Geologic Map of the Trampas Quadrangle, Rio Arriba, Taos Counties, New Mexico

Size: px

Start display at page:

Download "Geologic Map of the Trampas Quadrangle, Rio Arriba, Taos Counties, New Mexico"

Godfrey Waters
6 years ago
Views:

; Aby, Scott June, 2005 New Mexico Bureau of Geology and Mineral

(STATEMAP) under USGS Cooperative Agreement 06HQPA0003 and the New

New Mexico Bureau of Geology and Mineral Resources 801 Leroy

contained in this document are those of the author and should not

1 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, 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.

2 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 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 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.

3 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 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.

4 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

5 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 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.

SOUTH CERRO AZUL STRATIGRAPHIC SECTION. Upper Cerro Azul flow of the Servilleta Basalt (Tsbcau) Lower Sandlin unit (Tsl)

Figure A3-01 cobbles claysilt general grain size sand pebbles vf f m c vc SOUTH CERRO AZUL STRATIGRAPHIC SECTION Top at 18 m. Upper Cerro Azul flow of the Servilleta Basalt (Tsbcau) 15 Basalt: Very dark