CONGRATULATIONS TO BUDABELI, GRASSHOPPERS&ANT AND SANDPIC FOR THEIR CO-FTF!
Requirements
To earn credit for “finding” this Earth Cache, hike/navigate to Ground Zero along the northern part of the North Crest trail, make some observations, answer the questions, and take one picture as described below. Send your answers and picture to the CO via the geocaching site (but don’t include the picture in your cache log, or it will get deleted; then you can re-submit your log without the picture). If you were to take another picture showing how much “fun” you are having, I can show that to the Cibola National Forest Sandia Mountain Ranger District to thank them for giving permission to “place” this EC here.
Description
Webster and Kues (2006) published an impressive paper describing crinoid fossils found in abundance in Pennsylvanian-age rocks in New Mexico. They identified many new genera and species of crinoids from 26 localities in the state, with the closest investigation site to the Sandias being in Cedro Canyon in the Manzanita Mountains. Owing to the complex paleogeographic setting of New Mexico during the Pennsylvanian – with numerous marine basins separated by uplifts, and the repeated rise and fall of sea level, there was opportunity for an explosive growth of new types of crinoids, and many in New Mexico are endemic (native) to here. The subject of this Earth Cache are the abundant crinoids seen in the Pennsylvanian-age Madera Group limestones in the Sandia Mountains, right along the trails, and in particular the interestingly colored ones that appear to be more common in the northern part of the Sandias.
Do not attempt this cache if you anticipate snow to be on the ground.
Background
FIGURE 1 (next), taken from Wikipedia, shows crinoid anatomy for the type whose fossils are common in the Sandias. Note that crinoids were animals that lived in marine continental shelf environments. Crinoids tend to resemble plants, and consist of a “holdfast” to anchor themselves to a substrate, a long columnal stalk, and a crown of radiating feathery arms that performed filter planktonic feeding. When they died and collapsed onto the sea floor they would probably become easily disarticulated with current reworking and scavenging by other animals. Generally, the most abundant crinoid fossilized remains are calcified/limestone fragments of the columnal stalk. However, it is the aboral cup part of the crown that is used for species identification, and that cup can indeed become fossilized, but they make up a tiny fraction of the population of crinoid fossils found (Webster and Kues, 2006).
FIGURE 2 (next) shows a composite of gray crinoid fossils found in the Madera limestone from various locations in the Sandias, including along the South Crest Trail, the North Crest Trail, the Crest Spur Trail and along a trail in the Placitas side of the mountains. I think most of the Madera crinoid fossils in the Sandias are probably gray, similar to the usual gray of the Madera limestone itself.
Crinoids are invertebrates from the Class Crinoidea and Phylum Echinodermata (Wikipedia), along with starfish and sea urchins, which exhibit 5-fold symmetry in some body parts. But unlike most invertebrates, which have a skeleton outside the body, an echinoderm’s highly porous skeleton is tucked between tissue layers, meaning that it has live tissue in its skeleton, and that tissue can become trapped during fossilization and preservation (C. Gramling, 2007). Note that the carbonate-secreting crinoid’s skeleton contains magnesium-rich calcite held together by ligaments and muscles (Wikipedia). Modern crinoids (sea lilies) are often colorful, and some trace colors have been found preserved in fossilized crinoids as well. Researchers are investigating whether the ancient pigments in the fossils are actually remnants of original organic molecules of the once-live animals (C. Gramling, 2007). The few pictures I’ve seen of these show that the pigments have a faint purplish hue.
FIGURE 3 (next) shows a picture of a rock sample obtained along (or close to) the Strip Mine trail on the Placitas side of the sandias. This rock sample is a hunk of limestone that has been heavily colored with shades of red and ochre. The color is most likely due to coatings and permeation of iron oxide minerals, such as hematite (red) and limonite (ochre). There are many locations along trails in the northern Sandias, within the Madera formation, where the usually gray limestone has been colored various shades of red, orange, yellow and ochre. If you have hiked the Osha Springs and Loop trails to the Lands End Barite mine, you will see a large amount of this color limestone.
