Stratigraphy, Sedimentology, Petrology, and Basin Evolution of the Abiquiu Formation (Oligo-Miocene), North-Central New Mexico

Home , Abiquiu Formation, Cerro Pedernal, Hinsdale Formation, Los Pinos Formation, Tesuque Formation

Stratigraphy, sedimentology, petrology, and basin evolution of the Abiquiu Formation (Oligo-Miocene), north-central New Mexico

M. E. VAZZANA Exxon USA- Western Division, I800 Avenue of the Stars, Los Angeles, California 91)(167 R. V. INGERSOLL Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131

The Getllogical Society of America Bulletin, Part II, v. 92, p. 2401-2483, 24 figs., 4 tables, December, 1981, Doc. no. M11209

of Smith and described all three INTRODUCTlON units, informally, as members," but

Smith (1938, p. 945) first described did not attempt detailed petrologic

the "Abiquiu Tuff" of north-central New description.

Mexico,as being composed of three units. Since then, few workers have studied

A basal conglomerate which "grades from the stratigraphy, petrology, and.prove-

unsorted granitic talus to streawlaid nance of the Abiquiu Formation, which.

gravel, also granitic" underlies a "hard, crops out in Rio Arriba anr9andwal

I compact, limestone" which is overlain by Counties in northern New Mexico (Fig. *:i "tru,e tuff." Smith (1938, p. 947) also 1). Many workers'have summarized and

noted the "unique occurrence of a 5-foot extrapolated Smith's work (Bingler,

.bed of flint on the $opes of Cerro 1968; Church and Hack, 1939; Galusha

Pedernal about 175 feet above the base of and Bick, 1971'; Smith and Muehlberger,

the formation" (Fig. 1). Church and Hack 1960; Smith and others, 1970). Incon- .?, (1939, p. 621) further described this sistencies in published maps of this

''flint'' ("usuAlly a pearly white though region have developed due to the lack a flecks and stains of red, yellow and black of detailed petrologic and stratigraphic

. . . are common") and coined the name descriptions. Smith and Muehlberger

Pedernal Chert. They reiterated the work (1960) compiled the work of previous 2401

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I are0 of SCALE m U.S. Forss? Service Roads 0 10 20 krn Peaks

c I , Figure 1. Location map of geographic features in study area. Stippled pattern indicates

Abiquiu Formation outcrops. Average paleocurrent directions also are shown, with members

indicated as follows: L, Lower; T, Transitional from Lower to Upper; U, Upper.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/12_Part_II/2401/3419025/i0016-7606-92-12-2401.pdf by guest on 30 September 2021 authors for their "Geologic riap of the study. On the basis of five thin

Rio Cham Country." Galusha and Blick sections, he characterized the "upper

(1971) identified paleontologic material portion of the Abiquiu Tuff" as being

within Santa Fe Group outcrops, mistaken11 rich in quartz and feldspar, with

identified as Abiquiu Formation (T. generally low lithic content. He

Galusha , 1978, personal commun. ) and further stated that volcanic lithic

suggested that the formation is limited fragments make up 982 of the total I to the Espszola basin. Church and Hack lithic population. "Cement and

(1939) , Woodward and Timer (1979), and matrix'' constitute between 8% and 20%

Woodward and others (1974, 1976, 1977) . of the total rock.

have mapped Pedernal Chert and the basal The present study was initiated

conglomerate as far west as San Pedro to rectify the inconsistent and dis-

Peak and vicinity (Fig. 1). Kelley parate use of the term "Abiquiu For-

(1978) shows sediments formerly included mation." Fieldwork, completed during

in the Los Pinos Formation (Atwood and the summer months of 197thdj 1979,

Mather, 1932; Manley, 1981; Smith and and microscopy were threefold ,in pur-

Muehlberger, 1960) as Abiquiu Formation pose: to review and define the

without explanation as to why the map stratigraphy of the Abiquiu Formation;

units weqe changed. to compile an outcrop map (1:125,000)

The mast recent work which deals with using published maps and reconnais-

the Abiquiu Formation (Manley, 1979) sance and detailed mapping; and to use

briefly discusses radiometrically dated detailed sedimentary petrologic and

volcanic debris within it and suggests paleocurrent analyses of the Abiquiu

sources from within the San Juan volcanic Formation €or determination of prove- ' field. Wilson (1977) has done the only nance and regional lithowic rela-

detailed sedimentary petrographic work * tionships. These three types of

on the Abiquiu Formation prior to this investigation provide data for detailed

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basin analysis throughout Abiquiu time Regional Features and allow for integration of Abiquiu

sediments into a regional tectonic and Brazos Uplift. The Tusas Moun-

paleogeographic framework.. tains of northern New Mexico are the

The present paper is a condensation remnants of a southeast-trending,

of an M.S. thesis by the senior author Laramide block-faulted highland (the ...-a (Vazzana, 1980). The results have been Brazos uplift, Fig. 2) about-_ 40 k.ni ,I - presented orally by the junior author wide and 80 km long (Woodward and a (Vazzana and Ingersoll, 1980), who super- Ingersoll, 1979). .This highlgnd

vised the research. merges with the San Juan Mountains

in the vicinity of the Colorado-New REGIONAL SETTING Mexico border. Rocks exposed in Introduction this uplift are Precambrian; Mesozoic,

Outcrops of the Abiquiu Formation Tertiary, and Quaternary age. Pre-

trend south-southwesterly (Fig. 1) and cambrian rocks are dominantly quartz-

cross the boundaries of four major tec- ites, muscovitic quartzites, and

'tonic elements of north-central New feldspathic schists (Barker, 1958).

Mexico: the Chama basin (platform), The quartz$%es are commonly "compact.,

the- EspaGola basin, the Jemez volcanic hard, coarsp-grained, gray f&d]

field, and the Nacimiento uplift (Fig. vitreous (Tjingler, 1965, p. 19).

2). Geometric and kinematic analyses The dominant type is 95% pure quartz

of each tectonic clemen't within and with acruate layers of specular hema-

adjacent to the depositional basin pro- tite which may represent relict cross-

'vide a background essential to the bedding; this quartzite was named

interpretation of Abiquiu sedimentation. Ortega quartzite by Just (1937). It

can be traced from La Madera Mountain

no rthwes twa rd through the Ortega

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7 0 10 20 30 40 Km

,Figure 2. Schematic tectonic map of north-central New Mexico

(after Smith and other, 1961). The modern Tusas Mount'ains are

the remnants of the Brazos uplift.

