Geoscience Reference
In-Depth Information
base of the Cucaracha Formation is marked by a distinctive pebble conglomerate bed
that lies unconformably over the Culebra Formation (Figure 6). This conglomerate
bed is widely distributed and contains volcanic pebble clasts with rare fragments of
carbonized wood (without teredinid borings) and oysters. This and other pebble con-
glomerate beds higher up in the Cucaracha Formation commonly become finer upsec-
tion, grading into lithic wacke, siltstone and claystone. Medium to coarse-grained,
lithic wacke beds are commonly cross-bedded, which show an average paleocurrent
direction to the east (N87°E ± 6.4° (95% confidence cone, Fisher analysis)). These
interbedded channel deposits contain permineralized logs of up to 1 m in length and
30 cm in diameter oriented parallel to bedding. Pebble conglomerate and lithic wacke
also contain rare fossils of land mammals. Olive-gray to blackish red claystone is
the most common lithology in the Cucaracha Formation. This claystone is commonly
structureless to slickensided, but may contain mottling and drab-haloed root traces.
Horizons of calcite nodules and rhizoconcretions are common throughout the clay-
stone. Two horizons contain spherical to platy barite nodules (~2 cm in diameter) in
olive-gray claystone. Fossils of land mammals, turtles, fish, crocodiles, and gastro-
pods (
Hemisinus
(Longiverena)
oeciscus
) are present locally in claystone, as noted
by Whitmore and Stewart (1965), Woodring (1957-1982), and MacFadden (2006).
Four lignite beds are present in the upper half of the Cucaracha Formation (Section 8;
Figure 11). The Cucaracha Formation contains a distinctive bed of welded tuff 4.3-7.7
m thick (also known colloquially as the “ash flow” (Woodring, 1957-1982; Woodring
and Thompson, 1949)), which is broadly distributed and serves as a useful marker bed
(Figure 6).
The Cucaracha Formation represents a coastal delta plain that consists of channel,
levee, fl ood plain and marsh deposits (Figure 11). Abundant paleosols indicate that
soils commonly developed on these deposits. Retallack and Kirby (2007) recognized
12 different pedotypes that represent as many vegetation types, including mangrove,
freshwater swamp, marine-infl uenced swamp, early successional riparian woodland,
colonizing forest, dry tropical forest, and woodland. Oxygen and carbon isotopic anal-
yses of land mammal teeth are consistent with these interpretations, as they indicate
diverse, C3 plant communities, possibly ranging from dense forest to more open wood-
land (MacFadden and Higgins, 2004). The pebble conglomerate bed at the base of the
Cucaracha Formation represents a fl uvial-channel deposit that is broadly distributed
(based on its geometry and sedimentology, which are typical of fl uvial-channel de-
posits (Miall, 1978, 1992)). Incision of this channel into underlying marine mudstone
and sandstone of the Culebra Formation indicates that part of the underlying section
has been eroded by the channel. The pebble conglomerate contains fragments of wood
that show no evidence of teredinid borings (unlike the wood found in the underlying
Culebra Formation), suggesting that this basal conglomerate was deposited above sea
level. The presence of oyster fragments probably represents reworking of the underly-
ing marine Culebra Formation. Interbedded lenses of pebble conglomerate and lithic
wacke further upsection represent small fl uvial channels, based on their lenticular ge-
ometry and sedimentology (Miall, 1978, 1992; Pettijohn et al., 1987). The small ratio
of channel deposits to claystone (the sandstone/claystone ratio for the entire formation
is 18.4%) suggests that these were small meandering channels (there is generally a
