (opening of the shell). In general, these shells were considerably worn, and the shell was much thinner than normal.


Ficus ventricosa (Sowerby) is a shallow-subtidal (offshore) gastropod. This species was very rare (only one  found) in the strand-line assemblage. The protoconch was present, but the outer lip is broken


Concerning the taphonomy (what happens after a lifeform dies), the mudflat-dwelling Mulinia and Chione shells, and probably the Nassarius shells,  were likely transported reworked a few km and concentrated by the tidal currents and storm waves at the strand line. The nearshore-marine shells of Phalium (Semicassiscentiquadrata and Ficus ventricosa were likely transported as much as 10 to 13 km distance by a tidal bore and/or storm waves, which moved across the low-relief sabkha surface. The absence of a barrier-island system in this area allows for unrestricted flooding, which enhances currents capable of transport the remains of marine organisms.  

Given enough burial and time, strand-line remains will become part of the sequence of coastal tidal-flat sediments consisting of muddy siltstone and claystone, with lenticular beds of gypsum and halite, as well as interfingering lobes of alluvial debris. This sequence has all the characteristics of an arid inland basin, except for the intertidal and marine shells. Normally, one would not expect marine shells to be transported 10 to 13 km (6 to 8 mi.) and to be in such good shape, but the evidence shown above proves otherwise.


Mixed assemblages of marine and terrestrial organisms are important in distinguishing between ancient examples of coastal sabkhas and continental sabkhas.

Monday, May 7, 2018

Coastal-sabkha strand line near San Felipe, Baja California Pt 1



On August 8, 1976, while on a field trip to the San Felipe area, northern Baja California, I and my colleagues spent a day looking at the biology of a strand-line (shoreline) assemblage at the westernmost limit of a large coastal sabkha (uppermost super tidal-flat environment). This assemblage, which is shown below, was collected at a locality 3 km (2 mi.) east of Mexico Highway 5 and about 60 km north of San Felipe.

The locality is on the west side of the Colorado River Delta. The delta has undergone significant environmental change since the building of dams across the river and subsequent diversion of water. The influx of freshwater into the delta region was greatly reduced and, therefore, there has been increased salinity in the region.



Location of the sabkha (white area) and locality area, plotted on a Google Earth (2018)-generated image, is shown above. Spring tides (new and full Moon phases) in the northern Gulf of California (present day) are known to be some of the highest in the world (and most dangerous). They can have a spring-tidal range of up to 10 m, with an accompanying tidal-bore front as much as 1.5 to 3 m high in an almost vertical wall of water moving at about 2 knots.




View due east (toward the ocean) of the salt-incrusted mudflat of the sabkha just east of the strand-line locality. The open waters of the Gulf of California were several kilometers (10 to 13 km) away, but standing water with salt covering the water occurred about 300 m east of the strand line. This low-lying sabkha usually floods in the summer during very high spring tides. The grooves are salt-encrusted tire tracks, made by off-road enthusiasts driving across the very muddy surface. This would have been a highly risky undertaking because of the high likelihood of getting stuck. 

Closeup of the moist, muddy ground in the immediate area of the strand-line locality. Pen is 13 cm length.



View of the August 8, 1976 strand-line locality. The week before (July 25–28) I visited the site, there was a spring tide (new moon), associated with high tides.
 


Remains of fauna and flora found along the strand-line zone (about 20 m wide) include scattered pieces of wood, gourds, beetles, bird eggs, a few nearly complete birds with feathers intact (not shown above), bones of the brown pelican [Pelecanus occidentals californicus, not shown here], land-mammal leg bones and jawbones, algal scum, whole fish (up to 20 cm long), whole crabs, single and broken valves of bivalves, gastropod shells, and also some glass floats used by commercial fishermen. The scale is 25 cm long.  The fauna and flora represent a mixture of some terrestrial life and abundant marine life, all confined to the 20-m wide zone. Immediately east of the strand line, the salty mudflat was essentially barren of lifeforms.

To the south of where the remains were found, a thin lobe of alluvium extended into the strand-line zone. The lobe was related to a flash flood, which transported small pebbles to large cobbles of igneous rocks (including pebbles of pumice), large wood fragments (see image below), and beetles into the strand-line zone. West of the strand line there was sparse chaparral vegetation (e.g., some ocotillo plants).

The following images show closeups of some of the strand-line biota. 
gourd with seeds inside (scale is 5 cm)


bird egg (30 mm length)


Cryptoglossa verrucosa ground beetles. They were young adults to adults and complete with legs, mandibles, and antennae. Their soft parts had been mummified.



Mugil cephalus, a mullet fish (encrusted by salt residue). This fish occurs in coastal estuaries and lagoons throughout the region. Specimens found at the strand line were mostly complete and well preserved, with skin and fins present. The specimens ranged from tiny juveniles to large adults. 

Callinectes bellicosus, a "swimming crab" (largest specimen approximately 12 cm wide). This crab lives in mudflat channels and nearshore areas. The specimens found at the strand line showed excellent preservation: Most  are complete with legs, chelipeds (claws), numerous sharp spines along the edge of the carapace, and both young adult and adult specimens were found. None showed any signs of abrasion. Some were bleached. Locally, concentrations of only the chelipeds were present.


Large glass floats (each one about 16 cm diameter), used by commercial fishermen for their fishing nets.

PART 2 OF THIS POST = THE NEXT POST

note: All the pictures of the remains were taken 42 years ago, as Kodachrome slides, which I scanned and converted into digital images. 

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 a 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 a road cut along Interstate 15 (see index map in preceding post) indicates that the spheres do not require geological time to form. Interstate 15 was completed in 1964, so the road cut containing the spheres was 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 the road cut 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.