THE TECOLOTLAN GRABEN: A RECORD OF SEDIMENTATION AND VOLCANISM FROM CRETACEOUS AND TERTIARY VOLCANIC ARCS

The Tecolotlan valley is a small graben located about 112 km SW of Guadalajara, located in the NE corner of the Jalisco block in west-central Mexico. The graben is 10 km wide and 20 km long, with about 500 m of topographic relief. It is bounded to the east by a horst of Cretaceous (Albian/Cenomanian) limestone and to the west by an uplifted block of Late Cretaceous ash-flow tuffs (one has been dated at 77 Ma) and volcaniclastic sediments. The valley contains Miocene to Recent sediments that are important for their diverse vertebrate fauna. Similar fossiliferous sediments are found in several other localities across central Mexico. They record the first appearance in North America of several South American immigrants and suggest that the land bridge between the continents formed earlier than has previously been supposed. Tertiary basin fill can be divided into older, Hemphillian age sediments (about 50-60 m) that are mostly fine-grained fluvial and overbank deposits, and younger, generally coarse grained, Late Blancan sediments (up to 30 m) that consist of poorly bedded gravels, sands, and mudstones grading upwards into discontinuous gravel-filled channels cut into finer grained siltstone and sandstone. Intercalated with the basin-fill sediments are isolated basaltic, andesitic, and rhyolitic lava flows and pyroclastic-fall ash beds, ranging in age from 2-5 Ma. Chemically the lava flows are transitional between akaline and calc-alkaline, subduction-related magmas and are probably related to extension of the plate overlying the subduction zone.

The rocks found in the western horst and directly underlying the Tertiary basin fill record a history of arc volcanism and sedimentation in this region during the Late Cretaceous. The sequence (over 300 m) consists of subduction-related rhyolitic ash-flow tuffs and trachytic lava flows (lacking pillow structures) interbedded with volcanic debris flow deposits, arkosic fluvial sandstones, and volcanic conglomerates. These rocks differ from Early to Middle Cretaceous volcaniclastic sequences of the Guerrero terrane that are dominated by marine sediments and pillow basalts. This suggests that by Late Cretaceous time the arc was emergent, but probably still low-standing as it continued to accumulate sediment after the closure of the Arperos oceanic basin and accretion of the Guerrero arc terrane to Mexico.