According to Dr. Virgil Lueth (2022), large replacements of limestone by chalcedony/jasper (types of silica, or micro-crystalline quartz) are not uncommon near areas of significant barite-fluorite mineralization up and down the Rio Grande rift. The source of these introduced minerals is likely caused by hydrothermal fluids [circulating through faults, fracture, cracks, bedding planes and pores of the limestone] introduced during the rifting activity that raised, for example, the Sandias up above the Rio Grande basin (again, Dr. Lueth). Additionally, red-colored fossils, where the original gray calcareous material was replaced by chalcedony colored by hematite, have been reported in the Madera Limestone (yet again, Dr. Lueth). FIGURE 4 (next) shows a jasper-like color in part of a chert inclusion in limestone (Osha Springs trail area).
The phenomenon by which originally calcareous, or calcite-rich, or magnesium-calcite-rich fossils become replaced by silica (micro-crystalline quartz) while the surrounding matrix, or background, limestone does not become replaced, is a well-observed phenomenon (Malva, R. G. and R. Siever, 1988). This process is complicated, having to do with geochemical and physical processes. What is important is that the composition of the calcite of the fossils has to be sufficiently different from that of the surrounding limestone matrix so as to allow circulating silica-rich fluids to replace the calcite in the fossils and not the limestone matrix (Lueth, V. 2022).
Tool of the Trade
You might like to bring a hand lens, as shown in FIGURE 5 (next), or a light-weight magnifying glass, although this is not strictly necessary.
Hike to Ground Zero
Hike to Ground Zero along the northern part of the North Crest Trail. The optimal parking location is the one at the south end of Tunnel Springs Road, at the northern terminus of the Crest Trail. You can hike up either the North Crest Trail or the shorter (and steeper) Ojo Del Orno Trail (at the top of which you would join the NCT).
Do not disturb any of the fossils, for example by trying to chip them out of the rock. Leave them for everyone to see and enjoy.
Questions
After hiking to Ground Zero, try to answer the following questions to the best of your ability, based on your visual observations and what you have read in the “Description”.
What kind of fossils do you find in abundance at and around GZ? Take a picture of the fossils that includes some sort of unique object of yours that illustrates the scale of them. Send your picture to the CO via the geocaching site email or text functionality, but don’t include it with your log of the find.
Do these fossils include the whole animal? How would you describe the physical characteristics of the fossils?
In what kind of environment did the original animals live, and why are they located at and near the top of the Sandias? The “Description” only briefly gives a hint that would help you answer the latter question. If you need some more background understanding, you could read the material found in other Earth Caches in the Sandias, including “Sandia Crest Earthcache” (GC3KFAF), “The Great Sandia Un-Conformist EarthCache” (GC1YK8V), and“Great Sandia Un-Conformist, down-low” (GC6V0J8).
What color(s) are the fossils? Can you tell whether the coloration is just a coating, whether the color is original preserved pigment left over from the living animal, or whether these fossils have the coloration throughout? How do you think they obtained this color?
References
Aubele, J., L. Crumpler, J. Deal, S. Lucas and P. Stubbe, 2005, Geology of the Sandia Mountains, Chapter 5, Field Guides to the Sandia Mountains, Editors R. Julyan and M. Stuever, University of New Mexico Press.
Connell, S. D., 2008, Geologic Map of the Albuquerque – Rio Rancho metropolitan area and vicinity, Benalillo and Sandoval Counties, New Mexico, Geologic Map 78, New Mexico Bureau of Geology and Mineral Resources.
Fossilera.com, 2022, About Crinoids
Gramling, Carolyn, 2007, The Color of Crinoids, Geotimes
Lueth, Virgil, 2022, Professor Emeritus – Senior Mineralogist/Economic Geologist, New Mexico Bureau of Geology and Mineral Resources, personal communication.
Maliva, R. G. and R. Siever, 1988, Mechanism and Controls of Silicification of Fossils in Limestones, The Journal of Geology, 96, 4, 387-398.
Webster, G. D. and B. S. Kues, 2006, Pennsylvanian Crinoids of New Mexico, New Mexico Geology, 28, 1 pp. 3-36.
Wikipedia, The Free Encyclopedia.