By Don Vaughn October 3, 2009
It is one of nature’s convenient ironies that as we appreciate the variety and beauty of the plants and animals that inhabit our tidepools, the pools themselves and the rocks that surround them record the lives of similar creatures that preceded them—and us—a long long time ago. While fossils of the actual organisms that lived in the sediments that formed the rocks around the tidepools can be found, the record of their daily lives and pursuits (what they did for a living) is far more common. More common than shells or carapaces, bones or skulls are their “trace fossils”; also referred to as “shadow fossils, lebenspuren or ichnofossils” (as if calling something by their German and Latin names makes anything easier). So, we’ll stick to calling them trace fossils!
Trace fossils are typically burrows, borings and footprints, trails or tracks, roots or even the fecal remnants of the organisms. These can also be called “biogenic sedimentary structures.” These fossils have distinctive forms and shapes. They have been studied by geologists, biologists and paleontologists as indicators of the ancient environments in which the organisms lived, what are referred to as “the paleoenvironment’’. Like other fossils they have been given formal scientific names and classified by type.
There are several readily identifiable trace fossil types visible in the rocks in the tidepool area; but first let’s get an overview of the rocks around the tidepools.
There are four rock units that form this end of the Point Loma peninsula but generally, there are two basic rock units, or formations, at the tidepools: the Upper Cretaceous (71- to 83-million-year-old) Point Loma Formation and the Pleistocene (150,000-year-old) Bay Point Formation. The contact between these units is what’s called an “angular unconformity”, as shown here. Geologists can interpret a lot of things from an angular unconformity: the deposition of the 75-million-year-old Point Loma Formation sediments, the uplift and tilt of the rocks they became, the erosion of whatever was deposited over these rocks, the planation (leveling) by wave action of the rocks and the deposition of the much younger (Pleistocene) Bay Point Formation (the unconformable part). So there’s a lot—a whole bunch—of time represented by this configuration, this “unconformity”. The tidepools are actually in the Point Loma Formation and we’ll concentrate on some trace fossils visible in these rocks.
The Point Loma Formation consists of interlayered rocks: sandstone and shale, about 80 meters worth, with the lower 40 meters characterized by the interlayering of the sandstone and shale; and the upper part, which is mostly shale. Sandstone is derived from, ta da, sand! Shale is derived from finer grained materials called silt and clay, or mud. And their differences are readily apparent.
The sandstone appears rough, blocky and red or orange. The shale layers appear thin, gray and wavy. These rocks were originally soft sediments deposited into the ocean via a river.
The coast of San Diego was about 9 kilometers farther east than today and so this picture is very representative of what we would have had back then: a river carrying sediments to the sea, an alluvial fan, littoral drift of the sand sediments into a submarine canyon through the continental shelf and deeper undersea fans of sediment accumulation. The sediments flowed downhill in different fashions forming the large undersea fan, but came to rest in this great pile that now is represented by the exposed layers of rock. These materials were deposited below sea level in what is called the “bathyal” environment (depths of 200 to 2000 meters) and at shallower depths. If you look closely you can actually see the thickness of the sandstone layers (referred to as “beds”) varying within a relatively short distance because the sediment, in this case sand, flowed outward from a channel and came to rest. The sand is thicker where it flowed from and thinner away from that point, actually pinching out.
The trace fossils are best observed in what’s called “plan view”; that is, looking directly down onto what is called the “bedding plane”, perpendicular to the sand and shale beds. Some trace fossils can be seen on the edge or “section” of the bedding planes, but what is really impressive is looking down on a whole slab full of these features.
There are two main fossils that are quite abundant in the tidal zone based on my recent experience. You can find them easily today and remember them forever! They are called “Ophiomorpha” and “Thalassinoides”.
They were most likely made by the same animal: a decapod crustacean, (a shrimp or lobster). Why do they have two names if they were made by the same animal? Well, if you think about it, when you walk on the beach you leave different kinds of marks, don’t you? Footprints and, let’s say if you sit down, you also leave a butt print. These marks are obviously different. So, we’ll call one a “footprint” and one a “butt print”. With Ophiomorpha and Thalassinoides we have an animal that burrowed through both mud and sand and the burrows have different features. For one, Ophiomorpha is a sand-filled burrow in the mud (shale) lined with fecal pellets. Thalassinoides is a mud-filled burrow in the sand. So they have different names! Frequently they connect and there’s a nice diagram of this relationship in one of the references I have. These fossils are indicative of shallow, littoral (meaning near the shoreline) and shallow sublittoral environments.
Here’s a nice diagram of the typical seabed fauna from the Jurassic/Cretaceous time period. We can see the shrimp burrow on the right. There is also another burrow here that we find as a trace fossil now called “Chondrites”, made by a worm. There is also another trace fossil here that hasn’t been studied very well and is still called an unidentified fossil of a mud-filled burrow, darker mud fillings in lighter mud, believed to be packed with fecal pellets.
I have two references prepared by the paleontologists who actually studied the trace fossils on Point Loma. These include the definitive study of the trace fossils in this monument. You can actually use it as a field guide because one of the pages has a number of drawings of the various trace fossil types. I throw one in my TPERP backpack. See Don if you would like a copy. All in all, the article indicates there are eight identified and three unidentified fossils in the interbedded sandstone and shale sequence, comprising the “trace fossil assemblage” of the Point Loma Formation.
The other handout I have addresses the role of marine rock burrowers in the erosion of submarine rocks. The article was written from research on the local Scripps Submarine Canyon off of La Jolla. There’s a brilliant photograph, x-ray and drawing of a rock just chock full of rock borings made by a wide variety of animals. The article concludes that “In many localities erosion by animals is more important than erosion by physical and chemical processes.” So, we can see there is tremendous impact that animals can have on the rocks in the tidal and submarine environments, especially given lots of time. So all those little rock-hugging, boring, burrowing, skittering critters we know and love and see during low tide, and their deeper cousins, have a tremendous role in the environment and effect on the sediments and rocks in which they live. And they leave the record of their lives behind for us to see.
Animals, plants, waves, rocks, and…time! They’re all there for us to find!
Last revised 17-Aug-13