Part II: The southeastern (Oaxacan) faunas

Abstract

The areas and faunas treated in this part of the study are: (1) Suchilquitongo, northwestern Oaxaca Valley; (2) Matatlán, southeastern Oaxaca Valley; and (3) Nejapa, eastern Isthmus of Tehuantepec region.
In Suchilquintongo, the Tertiary sequence unconformably overlies a basement formed by the Proterozoic Oaxaca Complex (schistous and gneissic bodies) intruded by Paleozoic granites, overlain by the Plate Jurassic ?Yogana Formation (red arkisic sandstone), in turn unconformably overlain by Cretaceous marine sedimentary units (sandstones, shales and limestones). The sequence includes an ?Oligocene informal unit (basaltic flows, and conglomerates), unconformably covered by the Suchilquitongo Formation (tuffs, tuffaceous sandstones and silts largely of lacustrine desposition), which bears the latest Hemingfordian- earliest Barstovian (medial Miocene) homonymous local fauna; this unit is unconformably overlain by a late Tertiary conglomeratic one, in turn covered by Quaternary deposits. The Suchilquitongo local fauna includes: Rhinocerotidae Gen et sp. indet.; the tridactyl high crowned horse Merychippus sp.; the protoceratid cf. Paratoceras sp.; and the oreodon Merychyus aff. M. minimus.
In Matatlán, the Tertiary sequence unconformably overlies Cretaceous units (limestones and marls); it consists of rhyolitic vitric tuffs, tuffaceous-arkosic fluviatile sandstones, and associated aphanitic to porphyritic andesitic bodies; the sequence is unconformably covered by Quaternary deposits. The sandstone strata bear the homonymous local fauna, whose age falls in the late Hemingfordian-Clarendonian (medial to late Miocene) intervale, and it consists of: Rhinocerotidae Gen et sp indet.; Merychippus cf. M. primus; Camelidae Gen. et sp. indet.; and a tylopodan.
In Nejapa, the Tertiary sequence unconformably overlies a basement of silicic metatuffs and ?andesitic volcanics and metavolcanics, both of very restricted outcrop area and unknown Precenozoic age; the sequence includes rhyolitic vitric tuffs and fluvio-lacustrine tuffaceous-arkosic sandstones that bear the El Gramal and El Camarón local faunas of late Hemingfordian-early Clarendonian (late medial Miocene) age; it is unconformably covered by Quaternary deposits. The El Gramal local fauna consists of: The primitive mastodon Gomphotherium sp., the common Miocene horse Merychippus (s. L) sp., seemingly represented by a population with simple patterned cheek teeth and another one with complex patterned ones; Camelidae Gen. et sp. indet., and ?Protoceratidae Gen. et sp. indent. The El Camarón local fauna consists of ?Gomphotherium sp., the mustelid carnivore Plionictis oaxacaensis sp. nov., Merychippus (s. 4) sp., represented by a population of horses with a simple occlusal pattern in the cheek teeth, and Antilocapridae Gen. et sp. indet.
To sum up, the Tertiary sequence remains undifferentiated in two of the three studied areas; it occupies grabens limited by horsts made up of the Precenozoic basement; it is affected by extensive faulting and fracturing; it attests extensive-largely silicic- Mid-Tertiary volcanic activity; in one area, Suchilquitongo, radiosiotopic data allow a measure of age-calibration; the mammalian record includes exclusively taxa of strict North American affinites, and its faunas closely correlate to well known Hemingfordian and Barstovian faunas in North America, and to the only seemingly Hemingfordian Central American fauna of Panama; in fact, the Oaxacan faunas appear to represent partial samples of a large and continuos medial Miocene North-and-Middle American therofauna.