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.   



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.