Page 85 - Land Snails of New Mexico

Cather (1989, 1990) found evidence of the persistence of Laramide upliftsthatstillwere shedding sediments into the Baca Basin up to the time of late Eocene volcanism. Datil Group volcanics ranged in thickness from 1 km, where deposited in basins,toonly300m whereonlappingLaramideuplifts(Cather,
Oligocene
According to Elston (1984:229-230), Oligocene extensional activity produced conditions in which low-angle detachmentfaultingprevailed.Hesuggestedthattheregionnow recognized as the Basin and Range Physiographic Province (includingNew MexicoexceptfortheColoradoPlateauandthe eastern plains) possibly doubled its width during this period, whileexperiencingupliftoffrom 1 to2 km. Inthisearlierphase o f B a s i n a n d R a n g e activity, b a s i n s w e r e o r i e n t e d o b l i q u e l y w i t h relationto present basins of the region, were broader, and were bordered by faults with lower angles than is the case with present basins (Baldridge and Olsen, 1989).
1990:1447). This indicates something of late Laramide
topographic relief in that area. Cather noted (1990:1449) that
"blocks of Paleozoic limestone, some nearly a kilometer in
length" occur in lower Datil Formation debris-flow deposits.
This seems to indicate existence of scarps formed on outcrops of
Paleozoic sedimentary rocks in foundering Laramide highlands.
Cather (1991:273) also found evidence in sediments of Eocene
ageintheCub MountainFormationoftheSierraBlancaBasin .
ofan"unroofing"ofLaramideupliftsinthatarea,which ThisearlierphaseofBasinandRangedevelopmentmust
comprised crystalline basement as well as Paleozoic carbonate rocks. It seems likely that uplifts with outcrops and scarps formed on Paleozoic limestones and associated carbonate rocks
could have provided especially congenial habitats for land snails, as do the Sacramento Mountains today.
Several authors have recognized a marked climatic shiftin NorthAmericainthelaterEocenetowardsacooler,driermode
that w a s to persist, with s o m e fluctuation, for the remainder of the Tertiary. This was the "terminal Eocene event" of Wolfe (1978:694). In a symposium edited by Prothero and Berggren (1992),severalauthorsexploredaspectsofthelateEocene and Eocene-Oligocene Transition. In general, evidence was presented suggesting periods of global cooling in the late m i d Eocene and at the time of the Eocene-Oligocene transition (Berggren and Prothero, 1992:1). There isevidence, insome
cases, that these periods of cooling were associated with increases in aridity. Using paleosol records from the Badlands of South Dakota, Retallack (1992:382) found evidence for a decrease in precipitation and increase in a more arid-adapted flora between 38 and 30 Ma. Leopold et al. (1992:404, Fig. 20.2), in an examination of palynological data, found evidence intheRockyMountainregionofColoradoandWyomingfora shift from mid-Eocene broadleaved evergreen and deciduous forest to a latest Eocene "woody" (non-grassy) savannah. Significant amounts of pollen of Oligocene age in the Rocky Mountain/High Plains region were lacking.
Evanoff et al. (1992), in an analysis of the White River Formation of east-central Wyoming, described a well-defined change in land-snail faunas between the latest Eocene ChadronianandearliestOligoceneOrellanLandMammal Ages. These authors interpreted (p. 127) Chadronian snails to have lived in a subtropical woodland with seasonal precipitation. ProminentgenerawereAshmunellaandSkinnerelix,thelatter
a humboldtianid genus, described by Evanoff and Roth (1992). Orellan genera, on the other hand, were considered to have inhabitedaprogressivelymore open and more aridwoodland or "bushland" environment and comprised a fauna of smaller s h e l l e d g e n e r a o f m i x e d e a s t e r n , n o r t h w e s t e r n , a n d s o u t h w e s t e r n affinities.
ItseemslikelythatthebiotaofNew Mexico,includingits molluscan components, was also affected by climatic changes of theEocene-Oligocenetransitionmuch asinotherpartsofthe Rocky Mountain region, discussed above.
h a v e p r o d u c e d c o n s i d e r a b l e t o p o g r a p h i c relief. H o w e v e r , s u c h reliefwas probably dwarfed by the building up of volcanic piles during the time from late Eocene to earlier Miocene, but especially in the Oligocene. Elston (1984:229) noted that in New Mexico"someofthehighestmountainsofNorthAmerica, up to 4.3 km high, rose through resurgence of ignimbrite cauldronsandisostaticupliftofunderlyingplutons." These volcanoes must have been imposing features of the Oligocene l a n d s c a p e , j u s t a s a r e s o m e o f t o d a y ' s m o u n t a i n s ( S m i t h e t al., 1985:308), which descend from their roots (e.g., Latir Peak, Sierra Blanca, and the San Mateo and Organ Mountains). Volcanicactivityaffectedmuch ofthewesternandnorthernpart ofNew Mexico.Smithetal.(1985:293)wrote:
In Oligocene time, volcanism was widespread in western N e w Mexico. The southwestern quarter of thestate,inparticular,became a volcanic highland, consisting of thousands of meters of calc-alkalic lavas and pyroclastic flows. The landscape was dominated by resurgent domes of ash-flow tuff (ignimbrite) cauldrons and by andesitic
stratovolcanoes.
In southern New Mexico, McIntosh et al. (1992b)
recognized three mid-Tertiary volcanic fields: Sierra Blanca, Boot Heel, and Mogollon-Datil. The latterfield was, by far, the mostextensive,includingsome 10cauldronsoccurringfromthe Las Cruces and Socorro areas on the east to the Mogollon Mountains on the west. Pulses of ignimbrite activity were dated ("Ar/*Ar) at between 36.2 and 24.3 M a within this field (McIntosh et al., 1992b:Fig. 12). Moore et al. (1991:139) reported evidences that volcanic activity occurred in the Sierra Blancafieldbetweenca.37and26Ma.
S m i t h e t al. ( 1 9 8 5 : 3 0 8 ) n o t e d t h a t t h e r e s u r g e n t d o m e s o f t h e Bursum and Emory cauldrons in the present Mogollon Mountains and Black Range, respectively, were the largest cauldronsknowninNew Mexicoandwereamongthelargestin the world. Axelrod and Bailey (1976) discussed a fossil flora preservedinmoatsedimentsoftheEmory Cauldron,interpreting it a s h a v i n g a s u b a l p i n e a s p e c t , w h i c h f u r t h e r i n d i c a t e s t h e l o f t y heightsattainedbysome ofthesevolcanicmountains. Probably, taxa of land snails that were adapted to such montane forests were also present. Given the sedentary tendency in dispersal, w h i c h c h a r a c t e r i z e s l a n d s n a i l s , it m a y b e t h a t s p e c i e s a n c e s t r a l to present denizens of the mountains of west-central N e w Mexico were already present in these Oligocene-Miocene
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