Geologic Map of the
Trampas Quadrangle,
Rio Arriba, Taos Counties, New Mexico
By
Bauer, Paul W.; Helper, Mark A.; Aby, Scott
June, 2005
New Mexico Bureau of Geology and Mineral Resources
Open-file Digital Geologic Map OF-GM 104
Scale 1:24,000
This work was supported by the U.S. Geological Survey, National Cooperative Geologic
Mapping Program (STATEMAP) under USGS Cooperative Agreement 06HQPA0003
and the New Mexico Bureau of Geology and Mineral Resources.
New Mexico Bureau of Geology and Mineral Resources
801 Leroy Place, Socorro, New Mexico, 87801-4796
The views and conclusions contained in this document are those of the author and
should not be interpreted as necessarily representing the official policies,
either expressed or implied, of the U.S. Government or the State of New Mexico.
Qal
Quaternary alluvium.
Active channel alluvium, floodplains, low (young) alluvial terraces, tributary-mouth fans and some valley-slope colluvium.
Often brownish and/or reddish, poorly-to-moderately sorted, angular-to-rounded, thinly-to-thickly bedded, loose silt and silty sand
with subordinate coarse lenses and thin-to-medium beds of mostly locally derived clasts. On this quadrangle Quaternary alluvium
includes a diverse suite of deposits ranging from the active channel alluvium of the Rio Grande to small ‘pockets’ of alluvium in
upland positions.
Charcoal sample submitted for dating
Qc
Quaternary colluvium
Hill slope and valley-margin colluvium found in diverse settings are composed of locally derived (?), light-to-dark brown,
orange, and rarely reddish; poorly-to-moderately sorted; angular-to-well rounded silt-to-sandy conglomerate/breccia with clasts
locally to >1m. Large areas of the quadrangle are covered with a thin veneer of colluvium that is indistinguishable in the field from
poorly exposed parts of the underlying sedimentary bedrock units. Colluvium has only been mapped where it is clearly
distinguishable from either by fortuitous good exposure or by inclusion of unique clast assemblages (e.g. where rounded clasts of
Quaternary gravel have been recycled into these deposits). Colluvium is also distinguishable where it overlies Quaternary terrace
gravel (Qg) deposits, as on relatively low terraces on both sides of the Rio Embudo. South of the Rio Embudo narrow strips of
colluvium along ridges below the Mesa Cejita define a former graded hill slope surface that appears to grade from the top of the
mesa(?) to the top of terraces approximately 35-40 m above the Rio Embudo. This dissected geomorphic surface is best seen by
looking south from a high vantage north of the Rio Embudo.
Qla
Quaternary ‘landslide’ alluvium
Alluvium found on toreva (rotational) blocks associated with landslide complexes found northwest of the Rio Grande. Alluvium
as exposed by shallow (<2m), discontinuous gullies is light-to-dark brown and dominated by very thin-to-medium bedded, loose,
massive sandy-to-silty beds with thin, discontinuous layers of pebbles and rare cobbles (to ~15cm). This alluvium is often
discontinuously incised at present but appears to have been deposited in closed depressions created by rotation of blocks during mass
movement. Some Qla is still contained within such closed depressions, particularly near the head of landslides.
Previous workers have interpreted these deposits as either playa (Bauer and Helper, 1994) or mixed alluvial and eolian
(Koning and Aby, 2004) in origin. Although some fine material in these deposits may be eolian in origin (especially as this area is in
the lee of the dominant southwest winds), we see no evidence for primary eolian deposition and no evidence of ephemeral standing
water (playas) and therefore interpret them as primarily alluvial in origin.
Charcoal sample submitted for dating
Qtr
Rio Grande terrace gravels
Pebble-to-cobble size gravel deposits preserved as small remnants inset into Quaternary landslide deposits north and
northeast of Rinconada. These gravels are identifiable as Rio Grande alluvium by the inclusion of Proterozoic Glenwoody Formation
clasts (derived from north of the Quadrangle boundary near Pilar) and rounded cobbles of Sevilleta Basalt. These clast types also
distinguish them from presumed Pliocene gravels derived from an ancestral Rio Embudo and preserved beneath(?) Sevilleta Basalt
north of the Rio Grande (See Ql discussion) but only exposed in Ql deposits at present.
