San Clemente de Térapa, Sonora, Mexico

An International, Multidisciplinary Project

Marci Hollenshead (QSP '02) with kids.

The Opata village of San Clemente de Térapa is located immediately south of the town of Moctezuma, in northeastern Sonora, México. Residents call themselves the ‘Heves’ or ‘Tegüimas,’ but the Spaniards picked the Pima word for them which was altered to ‘Opata.’ The village of San Clemente de Térapa is located on an ancient river terrace overlooking the Río de Moctezuma.

Known vertebrate and invertebrate faunas of the Rancholabrean North American Land Mammal Age (late Neogene [Quaternary], latest Pleistocene, the most recent approximately 200,000 years) are exceedingly rare in the northern México state of Sonora. In fact, only 17 Quaternary mammalian localities are reported for Sonora, which represents approximately 2% of all Mexican localities. A review of literature indicates a near void of published data about the Rancholabrean of Sonora. Because of this paucity of Rancholabrean localities in northeastern México, little is understood about the Mexican Faunal Province, if it even existed during the late Neogene. We have begun a project about a fossil-rich marsh deposit (here named Térapa) from which we recovered more than 60 zoological taxa, many with tropical affinities, of Rancholabrean age from along the Río Moctezuma in northeastern Sonora (29º41’N latitude, 109º39’W longitude, 605 m elevation).

The Río Moctezuma begins just north of the town of Nacozari and flows south through Térapa to join the Río Bavispe and the Río Yaqui, which then empties into the Gulf of California midway between Guaymas and Ciudad Obregón (28ºN, 111ºW). The fossil marsh deposit is situated between the villages of, and on land owned by, San Clemente de Térapa and El Llano, 13 km south of Moctezuma (225 km south of the Arizona-Sonora border) and approximately 350 km upriver from the mouth of the Río Yaqui at the Gulf of California. Lowlands and river corridors of northern Sonora are occupied today by plant and animal communities aligned with arid, semi-arid, and semi-tropical desertscrub and thornscrub habitats, with woodland and forest communities in the higher, mountainous elevations. The immediate biotic community around Térapa today is a foothills thornscrub.

Fossil Locality Development and Water Source

Many basaltic lava flows have erupted from monogenetic vents in the Moctezuma volcanic field. These flows have periodically dammed or diverted the Río Moctezuma since the late Tertiary, creating numerous and apparently short-lived marsh deposits.

Lava flows from Volcán Cerro Blanco are thought to be the youngest eruptions in the Moctezuma volcanic field, with the most recent flow yielding a K/Ar age of 0.53 + 0.2 Ma (millions of years ago, megannum). These flows surged south and west to cascade into the Río Tepache and Río Moctezuma south of Térapa. Although the more southern flows around Tepache are understood, additional work is needed on the basaltic flows entering the river valley north and east of Moctezuma – up-valley from Térapa. Our preliminary investigation indicated that one lava flow (informally named Tonibabi) entered the valley from the east to flow south 12 km and terminate immediately south of Térapa. How this flow relates to the tholeiitic lava flows of Paz-Moreno et al. (2003) is not well understood at this time.

The Tonibabi flow created at least one catchment basin approximately 1 by 2 km, bound on the north, west, and south by basalt, and on the east by older, higher fluvial sediments, perhaps the ‘unconsolidated alluvial fan deposits’ of Paz-Moreno et al. (2003). Medium to fine-grained sediments containing abundant fossils (presented here) overlay and abut the Tonibabi flow. One sample of the Tonibabi flow (from a depth of about 5 m from the surface as exposed in the incised canyon of La Carbonera) produced an 40Ar/39Ar age of 0.44 + 0.13 (2s) Ma. This implies that the lava flow created the dam and impounded sediments at some point in time between 570,000 and 310,000 years ago. Additional age refinement is direly needed.