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Mountains (Fig. 2). Muscovitic quartzite, 2,130 m of structural relief (Woodward

aluminous schist, kyanite-muscovite schist, and Ingersoll, 1979).

feldspathic schist, and granitic gneiss COI Nacimiento Uplift. The Nacimiento

pose the remaining Precambrian rock types. uplift consists of an east-tilted

These are covered by a thin veneer of Palec fault block, about 80.km long and

gene sediments, including 38- to 28-m.y.- from 10 to 16 km wide. It is bounded

old volcaniclastic detritus of the Conejos on the west by the Nacimiento fault,

Formation (Potosi volcanic group) (Butler, which has at least 3,000 m of struc-

1946; Lipman and others, 1970). tural relief with respect to the San

The uplift was topographically high Juan basin to the west (Woodward, 1974;

during most of the Laramide time (latest Woodward and others, 1972). Sediments

Cretaceous through Eocene), but the shed westward clearly indicate that

presence of the voluminous Oligo-Miocene the uplift was topographically high

Los Pinos Formation suggests that it sub- during Laramide time (Baltz, 1967;

sided and/or was eroded thereafter Chapln and Cather, 198l), although it

(Barker, 1958). Some northwest-trending probably owes some of its structural

faults which were downthrown to the east relief to Neogene deformation related

during Laramide time experienced reversed to formation of the Rio Grande rift

relative motion in the Neogene (Muehlberger (Kelley, 1950). Paleozoic and younger

1960). Bingler (1968) cites eastward dips sediments on the eastern flank are

of from 3' tQ 5' in Neogene sediments. obscured by thick deposits of the

The BrazoS,uplift is bounded on the Jemcz volcanic field (Smith and others,

east and south by thaSan Luis and 1970). Prior to Jemez activity, the .

Espahola basins, respedtively, of the Nacimiento uplift was beveled, and * Rio Grande rift. The/Chama basin forms , rocks of Oligocene and Miocene age

an irregular, arcuatg western margin and (including the Pedernal member of-the

is separated from the Brazos by as much as I Abiquiu Formation, see below) were

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deposited on Precambrian, Paleozoic, . is a north-trending, ovate basin

and Mesozoic rocks (Churchand Hack, 1939: about 38 km across and 80 km long. I ,I San Juan Basin. The San Juan basin It is nestled among the Gallina-

(Fig. 2) of Laramide age (Kelley, 1950) Archuleta arch on the west, the

is ovate, with a 290-km long axis and Nacimiento uplift to the southwest,

a 215-km short axis. It is bounded on the Jemez volcanic field to the south,

the east by the Nacimiento uplift and the Espagola basin to the southeast,

the Gallina-Archuleta arch. Its north- and the Brazos uplift to the north

west-trending axis is doubly plunging and east (Fig. 2). Tilted upward

and slightly bowed to the northeast. The Paleogene and NeoBene sediments lie

basin is part of the Colorado Plateau with angular unconformity above L (Kelley, 1955) and contains rocks ranging tilted Paleozoic*..Mescyzoic, and

in age from Paleozoic through Quaternary. lower Paleogene rocks.

Gallina-Archuleta arch. The Gallina- Because the Chama basin lies at

Archuleta arch is the northward continu- an intermedia* structural level

ation of the Nacimiento uplift and sepa- between the/San Juan basin and the

rates the San Juan basin from the Chama Brazos uplift , Muehlberger (1960,

basin. At a point near the town of 1967) refers to it as a "platform."

. Gallina, the Nacimiento fault flattens The dominant structural feature, how-

and becomes a faulted anticline which ever, is the medial, broad, open

broadens to merge with the north-plunging Cham pcline.

anticlinorium (Woodward, 1974). Struc- Rio Grande Rift (Depression). The

tural relief between the arch and San Rio Grande rift is actually a series

Juan basin is -2,600 m, whereas between of linked, en echelon, tilted graben

the arch and Chama basin relief is only which run 1,000 km from central Cow - 460 m (Woodward, 1974). rado to southern New Nexico (Chapin 5 Chama Basin Platform. The Chama basin (1979). For this reason, it has been

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Figure 3. Measured stratigraphic sections and correlations.

Figure 3 appears on the following frames.

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n JEMEZ MTNS

0 \ area of map NEW MEXICO

Strotigraphic columns SCALE _L -...-.. USFS roads 10 0 *Okm a Peaks SANXA FE&'

Locations of. measured stratigraphic columns

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I801

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STRATIGRAPHIC CORRELATION FOLR THE ABIQUIU FORMATION by: Michael. E. Vazzana

I980

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/12_Part_II/2401/3419025/i0016-7606-92-12-2401.pdf by guest on 30 September 2021 referred to as a "depression" by some of the present topographic basin

authors (Kelley, 1952). The width of are marked by basalt flows of the

the rift varies from basin to basin, Servilleta Formation and the Cerros

reaching as much as 80 km (Chapin, 1971), del Rio volcanic field, respectively

and generally increases from north to (Manley, 1979).

south. Jemez Volcanic Field. The .lemez

Tertiary sediments preserved within volcanic field consists of uppermost

the northwestern part of the Espasola Miocene to Quaternary extrusive

<. basin include the El Rito, Ritito, rocks which flowed across the western

Abiquiu, Tesuque, and Chamita Formations margin of the Rio Grande rift onto

(Budding and others, 1960; Galusha and the eastern flank of thq Nacimiento

Blick, 1971; Manley, 1979; Spiegel and uplift and the southern end, of the

Baldwin, 1963). The Tcsuque and Chamita Chama basin. Two major calderas

Formations are part of the Neogene have been identified within the field

alluvial and fluvial deposits of the by Smith'and others (1970). The

Santa Fe Group, which dominates basin older Toledo calderais smaller (7 km

fill (Baldwin, -1956; Baltz, 1978; across) than the Vailes caldera (18 km

Manley , 1979). across), though both formed after the

The Espasola basin is an asymmetric, collapse of large vents (Ross and

west-tilted graben about 40 km long and others, 1961). Rock compositions

65 km wide. The Espagola basin is sepa- 'range from older' (Mio-Pliocene) olivine

rated from the Chama basin by a series basalts-and rhyolites through younger

of high-angle, normal faults downthrown (Pleistocene) rhyolites, latites,

to tiie east (Kelley, 1978). Volcanic ash flows, and tuffs (Smith and

rocks from the Jemcz volcanic field others, 1970). On the eastern margin

cover much of the western half of the' of the field, Quaternary volcanic

basin. The north and south margfns rocks are cut by intrabasinal, rift-

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related, high-angle normal faults, down- Coney and Reynolds (1977), and Keith

throwrr.-to-the ... east. (1978), who noted the absence of large volwes of igneous rock from Paleogene Tectonics 70 to 40 m.y. B.P. Presumably,

Laramide tectonism began during latest the shallow angle of subduction during

Cretaceous time in response to plate- the Paleocene-Eocene prevented the

tectonic interactions on the western descending slab from reaching a depth

margin of North America (Dickinson and at which melting could take place.

