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000
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/subangular 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 west-northwest 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.
36°7'30"N
421000
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105°52'30"W
423000
424000
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426000
105°50'0"W
427000
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430000
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Geologic Map of the Trampas 7.5 - minute Quadrangle
Base from U.S.Geological Survey 1953, from photographs taken 1952 and field checked in 1953.
1927 North American datum, UTM projection -- zone 13N
1000- meter Universal Transverse Mercator grid, zone 13, shown in red
by
Paul W. Bauer, Mark A. Helper, and Scott Aby
Taos Junction
Carson
Taos SW
May 2005
Velarde
Trampas
Penasco
Magnetic Declination
May, 2005
9º 42' East
At Map Center
1:24,000
0
Chimayo
Truchas
0.25
0.5
1
1.5
2
Miles
El Valle
0
0.25
0.5
1
1.5
2
Kilometers
CONTOUR INTERVAL 20 FEET
DRAFT
NMBGMR OF-GM 104
NATIONAL GEODETIC VERTICAL DATUM OF 1929
COMMENTS TO MAP USERS
A geologic map displays information on the distribution, nature, orientation, and age relationships
of rock and deposits and the occurrence of structural features. Geologic and fault contacts are
irregular surfaces that form boundaries between different types or ages of units. Data depicted
on this geologic quadrangle map may be based on any of the following: reconnaissance field
geologic mapping, compilation of published and unpublished work, and photogeologic interpretation.
Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given
contact onto a topographic base map; therefore, the accuracy of contact locations depends on the
scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could cause
misunderstanding in the detail of mapping and may result in erroneous interpretations. Site-specific
conditions should be verified by detailed surface mapping or subsurface exploration. Topographic
and cultural changes associated with recent development may not be shown.
Cross sections are constructed based upon the interpretations of the author made from geologic
mapping, and available geophysical, and subsurface (drillhole) data. Cross-sections should be used as
an aid to understanding the general geologic framework of the map area, and not be the sole source
of information for use in locating or designing wells, buildings, roads, or other man-made structures.
The map has not been reviewed according to New Mexico Bureau of Geology and Mineral Resources
standards. The contents of the report and map should not be considered final and complete until
reviewed and published by the New Mexico Bureau of Geology and Mineral Resources. The views and
conclusions contained in this document are those of the authors and should not be interpreted as
necessarily representing the official policies, either expressed or implied, of the State of New Mexico, or
the U.S. Government.
This work was performed under the STATEMAP component of the USGS
National Cooperative Geologic Mapping Program. Funding for geological
mapping was provided by the U.S. Geological Survey and the New Mexico
Bureau of Geology and Mineral Resources, a division of New Mexico Tech.
New Mexico Bureau of Geology
New Mexico Tech
801 Leroy Place
Socorro, NM 87801-4796
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http://geoinfo.nmt.edu
This and other maps are available in PDF format from:
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or contact:
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This draft geologic map was produced from scans of hand-drafted
originals from the author(s). It is being distributed in this form because
of the demand for current geologic mapping in this important area.
The final release of this map will be made following peer review and
redrafting in color using NMBGMR cartographic standards. The final
product will be made available on the internet as a PDF file and in a
GIS format.
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.