Chapter 4: Soft Rock Talk



Sediments originate in three different modes on the solid Earth’s surface: physical, chemical, and biological. ‘Physical or mechanical’ means transport and deposition of debris from disintegration of existing rocks, including those that have been chemically altered during their decomposition, for example, from feldspar to clay minerals. ‘Chemical’ connotes deposition of solutes from solution such as salt and gypsum. And ‘biological’ implies an origin by accumulation of dead animals or plants. After being consolidated, compacted, and cemented, sediments become sedimentary rocks. The natural cement can be clayey, siliceous, or calcareous. Frequently a sedimentary rock can form by a combination of processes. For example, limestone can be a mix of chemical and biological modes, or even three modes altogether.

Sand is defined to have grain diameter between 1/16 and 2 mm. Coarser than this range is the regimen of granule, pebble, cobble, and boulder in order of increasing size; finer grain sizes belong in the domain of silt, clay, and rarely colloid (gel, sol, emulsion). Generally mechanically-derived coarse-grained deposits are laid close to the source of parental rocks while fine-grained sediments have been transported farther away from the source. No clastic sediment (i.e., mechanically originated) has uniform grain-size distribution (size uniformity is known as sorting); it is usually composed of different proportions of grain size ranges. Hence, gravel refers to unconsolidated, non-angular aggregates with grain sizes exceeding the coarse sand. The names of conglomerate, sandstone, siltstone, and shale are defined for consolidated sediments in terms of dominant grain size distribution. Furthermore, clastics (sandstone and shale) are subdivided according to their relative abundances among the trio anchor components: quartz, feldspar, and rock fragments. Visible identification without quantitative analyses can be problematic. Those subdivisions will not be pursued here.

The three processes mentioned above produce orderly layered structures. Chaotic deposits with angular grains or blocks (i.e., no clearly definable layers) can originate from glacier, debris flow, or landslide. Pyroclastic deposits, which originate from volcanic eruption, can be messy, chaotic too. They are distinguishable at outcrop scales but at hand specimen scale, their differences can be subtle to the unaided eye sometimes.

Soft rocks refer informally to sedimentary rocks, or any rocks that are not hard rocks. This chapter encompasses sandstone and mudstone because limestone has been presented in Chapter 2. All my collections were stand-alone pieces in the field. Those from the desert are poorly consolidated and cemented. Carved mainly by wind and water erosion, my desert pieces are fragile. At hand specimen scale, however, those sandstones mimic famous landscapes you may see in the national parks like Arches, Zion, etc. My sandstone specimens represent the destructive phase of landform evolution in the desert while the mudstones connote re-construction from debris in the desert washes. Both exhibit a delicate balance of natural causes.


Figure 4-1: Time markers on sand-peppered gypsum (width 10, height 7, depth 9 cm; from San Bernardino County, CA; purchased).

Figure 4-1

I was thrilled at my first sight of this specimen. It bears time markers for contemporary stratigraphic correlation.

Incorporating sand as impurity, this gypsum piece has grown out of solution into hexagonal columns. During the growth period of these columns, dark matter appeared, because of environmental and supply changes, to form distinct seams (one was traced here with red asterisks). Those dark seams registered contemporary water levels or growth lines across all columns. Recognition of a key marker is critical in stratigraphic correlation, especially in the field, for establishing contemporaries of geologic units that crop out discretely and sparsely.

The stratigraphic markers can be a volcanic ash bed, a bed with key fossils of wide-spread distribution but short species-life spans, a bed with certain trace element contents, a bed of certain isotopic characteristics, or a sequence of magnetic polarity changes. Most of those markers are, however, identifiable only after laboratory analyses of field samples.

The specimen here demonstrates a correlation principle at a local scale but it has little application in the field.


Figure 4-2a: Dolomitic mudstone (25x9x14 cm; from San Bernardino County, CA) formed in a tranquil bay or lake.


Figure 4-2b: Bottom view of Figure 4-2a.


