Standing in my brother’s southern Utah yard this sunny springtime afternoon, I got to looking around at his desert landscaping and started thinking (always a scary endeavor). If I had been in this exact location around 260 million years ago, I would have been looking at the great-great-great-great-great ancestors of these landscaping rocks. Most likely I would have enjoyed using a snorkel, mask, and flippers at the time.
The rocks are Permian limestones, laid down in warm, shallow tropical waters when Utah was about ten degrees north of the equator and the Kaibab Sea covered much of the western edge of ancient Pangaea. Over tens of millions of years, the future Utah would either have been submerged under a mere few meters of water or been barely above sea level.
I honestly do not know for sure if these blocks were quarried from the sedimentary rocks of the older Toroweap Formation or the overlying (and younger) Kaibab Formation, but I expect that the quarries are local. These two formations are quite prevalent in southwestern Utah near St. George, forming the massive Hurricane Cliffs along interstate 15 from near Cedar City south to Anderson Junction. Large tracts of Washington County countryside are Permian limestones, and the rocks can be followed further south into the extreme northwest corner of Arizona, again forming massive cliffs in the northern reaches of the Virgin River Gorge.
Chert nodules in Kaibab limestone
Chert is an interesting rock, and to me these chert nodules are spectacular. Chert is a very fine-grained sedimentary rock with a tough, splintery, conchoidal fracture and consists predominantly of interlocking crystals of quartz. It occurs mainly as nodules or concretions in limestones (CaCO3) and dolomites (CaMg(CO3)2) and less commonly as layered deposits or “bedded” chert.
Silica replacing limestone in nodule
But how does the quartz chert get into the limestone and dolomite, anyway? As I understand things, at some point in the distant past, groundwater containing dissolved silica (SiO2) moved through fractures in the limestone. It might have only taken one miniscule grain of silica to precipitate out of the water to form the “seed” or core of the nodule. Over time the nodule grew from that seed of silica. Another explanation is that the silica replaced calcium carbonate or CaCO3 as once again silica-rich fluids moved through the fractured rocks of the ancient Kaibab Sea.
Calcite is the common rock-forming mineral of calcium carbonate (or limestone). A drop or two from a small bottle of dilute hydrochloric acid comes in handy for checking the “fizz” of calcite and also limestone in general.
These limestones happen to be fairly fossil-rich, , too. Bryozoans, brachiopods, crinoids, corals, and sea urchins are commonly found. Bryozoans were (and still are) colonial animals which secreted a hard, limy skeleton. They are not the same as corals and are not plants.
These fossils in the chert nodule look like they might be bryozoans.
Brachiopods (below the penny) have been around for 500 million years. Many of the species became extinct at the end of the Permian Period (around 350( oops!) 250 million years ago) but others still thrive today. They are solitary marine invertebrates and have distinctive bilaterally symmetrical shells.
Brachiopod (below penny)
A few other items also caught my eye as I wandered about in this southern Utah yard… I could not identify this lizard, but wonder if that protrusion is its ear (between its eye and foreleg). Plus – check out those feet!