Snyder, 1978). C'ompressional strain was Dickinson (1979) has postulated

relieved via basement deformation in the that concurrent with the Paleocene-

Rocky Mountain region (Woodward and Eocene magmatic lull, the slibduc ting

Ingersoll, 1979). Kadiometric dates on slab shallcwed to near-horizontal,

rocks extruded during the Laramide causing it to scrape along the bottom

oroogeny show a reduction in volume and of the overriding plate. This caused

eastward migration from 80 to 60 m.y. "deep-seated shear" and resultant

B.P. due to flattening of the subducting upthrusts and transforms of typical

slab (Dickinson and Snyder, 1978). The Laramide style. (See Chapin and

magmatic lull evident in the western Cather, 1981, and Lucas and Ingersoll,

United States during the Paleocene- 1981, for further discussion.

Eocene (Damon and others, 1964) was the Neogerqe, Tectonics result. This trend reversed in direction

and volume from 40 to 25 m.y. B.P., By 30 m.y. B.P., the ridge s'epa-

resulting in renewed volcanism in the rating the Farallon and Pacific plates

Oligocene as the slab steepened and had begun to interact with the North

descended rapidly (Dickinson, 1979). American plate and the convergent

This hypothesis has found sbpport in margin began changing to a transform

the work of Damon and others (19641, (Atwater, 1970). The change has

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continued between two migrating triple manifestation of the Basin and Range

junctions, one to the north and the other province and has experienced a simi-

to the south of the latitude of New Mexic lar history of extension,. localized

(McKenzie and Morgan, 1969, p. 129); along existing crustal weaknesses

this interaction has promoted exteision (Chapin and Seager, 1975 ; Woodward

across the Basin and Range province and Ingersoll, 1979).

and the Rio Grande rift (Atwater, 1970); AGE OF THE ABIQUIU FOFGWTION Dickinson'and Snyder, 1979a). Dextral Regional Correlation shear along the'San Andreas fault system

(Atwater, 1970: Livaccari:, 1979) and . The absence. of fossils caused

9 the absence of a subducting slab Smith (L938) to rely upon the position

(Dickinson and Snyder, 1979b) has facili- of Abiquiu bedeelow tfi5 Santa Fe

tated extension, which has been controllec (Group) for age determination. Osborn

or at least modified, by inherited (1918) used paleontologic evidence

structural grain (Chapin and Seager, 1975; to date the overlying "Santa Fe For-

Woodward and Ingersoll, 1979). Dickinson mation" as transitional from Miocene

(1979) has proposed tha; as the detached to Pliocene. Using this and the litho-

lithospheric slab continued its descent a logic similarity between the Abiquiu

below the North American continent, the and Conejos Formationk, Smith (1938)

plastic asthenosphere experienced induced deduced a Miocene age. Valentinian-

upflow to fill the void left by the Clarendonian taxa found in the over-

,descending slab. Later stages of this lying Chama-El Rito Member of the

process are manifested in the crust by Tesuque Formation require that the

the extensional tectonics of the Basin Abiquiu Formation be older than

and Range province which has occurred Pliocene (Galusha and Blick, 1971).

from 25 m.y. B.P. to the present. The Using stratigraphic relationships,

Rio Grande rift is the easternmost Bingler (1965, p. 65) found the Los

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Pinos Formatioq to be upper Oligocene Fe and Abiquiu in his structure sections d to lower Miocene; he called outcrops (Kelley, 1978). LatCral and vertical

"Los Pinos" in the El Rito area that Smith grading among the Abiquiu, Los Pinos,

(1938) originally had named "AbQuiq Tuff.' and Chama-El Rito bedsin surface Bingler (19681, p. - 36-37) alsP stggested outcrops west of Ojo Caliente (May, that the "lower 300 feet of Preumbrian 1980) further complicate such projec-

clast gravel included in the Abiquiu Tuff tions.

are the lateral equivalent of the Ritito Recent palynological work (DuChene

Conglomerate," and, that the "buff siltstonc and o,thers, 1981) is consistent with I of the Santa Fe . . . intertongues with the interpretation that most of the the Los Pinos." Butler. (1971) points out Abiquiu is Miocene, with the lowermost

that Ritito lithologies vary in age at part.possibly being Oligocene.

different localities. All of the above Radiometric Dating information is consistent with an age

'determination of latest Oligocene to A basaltic dike (Cerrito de la

Mioceqe for the Abiquiu. Ventana) that cuts the upper member r. Kelley (1978) states that "projections has been dated by Sachman and Mehnert

of the Tesuque and the Abiquiu Tuff in a (1978) at 9.8 -+ 0.4 m.y. Manley and

structure sectio'n across the entire b1ehner.t (1981) report an age of 15.9 '+ c EspaGola Basin [suggest that J the Abiquiu -+ 0.9 m.y. for another dike cutting the thins in the subsurface by interfingering- ~Abiquiu. Two basaltic flows within the . . . with the Chama-El Rito, Pojoaque, upper member adjacent to Cerro Negro and Skull Ridge beds'of the Tesuque [For- (May, 1979, 1980) have dates of 18.9

mation] ." V.' C. Kelley (1979, personal -+ 0.7 m.y. and 22.12 0.6 m.6. commun.) attempted to demonstyate. the (Baldridge and others, 1980). Inter-

imprecise nature of these projections by bedded basalt flows in the Los Pinos

showing a saw-tooth contact between Santa ' Formation ih Colorado have been dated

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radiometrically at 26 to 2 m.y. (Lipman STRATIGRAPHIC RELATIONS and Mehnert, 1975). A daci,te clast near

the top of the Abiquiu has been dated at The Abiquiu Formation lies with

17.3 2 0.8 m.y. (Manley and. Mehnert, angular unconformity above rocks of 1981). Precambrian, Paleozoic,. Mesozoic, and Paleogene ages. The thickest exposed Summary section lies on the south side of the