Figure 4-2c: Rear view of Figure 4-2a.

 Figure 4-2a, b, and c

I picked this stone with little expectation of a complicated story to tell until I looked into the details.

This is an interesting piece of mudstone with rich history. On the front face, two cycles of gradational size distribution in the mud are recognized: with grain diameter from less than 1 mm at the bottom to finer particles toward the top of each cycle (grain size is beyond my photo resolution). The size-decreasing trend is also accompanied by fading of color from pink to pale white over each cycle’s thickness of about 2 cm.

The bottom one fifth of the section (top & front view, Figure 4-2a & b) is prominently marked by small, angular rock fragments ranging from less than 1 mm to greater than 5 mm across. Larger fragments appear at the bottom surface, with one reaching 3×2 cm2 on its exposed area (right in bottom view, Figure 4-2b). Some fragments have depressed the mud layering (rear view, Figure 4-2c). Most of the fragments are probably vermiculite (short, platy, mica-like). Note that one cavity (vug) about 5 cm in diameter appears at the bottom. It is partially filled with dirty, tiny, short, platy quartz (? or calcite).

Relics of surficial dissolution are abundant under the bottom surface and on the sides, including pit depressions, tiny spiky nipples, and ridges (hinges). Those dissolution relics coalesce into broader depressions and subdued ridges over the top surface.

Now, let us come up with a story to put various parts together. The cyclic deposition of size-gradating mud indicates that the mud was settled in a quiet depositional environment. The fragments are confined not only to the bottom fifth of the section but also appear randomly or erratically elsewhere. Erratic distribution requires erratic actions. By what means were the mud and fragments transported to the site? Water transport would not be able to bring together the finely gradated mud with angular, poorly sorted fragments. Airborne transport by wind could not account for dropping of large fragments. Underwater slumping would not permit cyclic size-gradation in mud and erratic presence of fragments in the already settled mud layers (rear view).

I envision the deposition site was under a lake or bay with restricted water circulation and limited sediment supply. The mud, as wind-borne dust or water-carried particles, was suspended in the water. Eventually, the dust or particles settled with the coarse ones (< 1 mm in diameter) ‘raining’ down first, followed by finer ones (<< 1 mm) to form a subsection with size gradation. At beginning of each cycle, the bottom water was aerobic as to yield pink pigment by oxidizing its iron constituents. As time progressed, the bottom water became anaerobic and such reducing environ led to the appearance of pale mud of finer particles. Then, minor climate or weather change ignited a new cycle and the change was sufficiently great as to cause more active, wind-driven water circulation, which replenished oxygen in the bottom water to yield pink mud again; later, the circulation slowed and the bottom water became anaerobic to complete the second cycle of deposition with pale, fine mud, as revealed in this specimen.

Rock fragments dropped from rock-carrying ice rafts that had detached from glacier on hillside and floated on the bay water. This hypothesis explains the erratic presence of fragments in the mud, and their angular shape and poor sorting. Also, the dropping accounts for down-warping of layering around and under some fragments. In short, my story is: Fine mud settled in tranquil water with occasional ice rafts that released rock fragments into the mud.

All mud deposits were later uplifted in association with regional tectonic activities. This piece of rock dislodged from its parent mudstone formation and subsequently its surface endured endless wind abrasion and water dissolution in the desert. Later, one big piece of fragment (5 cm in diameter) fell from the bottom and a vug (empty hole) was left behind. Consequently, secondary minerals (quartz or calcite) grew from fluids seeping into the vug, and partially filled it in the form of dirty, thin and short septa.

Finally, spot-dissolution of the exterior completes the story. The surface relief suggests the dissolution was performed by droplets of water. The droplets, derived from either dew or rain, prevail at the bottom face and sides; on the top surface, those droplets coalesce into bigger drops, yielding a surface with subdued relief. The difference in droplet sizes accounts for the disparity in appearance of dissolution relics. The preservation for those relics necessitates the mud be cemented with calcareous or dolomitic fluid to have not only succumbed to weathering but also survived from wind abrasion. I believe the mud is dolomitic because it does not respond effervescently to weak acid test.