Ql
Quaternary landslide deposits
Landslide deposits composed of Sevilleta Basalt (Tb), Pilar Mesa member of Chamita Formation (Koning and Aby, 2005 in
prep), and Ojo Caliente Sandstone Member of Tesuque Formation (To) northwest of the Rio Grande; and Sevilleta Basalt, upper
member of the Picuris Formation (Tpu), and Quaternary/Tertiary gravel (Qg and QTg) southeast of the Rio Grande. Northwest of the
Rio Grande these deposits also contain variable amounts of well-rounded, gravel-to-cobble sized clasts that are similar in
composition to terrace gravels associated with the Rio Embudo (Table1). These gravels often form mantles on slopes below
concentrations of basalt rubble and/or Tbl exposures within the landslide complex. Although the base of the Sevilleta basalt is not
exposed on the quadrangle, we believe these gravels to represent Pliocene deposits of an ‘ancestral Rio Embudo’ based on their
composition. Surface of landslides is often mantled with 10-3000 cm, angular/sub-angular blocks of Sevilleta Basalt. South of the
Rio Grande, landslide deposits do not form identifiable ‘lobes’ as they do to the north and may be partly(?) colluvial. Deposits poorly
exposed except in road cuts along highway just north of Rinconada. In these exposures, deposits are arranged in thick, apparently
tabular bodies of variable composition.
Qg
Quaternary gravel
Includes both primary Quaternary terrace gravel and reworked deposits derived from these(?). Two main types of Qg are
present, one entirely dominated by quartzite clasts and one containing ~10% well-rounded, well-lithified Paleozoic sandstone cobbles
(Table1). The latter are indicative of Rio Embudo provenance while the former are related mostly to the Rio de las Trampas drainage
system(????). No detailed correlation of various gravels inset below QTg level was conducted and the current mapping should only
be taken to represent the presence of gravel. Obviously equivalent terraces can in places be distinguished by similar landscape
position. At least one fill-cut terrace is inferred to exist south of the Rio Embudo near Montecitio about 25-30 m above present grade.
The reader is advised that most Qg deposits have some alluvial cover and this Qal and/or Qc is sometimes not mapped in order to
show all known Qg deposits. This alluvial cover accounts for the gentle streamward slope of the surface of most Qg deposits.
Qg and QTg deposits appear to define a sequence of progressively lower terraces associated with the Rio Embudo drainage
system. These terraces are found approximately 250 m (e.g QTg/QTf west-southwest of Cerro de los Marquenas), 180 m (e.g.
southwest of Cerro de los Aroboles), 120 m (e.g. southeast of Cerro Alto) and at several levels (approximately 75, 50, 35, and 20 m
above grade) along the modern Rio Embudo between Canoncitio and the western quad boundary. The reader is once again cautioned
that these terrace deposits have not been correlated, height estimates are taken from the topographic map, and no genetic
interpretation of terrace genesis is implied. The gravel deposits as a whole appear to reflect a pattern seen in other drainage systems
in northern New Mexico where the oldest deposits form broader deposits apparently developed during a period of relatively slower
incision rates while younger gravels developed in a period of incision (Newell, et al., 2004). This increase in incision rate may be
related to integration of the Rio Grande and subsequent increase in ‘stream power’/fall of regional base level. The oldest ‘terrace’
gravels (QTg) define a course of the Rio Embudo between Cerro de las Marquenas and Cerro Puntiagudo. Subsequently, the Rio
Embudo shifted southeastward to a course of Cerro de los Arboles.
If QTg deposits are correlated to the ‘Pliocene’ gravels believed to exist below Tb flows northwest of the Rio Grande, then
the ancestral Rio Embudo seems to have followed a coarse roughly parallel to the modern coarse and including the prominent
‘dogleg’ presently found ~ 1 mile southwest of Cerro de la Cruz.
Tertiary deposits (see below) indicate 5-10 degrees of south-to-southwest tilting in post-Miocene time, and apparently
onlapped a Miocene topography developed in Proterozoic rocks. The Tertiary/Proterozoic contact therefore roughly defines a
southwest sloping ‘ramp’. Progressive southward shift of the Rio Embudo may therefore reflect ‘tracking’ of this contact by the
fluvial system within the context of regional post-Pliocene incision and modified by Miocene paleotopography.