Dates from basalt flows and a clast Rio Chama west of Vkk town of Abiquiu.

high within the upper member make it It thins westward to San Pedro Peak

unlikely that the Abiquiu Formation is -'. (Fig. 1), where Pedcrnal Chert rests

much younger than 17 m.y? This cannot directly on Precambrian crystalline

be proven rigorously as yet because of rwks. Exposures on the southwest face

--% the Lack of a dated hor&on at the grada- of Sierra Negra gradc upward conform-

tional top of the formation. Using simi- ably to the pinkish tan, ,coarse- to larities in lithology. and depositional fine-grained, feldspathic sandstones of mode of parts of the Los Pinos, in union the Chama-El Rito Member of the Tcsuque

with dated basalts below the Los Pinos Formation. West of Ojo Caliente, May

Formation, an estimat>d maximum age of (1979, 1980) has mapped small exposures

26 q.y. is established. Palynological of the upper member of the Abiquiu For-

ages of the lower member south of Cerro mation which grades vertically and

Pedernaf indicate Miocene deposition. laterally into vdlcanic1asti.c sands

Thus, the Abiquiu began accumulating near and gravels of the Los Pinos Formation

the Oligocene-Miocene bohndary (24 m.y. (Atwood and Mather, 1932). The Los

B.P.) (Van Couvering, 1978) and deposition Pinos and Abiquiu are gradational and

ceased in the middle Miocene (-17 m.y. intertongue in the northeastern part of

B.P.).. the study area, as discussed by Butler

(1971) and biay (1979, 1980).

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Units 1ithologically and ternpora-lly El Rito Formation (Eocene) (see Logdson,

equivalent to the Abiquiu Formation probabl! 1981). The contact.is marked by a basal

include the Picuris Tuff (Cabot, 1938) to Abiquiu bed of laterally discontinu-

the northeast, the Bishops Lodge Mevber of ous, tan, siliceous iimestone (Fig.

the, Tesuque Formation (Spiegel and. Baldwin, 4) which varies from 0 to 0.75 m; it

1963) to the southeast near Santa Fe, and ranges from massive limestone to silici-

the Zia Sand Formation (Galusha, 1966; fied carbonate infillings in porous

GaAe, 1981) souihwest of the Jemez volcanic sandy conglomerate (Fig. 5). The

field (Manlcy, 1979; May, 1980). However, macrocrystalline (sparry) habit of

detailed correlation with any of thesc the calcite crystals is characteris-

units is difficult due to lack of continuit: tic of chemically precipitated deposits

of exposures. formed at an air-ground-water interface

Outcrops northwest of the town of within porous, permeable sediments

Abiquiu are mapped by Kelley (1978) as in arid to semiarid climates (J. W.

Ritito Conglomerate (Barker, 1958) but Hawley and S. G. Wells, 1979; persondl

are lithologically similar to rocks on commun .) .

L the Cerro Pedernal originally designated STRATIGRAPHY as part of the Abiquiu Formation by Smith Measured Sections (1938). .These outcrops are considered

herein to be part of the lower member of Because the Abiquiu Formation lacks

the Abiquiu. Thfs Precambrian-clast , non- any laterally continuous, discrete

volcanic,. sandy conglomerate grades vert i- horizons, correlation is tentative

cally into the upper member. west of Abiquiu (see Fig. 3). The chert horizons within

(see Fig. 3.); The base of the conglomer- the Pedernal member provide a broad'

ate lies with angular unconformity (as zone which may be used to correlate

much as 30") above hematitic quartz arenite outcrops in the southern part of the

and arenaceous quartzite conglomerate of study area, but they are absent northeast

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Figure 4. Laterally discontinuous bed of siliceous limestone (massive

light-colored unit) at contact between El Rito Formation (dark, poorly

), exposed unit) and Lower Member of Abiquiu Formation ifi Red Wash Canyon.

(This contact forms the base of column 6, Fig. 3).

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Figure 5. Siliceous carbonate (basal Abiquiu) resting upon lilted beds of

El Rito Formation in Red Wash Canyon (see Fig. 4). Carbonate may have developed

through replacement of a soil profile, as indicated by inclusions of cobbles and

sand stringers.

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of Cafiones (Fig. 1). The lenticular nature mineral content. Because of the

of deposits within the upper member and generally coarse gra'in size of the / the high degree of postdepositional crystalline lithic fragments, few

faulting make precise correlation within remain as discrete lithic grains in

\ this m&er impossible. Correlations the sand-sized fraction.

illustrated in the composite sttntigraphic Lower member coiigloirierates are eroded

column (Fig. 6) are estimates based on easily and readily form talus slopes of

lithology and regional trends of thickness. 35" to 50". This, in combination with

present elevations (2,100 to 3,000 m) Lower Member and vegetation typical of the Rocky

The lower member of the Abiquiu Forma- Mountain region, accounts for the

tion consists of 20 to 90 q of arkosic scarcity of well-exposed sections of

sandstone and gravel conglomerate. In the lower member. Outcrops below well-

general, these show few primary structures preserved, massive Pedernal Chert < other than local imbricate cobbles and commonly are well exposed and protected

poorly defined channels (Fig. 7). from erosion. Mesa Lagunas, Temolime 4 Cobble-sized material near the base Canyon, and Cerro Pedernal exhibit such

of the member consists of well-rounded, well-exposed sections of the lower

gneissic, granitic, and quartzitic (some member (for example, Fig. 7).

Ortega Quartzite) clasts and ochre-colored Fine-grained volcanic fragments,

Pennsylvanian limestone. The latter con- . dominantly vitric, exist in an interval

tains characteristic punctate-brachiopod, roughly 5 m thick below the lowest bed

crinoid, and bryozoan fgssil fragments. of chert. These fragments commonly

Coarse- and medium-grained fractions of the occur in the. lithic subarkoses.

lower member include orange to tan, poorly Locally, they are found in small, poorly

sortee moderately angular, lithic sub- preserved channels (1-3 cm deep). # arkoses (McBride, 1963) with minor heavy- The unsorted, sheetflow-type deposi-

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IIC

(1-17)

I O(

50 80 (2-54) ...... ,....a .. . 40 70

30 60 U 20 50

10 40

0 30

9 Batranca

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79 1 (grades to Chama-el rito Member)

Red Wash Canyon Madero Canyon to Sierra Negra~-

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Figure 6. Composite stratigraphic column based upon

lithology and regional trends of thickness.

FigurB' 6 appears 'on the following frame.

