Page 79 - Land Snails of New Mexico

These highlands seem to have incorporated riftfaults oriented roughly perpendicular to those of the present Rio Grande rift valley.The riftshoulderhighlandscontinuedwestward,withor without interruptions, into Arizona as the "Mogollon Highlands," asmapped by Bilodeau and Lindberg (1983:Fig. 6). Dickinson (1989:8) depicted the Mogollon Highlands as Early Cretaceous features, consisting of a zone of steep, faulted scarps associated withriftingnorthoftheBisbeeBasin.On theirnorthernflank, he visualized these highlands as sloping more gently toward the region of the present Colorado Plateau. It would appear that during the Early Cretaceous a landscape persisted, which was dominated by mountains or highlands bordering a sea--the sea, in this case, being an a r m of the Atlantic rather than the Pacific Ocean as during the earlier Jurassic situation, discussed above.
in a final regression, the sea withdrew from northeastern N e w Mexico in Campanian to Maestrichian time of the latest Cretaceous (Molenaar, 1983:Fig. 10).
D e p o s i t i o n i n t h e E a r l y C r e t a c e o u s rift b a s i n i n s o u t h w e s t e r n New MexicocontinuedduringmostofEarlyCretaceoustime, but, as described by M a c k et al. (1988:140) "Late Albian and e a r l y C e n o m a n i a n w a s a t r a n s i t i o n a l t i m e b e t w e e n t h e rift a n d foreland basins. The riftshoulder was onlapped by the Sarten and Beartooth Formations . . . derived from the northwest." These authors further noted (p. 140) that "the riftshoulder that separated southwestern N e w Mexico from the Midcontinent forelandbasininAptian,Albianand earlyCenomanian time was not a significant barrier by middle to late Cenomanian and ... theforelandbasinoccupiedvirtuallyallofNew Mexico." Thus, t h e w e a r i n g d o w n o f t h e rift s h o u l d e r a l l o w e d i n c u r s i o n o f t h e
Such an orogenic belt in Arizona is also involved in the model of Cumella, who wrote (1983:196): "I suggest that the movement of the Late Cretaceous shoreline in the San Juan
Cretaceous epeiric sea from the east, establishing the conditions thatwouldcharacterizetheLateCretaceousofNew Mexico.
developed, Lipman and Sawyer (1985) reported identification of a number of late Cretaceous calderas with an age range of ca. 70-75 Ma. They also noted alikely caldera in this age range near Hillsboro,SierraCo.,New Mexico,whichtheycomparedtothe Crater Lake volcano of northern California.
TheLateCretaceousEpeiricSea
Itappears, then, that in the Late Cretaceous, recurrent uplifts and volcanic activity created highlands and mountains in eastern Arizona that would have bordered the epeiric sea, probably producing islands and peninsulas. Throughout the Cretaceous, land snails of montane affinity probably continued to find satisfactory habitats in parts of Arizona, and would have been, asitwere,poisedtocolonizeNew Mexicoashabitatbecame available. As land emerged from under the epeiric sea, a tabula rasamusthavebeenproduced,whichwouldhaveinvited terrestrial colonists. In the back-and-forth succession of
The Late Cretaceous epeiric (epicontinental) sea stretched
from the northern to the southern margin of North America as a
broad seaway covering much of the region of today's Great
Plains and Rocky Mountains. In the northern United States, the sea extended as far east as western Iowa and Minnesota and as
farwestaswesternMontana(RiceandShurr,1983;Witzke,et al., 1 9 8 3 ) . T o t h e s o u t h , m o s t p a r t s o f t h e G u l f C o a s t s t a t e s w e r e inundated,withtheseaextendingacrossTexasandNew Mexico andintoArizonaduringmaximal stages.To thesouth,much of México was covered by the sea (Enos, 1983).
transgressions and regressions, periods of colonization must havealternatedwithextinctionscausedby inundations.Itseems, then, that the first land snails colonizing N e w Mexican soil duringregressionswouldhavecome intothesouthwesternpart of the state. Ithas been stressed, above, that southern Arizona (and adjacent México) seem to have had a long Jurassic and Cretaceous history as a mountainous or highland region--these highlands often being coastal in location. Thus, any colonists repopulating southwestern New México probably included species adapted to coastal habitats of the epeiric sea as well as montane species.
The epeiric sea was subject to recurrent transgressive and regressive cycles. Witzke et al. (1983) recognized five major cycles in the northern United States as did Molenaar (1983) in the area of N e w Mexico and Arizona. Molenaar found that the earliesttransgression(lateCenomanian toearlyTuronian) was
also the most extensive (his Fig. 7), extending westward approximately to a diagonal drawn from the northwestern to southeastern corners of Arizona. Except possibly for a small area in the extreme southwestern corner, and for any mountaintops that became islands, all of New Mexico would have been covered by this transgression of the sea. With its regression (later Turonian), the southwest quadrant of the state was exposed. In succeeding cycles, transgressions extended successively less far to the southwest and regressions successively farther to the northeast. The fifth transgression extended only about as far as a diagonal drawn from the northwestern to the southeastern corners of N e w Mexico, and,
Regional Aspects of the Cretaceous and Faunal Implications
In southwestern N e w Mexico and southeastern Arizona, regressions were related to a return of tectonic uplift in the western part of a retroarc foreland region where compressional orogenesis w a s taking place. In this regard, M a c k (1987:512) noted: "Erosion of the orogen produced a great volume of siliciclastic sediment that spread eastward, southeastward, and northeastward across the former rift basin and onlapped the former riftshoulder." That is to say that the later Cretaceous sea in southwestern New Mexico was a repository of clastics derived from an orogenic source that existed farther to the west.
Basin area was controlled primarily by tectonism in southeastern
Arizona. . . ." Cather (1991:272) summarized evidence
indicating that paleocurrent and provenance data from the
Mesaverde Group of Late Cretaceous age "throughout much of
New Mexicofavorderivationfromorogenicareasinsouthern
Arizona and southwestern N e w Mexico." In the same general area in southeastern Arizona where Jurassic volcanoes had
S o m e aspects of the Cretaceous that are of broader regional extentthanthosediscussedabovemay relate,inageneralway, to N e w Mexico and its fauna. These pertain to the nature of lands west of the epeiric sea (apart from Arizona, discussed above) and to the general isolating effect of the epeiric sea on
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