Tbl
Sevilleta basalt flows displaced by mass movements
Relatively intact blocks of Sevilleta basalt flows displaced and rotated by mass movements north of the Rio Grande.
Identified by coherent blocks of basalt forming arcuate ridges within landslide deposits and often with original surface of flows
identifiable by pahoehoe textures.
Tb
Sevilleta Basalt
Flows of the Sevilleta Basalt found mostly in northwest corner of quadrangle, northwest of the Embudo fault zone. An
isolated remnant of these flows ~ 4 m thick is found southwest of the Rio Grande and Embudo fault zone. This remnant has recently
been used to calculate post-basalt displacement across the Embudo fault zone (Bauer and Kelson, 2004), who estimate its age as
approximately 3 Ma. They also use data from Appelt (1998) to estimate the age of the base of the exposed basalt flows north of the
Rio Grande at ~2.8 Ma, suggesting that additional basalt flows are buried by landslide deposits. Exposed basalt flows are at least
50(?)m thick. The entire flow package is only ~10-15 m thick near the eastern end of Comanche Canyon (~ 2 km west and westnorthwest of the northwest corner of this quadrangle) indicating that the flow package thickens toward the Rio Grande in this area
(and then thins to ~4 m across the Embudo Fault zone). This observation agrees with isopach data of Gunn (1981) to the southwest
that show the thickest basalt packages originally underlie the present course of the Rio Grande (which follows the Embudo fault
zone in this area). The fact that the ~3 Ma basalt flow flowed across the Embudo fault zone while younger flows were restricted to
north of this structure may indicate a relation (at least chronologically if not genetically) between tectonism and volcanism.
QTg
High-level Gravels
Ridge and mesa-capping gravel deposits and partially stripped and/or reworked remnants of such deposits. Possible Pliocene age is
indicated by their position above deposits containing the Guaje pumice (~1.6 Ma) near Truchas and the apparent projection of their
‘surface’ to beneath Sevilleta basalt on Black Mesa (Manley, 1976; Smith, et al., 2004). Complex assemblages of ‘high-level’ gravel
are common on the El Valle Quadrangle to the southeast (Timmons and Aby, 2005 in prep.). Many of these deposits are associated
with an ancestral Rio Embudo drainage system and can be distinguished by the presence of 5-10% Paleozoic Sandstone clasts and +/1% Basalt derived from flows near Vadito on the Penasco Quad ( Bauer et al., 2003).
QTf
Quaternary and/or Pliocene alluvial and colluvial(?) material
Mostly coarse, basement-derived alluvium and /or colluvium associated with QTg deposits and partly burying them. Clast
types are derived from adjacent bedrock highs (Table 1).
Tq
Quartzite-rich Tertiary unit.
Mostly light-colored (buff to very pale brown), moderately (?)-to-poorly sorted, loose-to-weakly cemented, medium-to-thick
bedded (?), silty sandstone (?) to sandy cobble conglomerate composed of Quartzite and <25 % granitic clasts (Table 1).
This unit is more Granitic-rich to the south on the Truchas quadrangle. On the Truchas quadrangle this unit contains ash/pumice
layers 11.7 and 11.3 Ma (Smith et al, 2004).
Ash sample submitted for dating
Tc
Cejita Member of the Tesuque Formation
Buff-to-greenish, moderately–to-poorly sorted, medium-to-thick bedded sandy conglomerate-to- pebbly sandstone and
subordinate coarse-to-fine silty sandstone containing rounded-to-subangular clasts of Quartzite, Paleozoic sandstone, siltstone, and
limestone, and Tertiary volcanic clasts. Ovelies both the Ojo Caliente Sandstone and Dixon member, and is only distinguishable
from the Dixon member by tracing contact (which forms a slope break) from the area where this member overlies To.
Ash sample submitted for dating.