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chert

cross-beddiog

channel, cut-and-fill ,MRluq b ,overturned cross-bedding

atrrmx3 imbricate cobbles

e, characterized by increased volcanic deCrea8e'd pB crystalline clasts

Figure 6.

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Figure 7. View of north wall of Temolime Canyon showing conglomkrakic sand-

stone of Lower Member. Several cobbles have imbrl;cations.showing southgesterly

palebcurrent direction;. Exposed part; of trees'in foreground are 2 to 5 m

high.

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tion (Bull, 1972). The large lateral lies above the low? member, but on San

extent of this member sugges'ts multiple Pedro Mountain it lies directly on Pre-

fan systems which have coalesced into a cambrian crystalline rock.

broad low-angle piedmont deposit. In the transition zone between the

I The uppermost 1 to 2 m of the lower lower and the Pedernal members, there

member contain significant quantities locally exists a lithologic sequence

of weathered feldspars and heavy minerals, that probably is characteristic of a

thus indicating a period of weathering weathered horizon. This seqnence con-

between lower and uaper member deposition. tains a tan to buff, fine-grained sandy

claystone at the base; feldspars are Pede rnal Member weathered to kaolinite in a zone typi-

The middle .member of the Abiquiu For- cally 4 to 25 cm thick. The sandy

mation consists ok a variable number of claystone grades upward to -a 1- to 5-

chert layers interbedded wjth medium- cm zone' of pale green, fine-grained

grained gneissic and volcanic conglomer- sandy claflstone. Above this, and immedi-

ates (0-50 m). The chert horizons are ately below the chert, is a punky, white

known collectively as Pedernal Chert siliceous carbonate material 0.25 to

("pedernal" is Spanish for "flint"). 25 cm thick (Fig. 8). Commonly, it

The& layers1 originally were referred to oCcurs in finely laminated layers which

as "flinty layers'' by Smith (1938) and can be separated with a thumbnail. In

later named "Pedernal Chart" by Church the next 25 to 4 cm, limestone-coated

and Hack (1939). Commonly, one discrete nodules of chert occur in a zone of

layer exists; however, two, three, and white, punky carbonate; this zone grades

four layers exist in the vicinity OP Cerro upward into massive chert. Depending

Pedernal, and farther south in the Cerro upon locale, massive chert ranges from

Valdez area (Fig. 1). In regions north' 0.6 to 1.8 m thick. On the north side .east of the San Pedro Mountains, the chert of Cerro PedFrnal, the chert is 3.5 m

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Figure 8. Contact between chert in Eedernal Member (above) and a sandstone

of the Lower Member. Note white calcium carbonate present at the interface

and incipient chert nodules suspended within it. Width of image in photo’

gGph is approximately 0.5 m.

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thick. (See "Origin of the Pedernal Chert carbonate.

for a schematic illust.ration of this In areas where mdtiple chert layers

characteristic sequence' "from unweathered occur, the stratigraphically lowest

to weathered to chert'horizons.) No con- layer is the thickest and laterally

sistent color or shade variation is appar- most extensive. On Cerro Valdez,,the

ent within the massive chert. For example lowest chert horizon is 1.75 m thick,

in an outcrop on Cerw Pedernal, the whcreas the laterally discontinuous

color sequence is: dark red grading up layer 45 m up-section is only 0.1 to

to pinkish, then red-speckled white topped '1.1 m thick. If these laydrs represent

by paraffin-colored with swirls of yellow- siliceous replacement of calicke

orange, Less than 5 m west; the same bed horizons, then'this phenomenon suggests

shows yellow-orange swirls in mixed that the lowest weathering horizon was

paraffin- and black-colored chert which also the 'best developed.

lies directly above nodular chert. Here, Upper Member the top of the bed is black.

Commonly, a hard layer of gray to tan, A sequence ofapproximately 170 m of

massive calcium. carbonate rests directly pinkish-tan to white, fine- to medium-

upon the massive chert. In places, the grained volcaniclastic sandstone lies carbonate is absent, but more commonly it above the Pedernal member,- and is exists as 9 5- to 10-cm-thick layer, designated the upper member. Medium-

which is most likely a caliche horizon, to coarse-grained beds contain small

developed within a soil profile and sub (0.25 to 0.75 cm) andesitic fragments,

sequently replaced .by silica (see "Origin - which weather to 'form a pocked surface

of the Pedernal Chert"). It'may, how- typical of beds north and west of the

ever, represent only a zone alongtrhich town of Abiquiu (Fig. 9). Coarse sand-

groundwaters, which could noi penetrate stone beds rongc 'from 0.1 to 1.2 m in

the underlyivg chert, deposited calcium thickness and alternate vertically with

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Figure 9. Basaltic cobble resting in medium-grained, massive volcaniclastic c sandstone of Upper Member in western wall of Arroyo del Cobre. Pocked surface

is due to preferential weathering qf andesitic lithic fragment's. Box is

approximately 5 cm wide.

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thinner, f iner-grained beds. This alterna- between ihe Pedernai and upper members

tion of thick, coarse-grained sand beds is difficult to determine precisely in

and thin, fine-grained sand and mud beds . the, field. The subtle change from

(due to lateral migra'tion of drainage tanQish sandstones with granitic',

patterns on the piedmont surface) gives gneissic, and volcanic detritus to

outcrops a, banded appearance. Fine-grainec sandstones with exclusively volcanic

layers commonly are buff to tan and clayey detritus lies above the uppermost chert

did may contain aligned heavy minerals. of the Pedernal member. In the vicinity

\.!ell-defined channels with gentle meanders. of, and north of, the town of CaGones

exist locally within the sandstone (see (Fig. l), where Pedernal Chert is

"Pdeocurrents") , and isolated lag ab-senr, the contact between upper and

deposits of cobbles and boulders are low'er members is gradational. At

common; flat clasts locally show imbri- Blanca Mesa (near CaGones) , the lower

cation. member grades upward into a 3-m-thick

Locally, large basaltic and rhyolitic zone .of mixed volcanic and Precambrian

cobbles are present within otherwise crystalline clasts, above which i$ a

massive beds of bone white, medium- 0.1-m layer containing dark red chert

grained volcaniclastic sandstone (Fig. 9). (Pedernal Chert?) fragments. Two

It is difficult to envision flow conditions meters above the transitional zone

under which both size fractions might be rests typical upper member volcaniclas-

deposited by floyikip watbr. One possible tic sandsronc.

