 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; March 2002; v. 30; no. 3; p. 279–281; 4 figures. 279 70 Ma nonmarine diatoms from northern Mexico Elizabeth Chaco ´ n-Baca H. Beraldi-Campesi S.R.S. Cevallos-Ferriz    Instituto de Geologı ´a, Universidad Auto ´noma de Me ´xico, Circuito de la Investigacio ´n Cientı ´fica, Me ´xico D.F. 04510, Me ´ xico A.H. Knoll Botanical Museum, Harvard University, Cambridge, Massachusetts 02138, USA S. Golubic Department of Biology, Boston University, Boston, Massachusetts 02215, USA ABSTRACT Carbonaceous cherts of the Tarahumara Formation, exposed near Huepac, Sonora, Mexico, contain abundant diatom frustules occurring as benthic filamentous colonies. Stratigraphic and paleontological observations indicate that Tarahumara sediments ac- cumulated in a nonmarine setting; radiometric ages on encompassing volcanic rocks de- limit their depositional age to ca. 70 Ma. Tarahumara fossils therefore extend the pale- ontological record of nonmarine diatoms from middle Eocene to Late Cretaceous. Preserved populations include forms similar to species of the extant genera Amphora and Melosira, as well as filament-forming araphid pennates comparable to species of Fragilaria and Tabellaria. Tarahumara fossils indicate that by 70 Ma, nonmarine diatoms had achieved considerable environmental as well as taxonomic diversity. Keywords: Cretaceous, cherts, diatoms, frustules, silica. Figure 1. Map showing location of fossiliferous Huepac cherts in east-central Sonora, Mexico. INTRODUCTION Diatoms are among Earth’s most wide- spread primary producers, occurring as di- verse and abundant constituents of lakes, the open ocean, and nearly all intermediate eco- systems. Because they form siliceous skele- tons, or frustules, diatoms also constitute key elements of the contemporary silica cycle. Marine diatoms began to diversify early in the Cretaceous Period and by the Campanian (74.5–84 Ma) included relatively diverse cen- tric and rare araphid pennate forms (Strelni- kova, 1990; Harwood and Nikolaev, 1995). In contrast, unambiguously nonmarine diatoms have until now been known only from middle Eocene (ca. 39–50 Ma) and younger rocks (Bradbury and Krebs, 1995; Strelnikova and Lastivka, 1999). Here we report the discovery of abundant filament-forming centric and pennate diatoms in ca. 70 Ma cherts of the Tarahumara For- mation, Mexico. The encompassing sedi- ments were deposited under lacustrine to coastal brackish-water conditions, demon- strating that by the time diatoms became widespread as fossils, they had already un- dergone substantial environmental as well as taxonomic diversification. GEOLOGIC SETTING The 400-m-thick Tarahumara Formation, in the Sonora Province of northern Mexico (Fig. 1), consists predominantly of andesitic to rhyolitic volcanic flows, breccias, tuffs, and sandstones that unconformably overlie Upper Triassic and underlie Oligocene successions (Amaya-Martinez and Gonzalez, 1993; Jacques-Ayala et al., 1993). Locally, stromat- olitic limestones and intercalated cherts occur in the (informally designated) middle member of the formation. Rhyolitic tuffs immediately subjacent to the sedimentary member yield U- Pb zircon dates of 72.5  0.5 and 70.2  0.6 Ma; tuffs just above the sediments are 69.7  0.6 Ma (McDowell et al., 2001). Thus, the fossiliferous cherts are ca. 70 Ma. On the basis of sedimentological features and mapped distribution, the limestone-chert units have been interpreted as lacustrine (Gon- za ´lez de Leo ´n, 1994). Plant fossils, including abundant palms, a taxodiaceous conifer, isoe- talean lycopods, and an aquatic angiosperm, further support a freshwater to brackish-water interpretation (Cevallos-Ferriz and Ricalde- Moreno, 1995; Herna ´ndez-Castillo and Ceval- los-Ferriz, 1999), as does a newly described Te- traedron-like green alga (Beraldi-Campesi, 2000). The beds also contain ostracods, but no diagnostically marine invertebrates or protists. As described in the following paragraphs, sev- eral of the preserved diatom taxa appear to be related to extant diatoms found in nonmarine environments. DIATOM FOSSILS Diatom fossils were found in several cherty horizons at Huepac, in east-central Sonora, where they occur in abundance along with or- ganically preserved cyanobacteria (Chaco ´n- Baca and Cevallos-Ferriz, 1995; Beraldi- Campesi et al., 1997) and broadly spheroidal cells of likely protistan origin. For the most part, only the gross morphology of frustules is evident; fine structures, such as areolae, pores or vela, are rarely preserved, although col- lapsed cytoplasm occurs in many specimens. This circumstance limits systematic interpreta- tion. Nonetheless, preserved morphologies closely approximate frustules found in extant diatom clades. Preservation is clearly linked to early diagenetic silicification that entombed on January 23, 2016 geology.gsapubs.org Downloaded from