Cross bedding in beach sediments—then and now

I am always fascinated by pictures of the past that are engraved in rocks.

Whether it is an ancient inscription, a fossil, or even a tombstone, rocks are nature's photo album. As such, they can provide a faithful and direct link to some rather eerie moments in Earth history: a battle report, a last breath, a last word. Although we are oft at the mercy of geologic processes with regard to the "photo's" content, timing, and preservation (most are destroyed), each picture offers an invaluable resource in our futile—but well-intentioned and captivating—attempt to reconstruct a 4.5 billion-year tale.

Last week, I came across one such inscription. On a moderately populated beach in Santa Barbara, California, recent beach deposits have been preserved through a peculiar substitute for cementation: the tarry remnants of seeping oil. The photo on the right captures a mound of beach sand that has been cut by wave erosion and now stands above high tide. The grain composition is dominantly quartz, with a minor abundance of mica, so the sedimentary unit's dark color results from viscous oil that has found its way through the pore spaces to the surface.

Natural asphalt

The source of the oil is diatomaceous marl of the Miocene Monterey Formation, which was deposited between 17.5 and 6 million years ago. Such rocks form as a mixture of clay particles and diatom frustules accumulates in deep marine settings (typically at 200–2000 meters depth). Organic material—mostly the remains of surface-water algae and bottom-dwelling bacteria—is captured by adsorption onto the clay particles. Since the water was oxygen-depleted at depth, sufficient organic matter was preserved to make the sediments 'oil-prone'. More recently in Earth history, the sediments were buried deeply enough that ambient temperatures promoted the conversion of solid, residual organic matter (kerogen) to liquid hydrocarbons (bitumen, or what is commonly known as oil).

Because oil is less dense than water, which saturates porous rocks in the subsurface, buoyant force causes it to migrate toward the surface unless trapped by an impermeable barrier. The migration of oil is by no means a rapid process, taking years to thousands of years to move only a few meters (e.g. Bekele et al., 2002). In southern California, the oil is particularly viscous, thus natural oil seeps exist along the beaches of Santa Barbara even today.

"This far you may come and no farther; here is where your proud waves halt"


The most obvious characteristic of the sand pictured above is the prominent bedding (seen now as a linear pattern on the surface). Most of the bedding is planar (near horizontal), but a ~20 cm-thick bed of cross-bedded sand is evident near the middle of the photo. Planar bedding develops as wave after wave of ocean water distributes river-derived sand grains across the beach (aggradational stacking). Epsilon cross bedding (angled bedding with a 'curve' to it) and low-angle cross-stratification develop as channels and dunes migrate across the beach surface (progradational stacking). Both processes are typical in a nearshore environment, and so both features are diagnostic of nearshore environments in the rock record.

A link to the past

I imagine that many thousands of people have passed the outcrop above, but relatively few have considered its significance in historical geology. The bedding patterns stuck out to me in particular because I had seen them before. This was the first time, however, that I had witnessed such sedimentary structures forming in their 'native' environment. Below is a picture from my field experience in western Utah a couple years ago. Before I divulge any details, consider the similarities and differences between the outcrop in (A) Utah and (B) California.

First, the outcrop in Utah is well cemented (indurated) and appears as 'solid rock', whereas the beach outcrop is unconsolidated sand, only held in place by tar. Second—and not apparent from the photo—the rocks from Utah are comprised of carbonate (calcite) sand rather than quartz. Carbonate sand is more common in tropical, passive margin settings (like Florida, or the Bahamas). Related to this difference is the presence of coarse (pebble/cobble) clasts in (B), which is merely a function of the tectonic setting. The California coast is an active tectonic margin, in which river channels carry large clasts from mountainous terrane near the coast. Furthermore, California waves are currently forming an angular unconformity, since near-vertical rocks outcrop along much of the coast. The weathered remnants of those rocks are commonly preserved within the settling sand.

Despite these several differences between each outcrop, the genetic relationship is obvious. Sedimentary structures within both are most parsimoniously interpreted as having resulted from aggradational and progradational stacking of sand-sized sediments in a nearshore environment. In other words, both outcrops are perfect snapshots of historical beaches in the respective regions.

Deep time

The phrase "deep time" refers to the concept within geology that Earth history is incomprehensibly longer than human history. I would suggest, however, that when properly understood, the rock record is an effective tool in bringing this concept down to our level, just as a telescope peering into space captures comparably distant moments in the life of ours and other galaxies.

To wrap up the analogy I've set forth, consider that the rocks in Utah (outcrop A above) are part of the Cambrian Orr Formation, which was deposited some 500–493 million years ago. At that time, western Utah and Nevada actually marked the northern coast of what is now North America (that is to say, the continent has since rotated ~90° counter-clockwise). The region was also situated in the tropical zone, very near the equator, along a slowly subsiding passive margin, like the modern-day East Coast. Together, these facts account for the compositional differences between the outcrops noted above. But when set side by side with a recently formed analog, the time gap effectively vanishes.

And such is the allure of geology.

References

Bekele, E.B., Person, M.A., Rostron, B.J., Barnes, R., 2002, Modeling secondary oil migration with core-scale data: Viking Formation, Alberta basin: American Association of Petroleum Geologists Bulletin, v. 86, p. 55–74.