Figure 4-3a: Diapir-pierced siltstone (11x15x4 cm) — resulted from liquefaction. Dashed green line marks angular unconformity.
Figure 4-3b: Rear view of Figure 4-3a. The face was polished at 220th grid size and stained with clear oil for better visualization. Green line marks unconformity.
Figure 4-3c: Enlarged bottom section of Figure 4-3a, showing hematite or magnetite pseudomorphs after pyrite.
Figure 4-3d: Enlarged bottom section of Figure4-3b, showing dark crystals that cut layering.

Figure 4-3a, b, c, and d

Students of geology may explore their imaginations about this specimen, which was collected two decades ago from a now-forgotten source locality. It is a rare collection of hand-sized, solidified liquefaction specimen.

An unconsolidated layer of sediments could be mobilized, under overpressure, to creep, arch upward, and eventually puncture through its overlying layers. Petroleum geologists are familiar with salt-doming or diaprism that forms oil and gas reservoirs, as caused by the uprising of low density salt. Here, the density contrast between the silt/clay layers is insufficient to create such potential for diaprism. Instead, engineering geologists, geotechnical engineers, or seismologists may view this diapiric intrusion as a sign of liquefaction of an unconsolidated layer that was subjected to cyclic strong ground motion triggered by earthquakes, to an underwater land slumping, or to oscillation onslaught by tsunami or seiche near coast, bay, or lake. To be hazardous, however, the unconsolidated layer would have to be thicker, around one foot or more, and sandy, as exemplified in quick sand or sand boils. The two conditions, apparently, do not apply to the thin silt and clay layers here.

Anyone, letting sedimentologists or stratigraphers alone, can see the siltstone/shale banding, of which the shades or colors vary, depending on how the surface is prepared: upper left, unaltered field condition; upper right (rear side of the left), sandpaper smoothed and oil coated; lower (enlarged portion of the upper, respective pictures), smoothed and polished at 220 grid size (220th of one inch). Each of the approximately 0.5-cm thick layers consists visibly of many sub-mm laminae. Color contrast reflects subtle change in sediment supplies or depositional environment, which in turn implies seasonal or perhaps climate changes. All layers were arched and broken by the diapric piercing. The extent of arching and breakage intensified upward. However, without age dating, an absolute time relation for the deposition and diaprism cannot be established here.

Noticeable are two unconformity-like boundaries in this hand specimen, as marked by two dashed green curves, respectively. An unconformity signifies an invisible time gap in depositional sequence, typically the top part of the lower layer was eroded away before the new deposits were laid atop the eroded surface. Here, the lower (angular) unconformity demarcates the folded bottom layers from their overlying plane layers; while the upper unconformity seems to have separated the lower, layered deposits from the overlying, massive deposits. The ‘massive’ is here used to emphasize its unrecognizable layering for this hand-sized sedimentary section. This massiveness could be an extrusive product of the diapir.

The last but most intriguing in this stone is its magnetic signature. The specimen responds to a neodymium magnet, stronger at the bottom where magnetite crystals are visible and weaker at the top where magnetite is invisible. Sedimentary rocks can acquire magnetization through three modes. First, the magnetization inherits from tiny detrital magnetite particles that settle slowly and steadily through tranquil, deep water onto the depositional floor. Second, iron-consuming bacteria make magnetite to form the so-called magnetofossil. Both generate magnetization that orientates with the ambient geomagnetic field and thus provides the basis for studies of ancient geomagnetic field and tectonic evolution. The third, magnetization can be attained by reduction of hematite to magnetite in a deeply buried, anaerobic environment; or the magnetite can grow authigenically (i.e., self-made). The post-depositional magnetite contributes spurious noise to magnetization and the noise is routinely removed through magnetic washing (i.e., partial demagnetization) before the true, desired signal of ancient magnetization is utilized. Usually magnetization of sedimentary rocks is too weak to respond to a portable, static magnet. This piece is an exception and there are other exceptions, for example, banded iron-chert ores.