Downstream from Moctezuma, the Tonibabi lava forced the Río Moctezuma farther west to abut the Sierra Oposura, and separated the river from the newly created lava-dammed basin. This flow also diverted other drainages. La Carbonera parallels the Tonibabi flow on the southeast, and provides the incision and erosion of the fossil beds at Térapa. The location of this incised canyon through the lava flow appears to be related to a collapsed lava tube exposed next to the fossil deposits. Another larger drainage (Los Garambullos [=Nacoberachi]) parallels La Carbonera and also may have emptied into the Térapa basin (Fig. 2), heading in the Sierra la Madera about 20 km to the northeast and draining a catchment greater than 100 km2. Less than a kilometer south of Térapa is the confluence of another, larger stream system entering from the east. Here the Los Alamos, La Ardilla, and other smaller drainages merge, providing runoff from a large catchment area (approximately 290 km2). We propose that the Tonibabi lava flow collectively merged the Los Garambullos and La Carbonera streams, completely or partly funneling the runoff into the lava impoundment basin to create a marsh environment. We have no evidence either way that the Río Moctezuma contributed water or sediments to the impounded sequence described below (although see discussion below). The confluence of these side streams with the Río Moctezuma at the current configuration could occur only after the Térapa basin filled with sediments and fossils, forcing their flow immediately south of and around the Tonibabi lava flow and Térapa.

Stratigraphy

The geological package at Térapa is a 11-m-thick sequence of medium to fine-grained fossiliferous sediments that overlay and abut the Tonibabi lava flow. The stratigraphy of Térapa is reconstructed from 2 primary and 3 secondary sedimentary profiles. The entire sequence of deposition appears in the deepest portion of the basin at a sublocality named Camel Bill (southwestern area of locality). We propose that once the basin was formed by the Tonibabi flow, sediments and fossils began deposition. Rates of sedimentation have yet to be established for this infilling.

Fauna

The extraordinary fauna is both diverse and rich, and includes ostracodes, mollusks, fish, amphibians, turtles, a crocodylian, snakes, lizard, birds, and mammals, many with tropical affinities today. Most of the animals are either extralimital to the setting today (not found in the region today) or extinct. The recovery of Bison dictates a Rancholabrean Land Mammal Age for the fauna. Mammal species are abundant at Térapa, with additional species likely to be recovered with continued excavations and micromesh screen washing. Carnivores include Canis dirus (dire wolf), Lynxrufus (bobcat), and Procyon sp. (raccoon). Multiple forms of Equus and lamine camels were recovered. Numerous remains of cervids (deer) and antilocaprids (including Stockocerus and the diminutive pronghorn, Capromeryx) were recovered, along with single specimens of Platygonus (peccary) and Tapirus (tapir). Two different families of proboscideans, Gomphotheriidae (Cuvieronius) and Elephantidae (Mammuthus) are represented. Xenarthrans found thus far include Glyptotherium (glyptodont), Paramylodon (ground sloth), and the unusual Pampatherium (extinct giant armadillo, pampathere). Rodents are common including the marsh-loving Hydrochaeris (capybara) and smaller gophers, rats, and mice. Remains of bats and shrews are rare.

The occurrence of cf. Crocodylus acutus (a crocodylian; generic assignment uncertain) and Pampatherium, the giant armadillo, is unique in the northern interior Sonora setting. We speculate that a well-developed riparian corridor along the Río Yaqui, from the Gulf of California to the mountain-valley setting at Térapa, permitted the animals with tropical affinities to extend 350 km inland.

Resumen—Resumen - Recobramos fósiles Pleistocenos de un depósito de río represado por lava a lo largo del Río de Moctezuma, en el noreste de Sonora a la latitud 29º41’N, longitud 109º39’W, y a 605 m de elevación. Hoy la región es semiárida con matorral desértica en la falda de las montañas. El encierro que produjo la represa de lava resultó en un área pantanosa de poca duración y una sabana adyacente. La fauna extraordinaria es rica y diversa, e incluye ostracodos, moluscos, peces, anfibios, tortugas, cocodrílicos, serpientes, lagartijas, pájaros, y mamíferos, muchos de ellos con afinidades tropicales hoy en día. La mayoría de los animales están inexistentes en este ambiente hoy o extintos. La recuperación de Bison dicta una Edad Mamífera Terránea Rancholabreana para la fauna; 1 edad preliminar 40Ar/39Ar sugiere que el depósito está entre 570,000 y 310,000 años. La presencia de Crocodylus acutus (un cocodrílico; de asignación genérica incierta) y Pampatherium, el armadillo gigante, es único en el marco interior norte de Sonora. Especulamos que un pasillo ribereño bien desarrollado a lo largo del Río Yaqui, desde el Golfo de California al marco de sierra y valle en Térapa, le permitió a animales con afinidades tropicales extenderse unos 350 km tierra adentro.