explanation for this widely disparate, Summary bimodal distribution is tha: volcanic ejecta (bombs) from local vent;s underwent The Abiquiu Formation. includes about --in situ rounding; however, do local vents 300 m of chert and clastic sediment

are *exposed. principally composed of Precambrian

As mentioned previously, the contact crystalline and Tertiary volcanic

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detritus, is 70-90 m thick in the south. PhLEOCURRENT ANALYSIS Thicknesses of individual cobble-conglomei

ate layers are impossible to determine All three members of the Abiquiu

for lack of sharp, laterally continuous Formation contain sediments in which

breaks (Fig. 7). This member was deposit3 paleocurrent features have been pre-

on alluvial fans. served. However, the degree of i" The Pedernal member is variable in siliceous replacement (chert hor,izons)

thickness (0-50 m). Near Abiquiu, the nd the nonlithified nature of clastic

chert is absent, whereas as many as four layers (especially in. the. upper member)

chert horizons can be seen on the north obscure most primary structures.

wall of Temolime Canyon (Fig. 1). These The lower member is a poorly sorted,

are separated by mixed'volcaniclastic cobble conglomerate in which imbricate

sand and pebble conglomerate liyers. cobbles are abundant and can be measured

Local erosion and reworking of the Pederna with precision. Small (20-cm-wi,de)

member prior to upper member deposition st ream channels can be distinguished

reduced its thickness to only a few (Fig. lo), bu& because of the poor

meters; weathering processes resulted in preservation and highly variable

the formation of ,the chert horizons. orientation of channel axes, these

The upper member is approximately 170 m were not used in paleoc*-rrent measure- z thick and is exposed best in the vicinity, men t .

and dorth, of Abiquiu. The volcaniclaktic Pa1eocurrent.s were measured in the

sandstone'is massive and is interbedded Pedernal member only at Blanca Mesa,

with thinner, muddy beds. A few inter- where it is transitional in lithology

bedded ba:alt flows occur within this between-the upper and lower members.

member. Alluvial processes deposited The upper member consists of moder-

this member, as evidenced by numerous ately well-rounded, volcaniclastic

channels. sandstoGes in which little-vertical

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2133 20 I (2- 51)

? - 1 10 (2-52) L

0 Cerro Voldez (Mesa Lagunas)

(2-62)

San Pedro Mountains

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Cerro Pedernal (Temolime Canyon)

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scale in meters

I- 23) sample' location chert

limestone'( silicif iedb. paI eoc ur r ent deter minot ion showing azimuth mean and clay stone number of readings clayey sandstone

sandstone

conglomeratic sandstone

U Upper Member conglomerate I basalt P Pedernal Member crossbedding

channel, cut-and-fill stn cti re L Lower Member over turned crossbedding

T transitional between L imbricate cobbles and U .

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Figure 10.' View southeast of alternating thick- and ;thin-bedded sandstone

and siltstone in Arroyo del Cobre. Note lateral discontinuity of individual

beds and shallow channel in outcrop in foreground, indicating lateral facies

changes between channel and overbank deposits.

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,grading (upward fining) is visible. Some from imbricate pebbles and cobbles

grading can be seen in cobble- and pebble- were measured with a Brunton compass.

sized'material in channel lag deposits. The strike of the.flat surface (the

In Arroyo del Cobre and to the east in long-intermediate-axes plane) was

adjacent outcrops, these channels range assumed to be normal to flow. Flow ,

in size from 20 m wide and 7 m deep 4 direction is shown by the upstream dip

(Figs. lla, llb) to less than 0.5 m wide of these -flattened cobbles (Potter and

and 0.1 m deep. Their axes appear to be Pettijohn, 1977). Channels were observed

moderately linear and their trends can be in three dimensions, and the axis of

estimated. Local large-scale (1- to 2-m each was measured with abBrunton com-

cross-bedding can be seen from a distance, pass. Paleocurrent measurements were

but, more commonly, small scale (1- ta.2- rotated to horizontal using stereo-

cm) cross-bedding containing layers of graphic projections to determine true,

heavy minerals (as in Madera Canyon) is (horizontal) paleocurrent directions.

present. Howeber, such structures can Oscillation ripples noted by Wilson

be seen only on weathered surfaces, thus (1977) were observed in Arroyo del

making three-dimensional analysis diffi- Cdbre and Madera Canyon (Fig. 1) within

cult. Two 1ocations.est of Arroyo del 'fineJgrained mudstones. These ripples c Cobre contain soft-sediment slump struc-. are symmetric and cuspate and show

tures,in the form of overturned cross-beds no preferred transport direction.

which indicate paleocurrent direction Paleocurrent data from each of the

(Fig. 12a). Load structures also are seven outcrops in lower, Pedernal, and

present (Fig. 12b). Imbricate pebbles upper members were rotated to horizon-

and cobbles are ubiquitous in coarse- tal and plotted on rose diagrams (Fig.

gr'ained layers, and , theref ore, serve as 13). The number of readings (N) in

good paleocurrcnt indicators. .each rose diagram indicates the number

Paleocurrent orientations determined J;of out'crops studied; each re,ading

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Note cobble-sized lag deposits at base above J. Id. Hawley's hand.

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Figure 12. A. Overturned cross-bedding in zone of

soft-sediment deformation. View northeast along axis of

fold in arroyo east of Arroyo del Cobre.

B. Soft-sediment deformation (load structure) of

clay lens within volcaniclastic sandstone of Upper

Member.

Figure 12 appears on the following frame.

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Figure 42.

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Figure 13. Rose diagrams showing stratigraphic variation

in paleocurtent directions.

Figure 13 appears-on the following frame.

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- 'MADERA , CAhYON

' ARROYO del COBRE

----- a-I Z Q c

_----

BLANCA i MESA

rr W m RED WASH z CANYON w 247" ,

TEMOL I ME CANYON

16 MESA LAGUNAS

203"/ j Figure 13.

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represents an average df several measure- alteration (feldspar and heavy-

ments from each outcrop. A temporal mineral grains) have retained sharp

trend becomes apparent when the vector grain boundaries and internal homoge-

mean azimuths from each member are aom- neity.

pared. Lower member azimuths average The lower member is highly porous

203", 242', 247-0,, and 195'; Pedernal and permeable so that well-preserved

member (transitional) azimuths average outcrops contain high amounts of inter-

158"; and upper member azimuths average stitial calcite and sili&a cements.