With the unaided eye, one cannot tell the weak magnetization response at the top of the specimen is due to detrital magnetite or magnetofossil. The visible magnetite crystals at the bottom are surely post-depositional because those crystals are euhedral (the cubic corners of a crystal would have been rounded during sediment transport) and some bigger ones protrude across layering. The visible crystals appear only in the green or darker layers, of which the iron content is likely greater. (The weather-induced yellowish brown coating is surficial.) Despite the upward recurrence of dark green layers, the abundance of crystals declines drastically upward, from visible through sparse to invisible. To me, the environment seems to have evolved and become hostile for the bacteria growth. It is not clear whether the crystals nucleate from existing detrital magnetite or magnetofossil. Either way, those big magnetite crystals obliterate the host rock as a candidate for paleomagnetic studies. Note also that magnetite crystals, if present, are not visible in the diapric plume itself. Mineralogists may have something to say about the genesis of visible magnetite. A question, whether the crystallization is pre- or post-diapirism awaits an answer too.

The above narrative could be wrong. In view of the sediment color, it was likely deposited in a reducing environment. As such, pyrite could have been formed first and later oxidized to become hematite, which resumes the crystal form of pyrite (a pseudomorph), instead of magnetite. My opinion based on visual observation only is thus fairly inconclusive.


Figure 4-4: Mud nodules with veins and ‘sweat pimples’ (center, 7 cm across; from Imperial County, CA).

Figure 4-4

Note the crisscrossing veins at the left two pieces and the dark brown pimples on the middle and right pieces. Between the veins and crack walls, there lies a narrow air gap (which is barely observable from the pictures). The gap indicates either the veins shrank or the cracks widened as the veins and the mud nodules were dehydrating.

It is not clear whether the pimples were extruded from the interior or formed externally. I am inclined to believe they are ‘sweat pimples’, meaning: sweated or squeezed from the interior during the nodule’s dehydration contraction.

The cracks in the middle piece are hardly filled, in strong contrast to the piece on the left, of which the cracks are fully filled. (The emptiness in the cracks is not caused by falling off in vein material.) Hence, the veins are late arrivals to infill dehydration cracks. Unlike the occurrence of pimples, the veins are not extruded from the interior. In this scenario, the air gap between veins and crack walls had resulted from dehydration shrinkage of the veins. Alternatively, the gap could be indicative of immiscibility between the vain material and the mud.

But where did the dark-brown vein material originate and how was it segregated and emplaced so neatly along the cracks? Are the pimples and vein made of chert?

Compare those sweat pimples on the mud-nodule to the faux oolitic buds on limestone (Figure 2-9), to ‘peas’ dotted over sandstone in Figure 4-6a, and to oolitic limestone in Figure 5-5.


Figure 4-5a: Sandstone (Right: 28x8x25; left: 29x11x16 cm; from Imperial County, CA) showing preferential, deep-incisive erosion along bedding interfaces.

Figure 4-5b: Sandstone (34x15x10 cm), front and rear faces, showing excavation by erosion across bedding interfaces. 

Figure 4-5c: Sandstones (Left: 30x8x14 cm; right: 27x7x14 cm), showing contrast in erosion outcomes between massive and layered sandstones.

Figure 4-5d: Cross-bedded sandstone (Left: 18x13x14 cm; right: 15x13x8 cm). Cross-bedding builds toward downstream by carrier water or wind. Left-dipping cross-bedding signals the current flows to the left with newer sediments on-lapping leftward over older ones. Pattern changes as the carrier shifts direction.


Figure 4-6a: Sandstone nodule (22 cm long; from the estate sale of a deceased friend).

