Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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SMITH: MIDDLE TERTIARY MOLLUSKS
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Younger continental and volcanic rocks (~~"~*) Early to early middle Miocene Isidro Formation Late Oligocene San Gregorio Formation • Megafossil locality A K-Ar sample, age in million years (Hausback, 1984) Streams forming arroyos Major unpaved roads i i i i 0 3 Miles i i i i i i i 0 6 Kilometers TO VILLA INSURGENTES Fig. 2. Purisima Vieja and surrounding area, Baja California Sur, Mexico. Geology after McLean and Hausback (1984). of the formation (e.g., 81JTS8), especially near the type local- ity at the town of San Isidro in Arroyo La Purisima (Hertlein, 1925; Hertlein and Jordan, 1927; Smith, 1984). Species listed by Beal (1948) that were collected by W.S.W. Kew from the area between Pozo (Paso) Hondo and Purisima Vieja (USGS locality 9157) seem, from inspection of the matrix, to have been collected from down section and downstream from the palm grove rather than upstream in the direction of the younger, nonmarine rocks included in the basal Comondu Formation. Specimens of Turritella sp. cf. T. inezana bicarina Loel and Corey, 1932 collected by W.L. Watts (locality UCMP A-601) and illustrated by Merriam (1941) and Loel and Corey (1932) also came from this downstream section. Molluscan species from the upper part of the Isidro Formation at Purisima Vieja are listed in Table 1, and repre- sentative species are illustrated in plate 1. FOSSILS FROM ARROYO SAN IGNACIO Southwest of the town of San Ignacio an arroyo of the same name winds southwest to the head of San Ignacio Lagoon. Canyon walls are formed of white, fossiliferous cal- careous sandstones and siltstones of the San Ignacio Forma- tion, which is 60-100m thick (Mina, 1957), but obscured in places by talus slopes of volcanic boulders and by terrace deposits (Fig. 3). Throughout much of this area the San Ignacio Formation is disconformably overlain by a distinctive basalt cap referred to informally as the basalt of Rancho Esperanza (Sawlan and Smith, 1984). Originally described by Mina (1957), the San Ignacio Formation is known mainly from the molluscan fauna collected in 1921 by Marland Oil Company geologist B.F. Hake and described by Hertlein and Jordan (1927). Outcrops extend from the north side of Mex- ican Highway 1 near the turnoff to the town of San Ignacio to 5-10 km west of Rancho San Angel and at least 8 km down the arroyo, where an important Miocene shark fauna was found (S.P. Applegate, 1984, oral communication). Mina (1957) mapped outliers of the San Ignacio Formation as far south as Arroyo San Raymundo. An upper limit on the age of the San Ignacio Forma- tion fossils is provided by K-Ar ages of 9.72 and 10.1 m.y. on the

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