Geoscience Reference
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distributary-channel deposits. Carbonaceous mudstone beds represent interdistribu-
tary bay deposits with mangal associations of mangrove and oyster.
Land Mammal Biostratigraphy
Traces of land mammals are extremely rare in the Culebra Formation. We found three
molars representing three taxa in the uppermost part of the Culebra Formation in the
course of our work. These discoveries are the first definite report of land mammal fos-
sils from the marine Culebra Formation (although see Woodring (1957-1982), who
reported a partial ungulate metapodial from what he described as the transition zone
between the Culebra and Cucaracha formations). We found molars of the artiodactyl
Paratoceras wardi
and peccary cf.
Cynorca
sp. at the top of the Culebra Formation
in Section 3 (Figure 6), as well as a molar of the rhinoceros
Menoceras barbouri
in
the uppermost part of the Culebra Formation in Section 5 (Figure 6). Based on their
respective ages in North America,
Paratoceras wardi
is Barstovian (16-15 Ma), cf.
Cynorca
sp. is probably early Hemingfordian to Barstovian (18.8-14 Ma) and
Menoceras
barbouri
is Hemingfordian (18-17 Ma) (Figure 10) (MacFadden, 2006; Wright1 et al.,
1998). Taken together, these fossils suggest an age of between 18.8 and 14 Ma for the
uppermost part of the Culebra Formation.
Strontium Chemostratigraphy
In order to derive age estimates for the Culebra and La Boca formations, we collected
seven samples of fossil coral and bivalves from two sections containing the La Boca
and upper Culebra formations and analyzed them for their 87Sr/86Sr ratios (Figure 9;
Table 2). A fossil bivalve-shell fragment from the calcirudite bed near the base of the
La Boca Formation in Section 1 (i.e., the lower member of the Culebra Formation)
had an estimated age of 23.07 ± 0.53 Ma, based on its 87Sr/86Sr ratio. Two pectinid
bivalves from the “pectinid-spondylid” sandstone bed below the overlying E.L. had an
estimated age of 20.62 ± 0.58 and 20.99 ± 0.46. Two Acropora coral specimens from
the upper branching facies of the E.L. had an estimated age of 20.99 ± 0.71 and 21.24
± 0.44. In Section 3, two pectinid bivalves collected from a fine-grained calcarenite
bed (2 m below the conglomeratic sandstone containing
Paratoceras wardi
and cf.
Cynorca
sp. specimens) of the upper member of the Culebra Formation had an esti-
mated age of 19.12 ± 0.42 and 19.83 ± 0.39. Taken together, the samples from the La
Boca Formation are 1-4 m.y. older than the samples from the upper Culebra Forma-
tion (Figure 9, Table 2).
Cucaracha Formation
Lithostratigraphy
The Cucaracha Formation is about 140 m thick and consists mostly of claystone with
a minor amount of conglomerate, sandstone, lignite, and welded tuff (Figure 11). Len-
ticular beds of conglomerate and sandstone are more common in the lower half of the
formation below a distinctive welded tuff bed of volcanic origin, whereas tabular beds
of claystone and lignite are more common in the upper half above the welded tuff bed
(more specifically, the lower half of the formation has a sandstone/claystone ratio of
24.1%; whereas, the upper half has a sandstone/claystone ratio of only 5.7%). The