Figure 4-6a

Nodule is often interchangeably used with concretion. But the two are different. Concretion means in Latin to grow together from deposition around a seeding nucleus; whereas nodule connotes post-depositional replacement. In this sense, the well-known manganese nodules as being accumulated in the ocean floor should instead be named manganese concretions.

Figure 4-6b & c: Clay balls (left: diameter 32 cm, thickness 12 cm; right: 11x7x11 cm; from San Diego County, CA).

After falling from a clay sea-cliff onto beach, chunks of clay are constantly being eroded by water waves. Because the clay is fairly homogeneous and isotropic in physical properties, the chunks tend to be fairly uniformly eroded, particularly for the small ones that can be easily tumbled around by the back-and-forth splashing of sea water. Thus, small chunks are generally rounded to become ball-like and big ones end up more saucer-like. However, only a small fraction of all chunks are destined to balls or saucers; most of the chunks crumble into smaller pieces and disappear under the sea water eventually.

Alternatively and plausibly, the clay saucer or ball could originate from nodules before their dropping from the cliff. After the fall, those nodules are modified by the wave actions, rather than being carved from chunks of debris.


Figure 4-7: Black shale between layered chert (13x10x11 cm; from Los Angeles County, CA).

Figure 4-7

This deck- or mesa-like stone comes from a rocky coast. The thick black shale is sandwiched between two layers of white chert. Like the shale layer, both chert layers are not grainy to the eyes. The dull, grey bottom chert shows visible internal layering while the upper one is much lighter in color with more conspicuous internal layering. Also, the upper one is a little translucent with waxy luster; it is one variety of chert, namely porcellanite.

The black shale has a feeble odor of organic gas (or sea water residue?), when smelled with nose placed against it (even after soaking and scraping it in fresh water for days and letting it stand dry alone for months). Possibly it is a sample of oil source rocks as speculated on account that the sampling site, despite across a fault, is less than 10 miles away from an active oil producing field. If so, the chert layers hint periods of relatively cold climate when silica-bearing organism such as diatom and radiolarian were more productive than carbonate-bearing organism. (Note: carbonate is also more soluble in cold than in warm sea waters.)

The bottom chert layer had cracked, with cracks filled by the younger shale-forming, organic rich mud. The cracks are wider at the top and pinch downward. The largest crack-fill (in the rear side) reaching 0.5 cm wide also contains tiny shale fragments that are darker than the rest of crack-fill.  The upper chert layer does not have cracks that allow shale to ‘intrude’. The cracking and crack filling is how the orientation of the rock on the display is inferred.


Figure 4-8: Barite (left, 12x8x3 cm; from Los Angeles County, CA; right, purchased): Like gypsum (CaSO4.2H2O), barite (BaSO4) can occur in various forms in different depositional environments, including evaporation, but it is anhydrous. The right piece appears in radiated crystal form.

Figure 4-8

The one on the left (sitting on a slice of thulite) comes from southern California beach. It has a texture of frond- or leaf-like branching of ferns. Unlike dendritic texture or dendrites, the branches are slightly raised (about 0.5 mm) above their background. It is like a bas-relief or Basso-relievo in sculpture. A cut face at the base (not shown here) indicates the fern-like branching is not a surficial feature; instead, it ramifies internal, crystal growth pattern. (Dark spots are cavities resulted from boring by acid-secreting gastropods.)


Figure 4-9: A cobble (16 cm high, from Arizona).

Figure 4-9

This well-rounded cobble itself bears well-rounded framework pebbles as cemented by fine-grained brownish red siliceous matrix. Note that the clasts (big constituent grains) do not touch one another. It is paraconglomerate. In contrast, most of the frame-work clasts in orthoconglomerate contact one another. The paraconglomerate represents products in chaotic environment as if clasts ‘float’ in a matrix flow (e.g., debris flow or glacier till deposits); while the orthoconglomerate forms in aqueous currents. Note: analogous or equivalent patterns of gravel-cement relation can be seen in man-made concrete works.

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