Monday, November 25, 2013

Back to the western Caribbean, Pt. 1

Its been quite a while since the last post here at Caribbean Paleobiology. Lots of traveling and working hard on publishing parts of my dissertation as well as my current research projects here in Panama (stay tuned for more on this next year) have kept me extremely busy.

As I have mentioned previously, my work during this postdoc requires that I lead a group of interns (you can learn more about the internship here) as we search for terrestrial vertebrates in early Miocene deposits exposed on the Pacific side of the Panama Canal (see previous post). However, every now and then, as you may have seen in previous posts (here and here), we get to go to the Caribbean side of Panama in search of late Miocene marine vertebrates in the Chagres Formation. This is the first part of a series about our recent efforts to collect fossil marine vertebrates and to better understand the geology of the Chagres Fm.
Map of the northern part of the Panama Canal Basin. Here you can see the extension of the Chagres Formation and its members (map from Collins et al., 1996). (Click on the image to see a larger version.)

The Chagres Formation
This formation, exposed on the northern part of the Panama Canal Basin (see map above), generally consist of three distinct members or facies: Toro Member, silty sandstone facies, and Rio Indio facies (Collins et al., 1996). Age estimates for the deposition of the Chagres have been made using Foraminifera (which are extremely good index fossils). As a result, we known that the formation was deposited between 8.6-5.6 million years ago (Collins et al., 1996), during the final part of a geologic period known as the Miocene.
Toro Point, located southwest of the Caribbean exit of the Panama Canal, located within Ft. Sherman, which is a former US military base.
The Toro Member is the basal unit of the formation, and consist of cross-bedded coquinas and medium to coarse sands (see picture below). Coquina, is a term used to describe a sedimentary deposit that consists mostly, if not entirely, of shell fragments. In the case of the Toro Member, it is made up almost entirely of echinoid (sea urchin) and barnacle fragments, together with other less common bivalves and gastropods. Both, the types of invertebrates that make up the coquina, as well as the cross-bedding is indicative of high-energy, shallow marine habitats (Hendy, 2013)
Cross beds of the Toro Member, as exposed in Toro Point.
Disregarding what the geology and macroinvertebrates suggest, the Toro Member has been interpreted as being deposited at much deeper depths (several hundreds of meters), by a high-energy stream flowing from the Pacific Ocean towards the Caribbean sea, and thus representing the final connection between these two oceans (Collins et al., 1996). This interpretation, is based on the occurrence of deep-water fossils of Pacific affinities within the silty sandstone facies.
The silty sandstone facies of the Chagres Formation, as exposed in Playa Tortuguilla, located northeast of Fuerte San Lorenzo, and the mouth of the Chagres River. From left to right: James, Sarah, Elena and Zach (Fall 2013 interns) are studying the trace fossils of the Chagres. 
About 4-5 kilometers southwest of Toro Point, is where the main part of the Chagres Formation, the silty sandstone facies, is exposed. This is the most extensive of the members, going from southwest of Toro Point to about ~6 kilometers southwest of the village of Piña. Foraminifera (or forams for short) are not only used for estimating when marine units were deposited, but can also serve as index fossils for depositional depth, and environment. Forams collected from the silty sandstone facies of the Chagres were used for estimating the depth of this part of the formation, resulting in an estimate of somewhere between 200-500 meters (Collins et al., 1996).
Outcrop of the silty sandstone facies of the Chagres Formation near the village of Piña.
Forams are not the only fossils known from these units. Fierstine (1978) described a fossil marlin which he dubbed Makaira panamensis, an extinct species only known from this place and time. Other fossils found in these facies are fish otoliths. Otoliths are fish ear bones; they can be identified fairly accurately, and, similar to forams, they can be used as index fossils. A preliminary study of the otolith fauna of the silty sandstone facies collected near the village of Piña, suggests that these facies were deposited somewhere between 100-700 meters (De Gracia et al., 2012). This is a broader estimate, than that obtained using the forams, but still consistent with the idea that the silty sandstone facies represent relatively deep marine environments. These units, have so far, proven to be the most productive in terms of vertebrate fossils, thus most of our efforts have been in this area. In fact, back in 2011, I was near Piña, collecting a fossil dolphin* as part of the Pyenson Lab, and, more recently, with the Spring 2013 interns we collected a fossil sperm whale and parts of a marlin skull. (More about even more recent discoveries in the following iterations of this series).
*You can see more of the fossil dolphin collected during the Pyenson Lab 2011 expedition here!
Outcrop of the Río Indio facies of the Chagres Formation, somewhere south of La Boca del Indio. 
Towards the southwest, along the opposite site of the basin, between Palmas Bellas and Rio Gobea, is where we find the Rio Indio facies. These facies are characterized by siltstones and sandstones (Collins et al., 1996). Estimates of the depositional environment of these facies are variable, but generally much shallower than those of the silty sandstone facies (see below). Based on forams, it ranged from 50-80 meters, whereas estimates based on fish otoliths (= fish ear bones) it ranges from 0-100 meters (Collins, 1996; Collins et al., 1996; Aguilera and Aguilera, 1999). Just today was our first time visiting some of the Río Indio localities, and although we didn't find any vertebrates, we did find mollusks which are consistent with the shallower depth interpretations of previous workers.

So, stay tuned for the upcoming installments of this series!

Literature Cited

Aguilera, O., and D. R. de Aguilera. 1999. Bathymetric distribution of Miocene to Pleistocene Caribbean teleostean fishes from the coast of Panama and Costa Rica. Bulletins of American Paleontology 357:251-270.

Collins, L. S. 1996. Environmental changes in Caribbean shallow waters relative to the closing Tropical American Seaway; pp. 130-167, in J. B. Jackson, A. Budd, and A. Coates (eds.), Evolution and Environment in Tropical America. University of Chicago Press, Chicago, Illinois.

Collins, L. S., A. G. Coates, W. A. Berggren, M.-P. Aubry, and J. Zhang. 1996. The late Miocene Panama isthmian strait. Geology 24:687-690.

De Gracia, C., J. Carrillo-Briceño, W. Schwarzhans, and C. Jaramillo. 2012. An exceptional marine fossil fish assemblage reveals a highly productive deep-water environment in the Central American Seaway during the late Miocene. Geological Society of America Abstracts with Programs 44:164.

Fierstine, H. L. 1978. A new marlin, Makaira panamensis, from the late Miocene of Panama. Copeia 1978:1-11.

Hendy, A. J. W. 2013. Spatial and stratigraphic variation of marine paleoenvironments in the middle-upper Miocene Gatun Formation, Isthmus of Panama. Palaios 28:210-227.

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