164' and 143" (Fig. 13). This systematic These cements tend to obscure rock a. change from norf3eastern .derivation to composition and fab'ric and, ih some

J northwestern derivation.corresponds with cases, have altered or destroyed feld-

changes in petrology (provenance), spar and volcanic lithic fragments.

which are discussed below. Data are presented below to illustrate

the various lithic and feldspar grain PETROLOGY. AND PROVENANCE types found in the lower member, with Introduction the understanding that some of the

Diagenetic processes which occur in or:ginal detrital content has been

rocks exposed to arid and semiarid altered by diagenesis and weathering.

environments can mask and alter original Petrologic data discussed here may

detrital compo$ents of, sandstones. This be found in Vazzana (1980).

can lead to qsinterpretation of prove- Gravel Compositions nance (Walker and others, 1978), unless

steps are taken to ''see through" these Pebble- and cobble-sized clasts

alterations. Fortunately, rocks of the (6.0 to 25.0 cm) were counted in out-

Pedernal and upper members of tge Abiquiu crops of the lower merr.ber for provenance

Formation are relatively free of diage- determination and for comparison with

netic alteration. Prime targets for such sandstone data. The numbers of points

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counted in the 20-cm by 20-cm counting difficult,’ but Sangre de Cristo .aqd

grid were determined by the size of the Nacimiento uplifts are most likely:

outcrop, although the goal at each out- Volcanic rocks: rhyolitic to +de- I

crop was 100 data points. Cobble cate- sitic (no basalt); sources in the San

gories include gray quartzite, pink quart: Jaun or related volcanic fields.

ite, granite, gneiss, milky quartz, chert Sandstone Petrography: Methods . limestone, feldspathic arenite, diorite,

ampbibolite, phyllitc, rhyolite, and Medium-grained, massive sandstone

‘andesite, .and were recombined into the samples were chosen for petrographic

following categories (see Table 1) : analysis. Thin sections were made

Quartzites: gray and pink varieties, (friable samples were impregnated with

specular-hematite-bearing; sources in the epoxy), and a point-counting grid of ,

Ortega Mountains and vicinity. 0.5 mm by 0.5 mm was used. Four hundred

Precambrian cyrstalline rocks: granite points per thin section were counted

pegmatites, diorites, milky quartz, and using a mechanical stage. For each

gneissose rocks; sources in the Brazos- count, percentages of the various cate-

Sangre de Cristo uplift, and possibly gories listed below ere obtained along

the Nacimiento uplift. with the confidence limits for each, as

Metamorphic rocks: phyllites and outlined by Van der Plas and Tobi (1965).

amphibolites; sources in the Brazos- Grain categories include total

qangre de Cristo uplift. quartz (Q), feldspar (F), and lithic

Sedimentary rocks: dominantly ochre- grains (L) as described by Dickinson

colored Pennsylvanian dimestone, contain- (1970). Each of these is divided further

ing numerous brachiopod and bryozoan into subgroups: Q includes both mono-.

fossil fragments and minor arkosic detri- crystalline (Qm) and polycrystalline

tus; widespread nature of Pennsylvanian (including chert) (Qp); F includes K-

limestone makes source determination feldspar (K)’ and plagioclase (P) ; L

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TABLE 1. COBBLE PERCENTAGES (OF TOTAL COBBLES) FROM LOWER MEMBER IN RECOMBINED CATEGORIES. NUMBERS IN PARENTHESES INDICATE TOTAL "l3ERS OF COBBLES COUNTED. LOCATIONS MAY BE FOUND IN VAZZANA (1980).

~~ ~ ~~ location quartzites p6 xtalline detas seds volcs

2-66 52% 19% 19% 0 -11% (100)

2-63 . 70% 20% to 0 0. (81)

2-63a 66% 34% 0 0 0 (80)

2-64 41% 46% 12% 1% 0 (79)

2-64a 46% 43% 11% 0 0 (83)

1-27 5% 62% 8% 25% 0 (60) ~ . c. 1-27a 7% 52% 3% 38% 0 (60)

2-5 2 0 77% 0 23% 0 (75)

2-52a 0 75% 0 25% 0 (75)

2-62 3% 95% 0 2% 0 (81)

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includes sedimentary (Ls), metamorphic counted unless no more-suitable samples

(Lm), ..nd volcanic (Lv) lithic fragments. were available.

The latter category is dssigned on the Sandstone Petrography: Results basis o.f texture to one of four sub-

categories: vitfic (glassy or pumiceous) Lower Member. Sands within the

(Lw), felsitic (of generally rhyolitic lower member are generally porous,

or latitic composition) (Lvf), microlitic poorly sorsed, and angular to subangular,

(of generally andesitic and intermediate with 20% to 75% of the pore space

composition) (Lvmi) , and lathwork (of filled with calcium carbonate. Some

generally basalticcomposition (Lvl). destructionof feldspar grains (recrystal- Heavy minerals.. are divided into pyriboles lization to calcite) has occurred, but (Py), opaques (0), and other nonmagnetic most grains have retained moderately

heavies’(1I). Interstitial (I) includes sharp boundaries. Petrographic

cement (C), voides (V), and matrix (M). data-fr& the lower member show gener-

Rocks of the Abiquiu Formation contain ally high quartz content, intermediate ‘4 principhlly two types of matrix: epi- lithic content, and low feldspar cop-

matrix and peudomatrix (Dickinson, 1970). tent. Quartz occurs as both strained

Epimatrix is dominantly kaolinite ,and and unstrained varieties, lithic frag-

silica, but as these are also cementing ments are dominantly metasediments

agents, they are included in the 0 cate- (quartz-mica tectonites), and feld-

gory, Pseudomatrix includes easily spars include plagioclase and micro-

deformed rock fragments (usually vol- cline in nearly equal abundance.

canic rocks in the Abiquiu Formation) ;, Much of the quartz present in the I in Ao case are they so deformed as to lower member is strained, suggesting

prevent categorization into their proper metamorphic and/or plutonic sources

lithic types. Thin sectrions showing (Basu and others, 1975), an interpre-

high percentages of cement were not tatign consistent with;&he high amounts

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of plutonic and metamorphic quartz in the ing may have destroyed sand-sized

gravel-sized fraction. The low Qp/Q carbonate detritus both before and

values for the lower member and the after deposition. Postdepositional

lack of high lithic percentag&s*.imply destruction of carbonate detritus may

that the metamorphic soilrce(s) may have be,reflected, in part by large

been volumetrically less significar.t ,amounts of calcite cement.