To
Ojo Caliente Sandstone
Buff and very pale brown, well sorted, thick-to-thin bedded, well-rounded eolian and fluvially reworked eolian sand
interbedded with brownish, reddish, and greenish thin-to-thick bedded pebbly sandstones. On this Quadrangle the Ojo Caliente
Sandstone is a zone of mixed fluvial and eolian facies representing a zone of interfingering between the eolian dunes of the typical
To and the fluvial system of the Dixon member. The contact between the ‘pure’ To and this transition zone is near the western Quad
boundary but is believed to be largely covered by colluvium. Rather than define a new unit for these strata, we
Td
Dixon member of Tesuque Formation of Santa Fe Group (Middle Miocene)Red, tan, beige, and locally green sandy to clayey silt and silty clay beds ~.1-6(?)m thick, interbedded with tan, brownish,
reddish, and characteristically greenish; moderately to very poorly sorted; often preferentially carbonate cemented; thinly to thickly
bedded; conglomerates and fine to coarse arkosic sandstones between ~.5 and 5(?)m thick. Conglomerates contain abundant poorly
to moderately-well rounded clasts of Precambrian quartzite and Paleozoic sandstone, limestone, and siltstone (Table 1).
Sedimentary features other than plane lamination are not common but include ripple marks, cross beds, and lateral accretion
(point-bar) foresets. Contacts between beds are usually abrupt and bases of sandstones and conglomerates are commonly scoured,
with .01-1m relief. Imbrication of clasts is not common, but is locally moderately developed. Sandstones and conglomerates are
preferentially cemented with calcium carbonate. Carbonate cement sometimes forms a sparry white matrix between grains.
Recent mapping on the Trampas and Truchas quadrangles (Smith et al., 2004) shows that the Dixon and Cejita members of
the Tesuque formation are indistinguishable in the field where not separated by the Ojo Caliente Sandstone Member of the Tesuque
Formation.
Minimum thickness 250 m.
Ash sample submitted for dating
Tpu
Upper volcaniclastic member of the Picuris Formation
Volcaniclastic pebbly sandstone, pebble-conglomerate, and sandstone; minor overbank deposits of very fine- to fine-grained
sandstone, siltstone, and mudstone in very thin to medium, planar beds; also minor cobbles in the channel deposits. Individual
channel beds are commonly lenticular and very thin to thick. Many channel complexes fine-upward (Steinpresss, 1980). Pebbles
generally consist of felsic to intermediate volcanic clasts and subordinate quartzite, with minor Paleozoic sandstone and granite, but
basal unit (Tpb) includes a high proportion of locally derived crystalline basement detritus. Steinpress (1980) notes that
conglomerate is less common in the upper half of the unit. Sand is generally medium to very coarse, angular to subrounded, and
poorly to moderately sorted. Sand point counts by Steinpress indicates a composition transitional between feldspathic and lithic
arenite. Our paleocurrent data is consistent with that of Steinpress (1980) and shows a general southwest paleoflow. The paleoflow
and clast composition data indicate a source to the northeast, as interpreted by Steinpress (1980), and indicates either reworking of
the Picuris Formation in the Picuris Mountains or erosion of the Latir volcanic field in the Sangre de Cristo Range near Taos. In the
Trampas quadrangle the unit onlaps onto, and locally abuts, Proterozoic basement highs. Depositional environment was probably an
alluvial slope (see Smith, 2000, for discussion of alluvial slopes) because of the lack of sheetflood couplets and debris flows
characteristic of alluvial fans, and the lack of distinct floodplain deposits or evidence of channel meanders associated with a
meandering fluvial system. Upper contact is conformable (i.e., interfingering or gradational) with the overlying Dixon member.
Degree of cementation varies, but overall the unit is moderately cemented. Fossils collected from the lower part of the Chama-El
Rito Member near Rinconada are consistent with a Late Barstovian North American Land Mammal Age (Tedford and Barghoorn,
1993), this data and the interpreted age of the overlying Dixon member (12-14 Ma) indicates an age of 12.5-14.5 Ma. Steinpress
(1980) has measured a thickness of 480 m along Cañada Agua, which is compatible with the map data.
Tpb and Tdb
Basal Tertiary unit
Alluvial and colluvial material underlying Tpu and Td in many locations. Composition variable (Table 1) and everywhere
locally derived from Proterozoic units.