•Bond and García, 2002: Fine grained, calcareous sedimentary rocks and volcanic rocks.

•fluvial and lacustrine strata deposited in a rather narrow, approximately north–south running intermontane basin

•The base of this suc- cession consists of a channelized conglomerate followed by upward fining sandstones, siltstones, and mudstones with limestone intercalations (200 m). A coarse- to fine-grained, massive, greenish sandstone succession (containing calcare- ous mudstones, limestone, and tuff intercalations) overlies these deposits (225 m). The upper portion of the Chucal Formation consists of gray mudstones followed by cross- bedded sandstones and minor conglomeratic intercalations (172 m). The described succession reflects the following inferred environmental evolution: (1) fluvial or distal alluvial conditions grading to fluctuating lacustrine conditions ending in a well-developed lacustrine environment; (2) a stage of increased transport energy evidenced by the succession of green sandstones that filled the lake and expanded the fluvial plain environment; and (3) flood plain development grading to fluvial plain conditions. This evolution indicates two stages of basin development, one prior to deposition of the green sandstone succession and one after its deposition. Most of the Chucal fossils described in this work derive from the intermediate, green sandstone-bearing interval of the western flank (Member W3 of Charrier et al. 2005).