Late Jurassic invertebrate fossils from the Little Hatchet Mountains, southwestern New Mexico by Spencer G. Lucas, New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, NM 87104; Kate E. Zeigler, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131; Timothy F. Lawton, Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003; and Harry F. Filkorn, Department of Geology, Kent State University, Kent, OH 44242 Kr Howells Kuoc Abstract Kuh 24 Invertebrate fossils from the upper conglom- Well 30 35 erate member of the Broken Jug Formation, Tv Kr syncline the basal unit of the Bisbee Group in the Ts Kuh Little Hatchet Mountains of southwestern Km Khw New Mexico, are bivalves (cf. Gryphaea mexi- 60 19 Kuh Copper Dick cana Felix), gastropods (Nerineidae), and a fault coral (Thamnasteria sp. cf. T. imlayi Wells). 38 Kuoc 36 These fossils indicate a Late Jurassic age and support correlation of the Broken Jug Kd 67 30 Khw Pz Formation with the Kimmeridgian–Titho- Jbu Pz nian La Casita Formation of northern Mex- Jbl ico. The presence of Upper Jurassic marine Khu strata in southwestern New Mexico confirms 27 4487 39 Jbf identification of a Late Jurassic Mogollon Jbb paleo-highland in west-central New Mexico and east-central Arizona that separated the Kd Hachita Jbd Peak Bisbee rift basin to the south from the coeval Jbl fluvial depositional basin of the Morrison Formation to the north. Cottonwood Kmz 4470 Spring Jbd Introduction 31° 50´ Broken Jug Mesozoic marine stata underlie sedimenta- Pass Jbu 60 Jbl ry rocks of known Early Cretaceous age in Khu Sylvanite 43 the Little Hatchet Mountains of southwest- 55 ern New Mexico. A possible Jurassic age Jbf for these rocks, which are presently known 43 only from exposures east and southeast of 44 Hachita Peak (Fig. 1), was first suggested by Zeller (1970). Harrigan (1995) and Kd New Mexico 45 Lawton and Harrigan (1998) identified Gulch these strata as the Broken Jug Formation Jbb and assigned them a Jurassic age based on their stratigraphic position and similarity Cabin Kjhs Map location to Jurassic strata in southeastern Arizona Khw and Chihuahua, Mexico. Lucas and Stone Lawton (2000) followed these conclusions Ku 41 and noted the presence of Jurassic inverte- 42 brate fossils in the upper part of the Broken Jug Formation. Here, we describe these 56 Kuc fossils, determine their biostratigraphic Kus Kuo significance, and briefly discuss the paleo- geographic implications of the presence of Upper Jurassic marine strata in southwest- 26 Km Pz ern New Mexico. 57 Measured section Stratigraphy Winkler Ranch The fossils described here come from two Pz localities on the eastern slope of Hachita 0 2 km (abandoned) Tg Granite 1 1 108° Peak in the SE ⁄4 NE ⁄4 sec. 26 T28S R16W, Pass Hidalgo County (Fig. 1). P. Harrigan dis- 0 1 mi 25´ covered New Mexico Museum of Natural FIGURE 1—Geologic map of part of the southern Little Hatchet Mountains showing the fossil local- History (NMMNH) locality 4470, which ities (4470, 4487) in the Broken Jug Formation. Pz, undifferentiated Paleozoic strata. Bisbee Group: yielded the gastropods and coral described Broken Jug Formation—Jbd, dolostone member; Jbl, lower conglomerate member; Jbf, fine-grained here (also see Lawton and Harrigan, 1998, member; Jbu, upper conglomerate member; Jbb, basalt member; Hell-to-Finish Formation—Khu, fig. 2). S. G. Lucas discovered NMMNH undifferentiated Hell-to-Finish Formation; KJhs, Stone Cabin Gulch Member; Khw, Winkler Ranch locality 4487, which yielded the bivalves Member; U-Bar Formation—Ku, undifferentiated U-Bar Formation; Kuc, Carbonate Hill Member; described here. Locality 4487 is just west of Kuo, Old Hachita Member; Kuoc, Carbonate Hill and Old Hachita Members, undifferentiated; Kuh, locality 4470 and approximately 40 m (131 Howells Ridge Member; Km, Mojado Formation; Kr, Ringbone Formation; Kmz and Kd, cogenetic monzonite and diorite plutons; Ts, Skunk Ranch Formation; Tg, Tertiary granite; Tv, Tertiary vol- ft) higher stratigraphically. Both localities canic rocks. Modified from Lucas and Lawton (2000). are near the base of the upper conglomer- 16 NEW MEXICO GEOLOGY February 2001 ate member of the Broken Jug Formation into the whorl. A less prominent, thinner parallel alignment of branches on one side (Fig. 2; Lucas and Lawton, 2000, fig. 3). parietal fold occurs near the top of each of the sample indicates that the actual whorl along the columella. The lower height of the corallum was at least 120 mm Bivalves whorls are nearly twice as high as wide. (4.8 inches). Overall, the morphological We identify these specimens as nerineid details of the corallum are poorly pre- At NMMNH locality 4487, a coquina bed gastropods due to the presence of the served. Faint protuberances on the sur- yields many bivalves catalogued as palatal and parietal folds (cf. Cragin, 1905, faces of some of the branches are relics of NMMNH P-32956–32961. These bivalves pp. 96–98, pl. 20, figs. 1–3, pl. 21, figs. 4–5), the corallites. The septal arrangement was (Fig. 3A–H) are characterized by a small which are absent in turritellid gastropods. observed only in a few of the exposed left valve that has a thick shell wall, is tri- In addition, turritellid gastropods have corallites on one small part of a branch. angular in outline, and is clearly much convex whorls that are usually wider than Details of the columella were not deter- larger than the right valve. The left valve is they are high rather than much higher than mined. Skeletal microstructure and inter- moderately convex to deeply excavated wide as in the Broken Jug specimens. A nal skeletal architecture are not preserved. and is strongly arched dorsally. The beak is more precise identification is not possible, The morphology of NMMNH P-31555 bluntly hooked, and the presence of a sul- but it is significant that the gastropods is, with minor exception, the same as that cus from the beak downward to produce a from the Little Hatchet Mountains are not described for Thamnasteria imlayi Wells wing-like flare of the posteroventral mar- turritellids, as this gastropod family occurs (1942, pp. 127–128, pl. 21, figs. 1–3). Tham- gin is difficult to determine on most speci- in Cretaceous and younger rocks (Tracey et nasteria imlayi Wells was originally mens due to poor preservation; however, it al., 1993). In contrast, the Nerineidae have described from samples retrieved from the is visible on some specimens (e.g., Fig. a Jurassic–Cretaceous temporal range Jurassic (probably Oxfordian) Smackover 3D–E, H). The incurved beak is prominent (Tracey et al., 1993), so they are not precise Limestone in the subsurface of Columbia but incomplete on all specimens due to age indicators at the family level but do abrasion. Those specimens with preserved not preclude a Jurassic age. Hell-to-Finish external shell morphology show acute, Formation imbricate, concentric growth lines and Coral undulations (Fig. 3D). The shell is higher than long; left valve heights range from 30 We identify a coral from locality 4470, to 35 mm (1.2–1.4 inches), and left valve NMMNH P-31555 (Fig. 3K-L), as lengths are much more variable, ranging Thamnasteria sp. cf. T. imlayi Wells, 1942. from 10 to 30 mm (0.4–1.4 inches). The corallum of the single specimen is These specimens closely resemble speci- colonial, thamnasteroid, and ramose. The mens of Gryphaea mexicana Felix illustrated growth form is bushy to phaceloid in by Felix (1891, pl. 27, figs. 30–30a), Cragin appearance. Branches are cylindrical and (1905, pl. 3, figs. 1–6), and Imlay (1940, pl. relatively narrow, 3–7 mm (0.1–0.3 inch) 54, figs. 1–7). As Imlay (1940) stressed, left in diameter. Branch bifurcations are fre- valves of G. mexicana are highly variable, quent, dichotomous, and irregularly and the specimens from the Little Hatchet spaced. Newly formed branches initially Mountains fall within this range of varia- diverge from each other at an angle of basalt member tion. Therefore, but in view of the poor ~30°–45° but, with further growth preservation of the specimens from the branches from the same stem, often Little Hatchet Mountains, we tentatively exhibit a general trend toward subparallel assign them to that taxon as cf. Gryphaea orientation. The corallites are numerous, mexicana. Gryphaea mexicana is known from relatively small, closely spaced, and Upper Jurassic strata in Oaxaca and arranged perpendicular to the long axis upper conglomerate member Durango, Mexico, and the Malone Moun- of the branch. The calices are exclusively tains of west Texas (Felix, 1891; Cragin, monocentric, and the calicular margins 1905; Imlay, 1940). are generally circular in outline. Calice upper conglomerate member diameter ranges from 1.0 to 1.4 mm Late Gastropods (0.04–0.06 inch), and the distance be- conglomerate Jurassic tween the centers of calices ranges from invertebrate fossils NMMNH P-31554, collected by P. about 1.4 to 1.8 mm (0.06–0.08 inch). The sandstone Harrigan at locality 4470, in a grainstone septa are well developed, solid, about siltstone/ mudstone bed 5 m (16 ft) stratigraphically below 20–24 in number, and hexamerally sandy limestone P-31555 (Harrigan, 1995), includes parts of arranged in three cycles, the third cycle three high-spired, slender gastropods possibly incomplete. The septa of the first muddy limestone whose shells have been partially eroded and second cycles are subequal in size cherty limestone away to reveal their inner morphology and extended nearly to the corallite axis. dolostone (Fig. 3I–J). The specimens are thin-shelled The septa of the third cycle are weakly with slender, solid columella.