than the plutonic source(s) (Dickinson, Feldspar grains are spars= and

1970), or that metamorphic sources were are dominantly plagioclase. Mi$rocliqe.

coarse-grained and high grade (for example is the dominant potassium feldspar.

gneiss). Microlitic and vitric lithic grains

Lithic components of the sandstones (Lvmi and Lw) constitute 100% of

range from metamorphic and plutonic the total volcanic lithic content.

detritus at the bottom of the member to Microlitic grains diagenetically

volcanic and sedimentary constituents altered rapidly, and they look similar

nkar the top, whereas plutonic, meta- to alt6red vitric lithics; consequently, .w morphic, and sedimentary clasts-character- high vitric-lithic-grainp. percentages

ize the cobble-sized fraction throughout partly may reflect misident i fica tion

the member. Local braided channels of altered microlitic vol’crpic lithic

within the upper 5 m of the member con- grains.

tain high amounts of glassy material Calcite, silica, kaolinite, and

(Lvv). The absence of limestone detritus hematitic staining act (in varying

in the sand-sized fraction, although proportions) as cementing agents in

present as gravel clasts, may be a func- the lower member. The most common

tion of the rapid rate of chemical cement is sparry calcite, which fills

weathering -of carbonates in small grain pores and replaces some feldspar

sizcs due to their l+rger sui-face area grains. Silica tends to line pores

to volume ratios. This chemical weather- and commonly can be seen as thin films i

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between quartz grains. Kaolinite usually partially corroded quartz grains.

exists in thin gold-flecked films which Cement ranges from 6% to 25% in these

coat individual monocrystalline and poly- moderately porous rocks. Caicite

crystalline grains. also occurs locally as pore-filling

The lower member plots near the Q cor- cement.

ner of a QFL ternary diagram (Fig. 14). Monocrystalline quartz occurs as

This is to be expected due to the h.igh both we;l-rounded and highly angular

percentage of plutonic and metamorphic grains, suggesting the presence of both

constituents. The low lithic (L) per- reworked and first-cycle quartz. The

centage is apparently a tunction' of the . angular quartz, in' general, is un-

large grain size of plutonic and meta-. strained, which may indicate volcanic

morphic rock fragments. Individual sources.

crystals from these fragments eithgr Plagioclase and microcline are- .

have been broken into solitary grains or present, although the former is the -are coarse enough to be counted as mono- dominant feldspar. Increased contri- crystal,line grains (Dickinson, 1970). bution of andesitic volcanic detritus

Pedernal Member. The Pedernal member in the younger sediments (glass shards .,*' *-* is transitional between the subarkosic -are common) probably is responsible -I *- cbmposition of )&he lower member and the for the increase in the P/F ratio

volcaniclastic,composition'6f the upper from lower member through upper member.

member. The presence of paleosols (see Figure-14 indicates that the Pedernal

"Origil) of the Pedernal Chert") and the member has compositions which overlap

high silica content in the overlying those of both the lower and upper mem-

upper member have combined to formf.loca1- be"rs. In Figures.15 and 16 (Qn-h-Ls

ized, highly siliceous beds. In thin and Qp-Lv-Lsm plots) (Graham and others,

section, high silica content is evidenced 1976; Ingersoll and Suczek, 1979),

by pore linings and siliceous rinds on Pedernal member sandstones are skewed

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Figure 14. Quartz (Q), feldspar (F), lithics (L; ternery plot of point counts of

sandstones from Upper (circles), Pedernal (triangles); and Lower (squares) members

of the Abiquiq Formation. Note grouping of Upper and Lower Members, and wide

distribution of Pedernal Member samples, illustrating transitional nature of these

beds.

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Figure 15. Metamorphic lithic (Lm), volcanic lithic (Lv) , sedimentary lithic (Ls)

ternary plot showing uniformly high Lv content of Upper Member (circles) and variable

nature of Pedernal (triangles) and Lower (squares) Members.

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sedimentary and metasedimentary lithic (Lsm) ternary plot showing uniformly high Lvm

content of Upper Member (circles) and vari,able nature of Pedernal (triangles) and Lower

(squares) Members.

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volcaniclastic detritus being shed into , typically from 0.75 to 0.95. The

the Abiquiu basin starting near the end absence of zircon, tourmaline, and

of Pedernal time. rutile in more than trace amounts and \ Upper member. The upper member con- the sharp grain boundaries (Fig. 17)

sists of a series of fine-, medium-, and of heavies indicate little chemical

coarse-grained volcaniclastic sandstones ' or physical destruction of these grains

with as much as 59% volcanic lithic con- (that is, ZTR index = 0) (Pettijohn

tent. Silica is the dominant cementing and others, 1973). -Xagnetic separations

and pore-filling agent, although in show that magnetite comprises an esti-

some specimens, calcite is dominant. mated 30% by volume of heavy minerals.

Locally, siliceous limestones exist as Most volcanic lithic grains. are

lenticular bodies with limited lateral glassy (Lw), with substantial micro-

extent. Their origin is understood litic (Lvmi) and felsitic (Lvf) contri-

poorly, although petrographic textures butions. Elos t samples contain abundant

suggest silica replacement 0-f calcite. glass shards (Fig. 18), squashed micro_

Quartz exists as both angular and litic lithic grains (Fig. 19). Previous

moderately well-rounded grains, as in workers have reported high matrix per-

t!ie Pedernal 'member. centages (see DuChene, 1973; Timmer,

Heavy minerals, such as common horn- 1971; Wilson, 1977) which probably

blende, basaltic hornblende, hypersthene, included such squashed and altered vol-

enstatite, magnetite, and garnet, occur canic lithic detritus (properly placed

in greater percentages than in the lower in lithic categories in the present

and Pedernal members; some thin sections study). The upper member has a higher

contain as much as 14% of the total lithic (L) component than the lower

framework percentage. Hornblende is the member (Fig. 14).

dominant heavy mineral, with Py/H

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Figure 17. Photomicrograph (crossed nicols) of Upper Member from Madera

Canyon illustrating hornblende grain (H) with sharp unaltered edges. Altered

vitric lithic grain (Lv) is confussed easily with true marrix (Dickinson, 1970).

Other grains.are mostly quartz, feldspar, and volcanic lithic .fragments. Long

dimension of photo equals 1.07 mm.

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Figure 19. Photomicrograph (cros$Fd nicols) of Upper Member sandstone

showing devitrified rinds enclosing microlitic lithic grains. Long dimension

of photo equals 0.7 mm.

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Church and Hack (1939) originally cited detritus. A zone 1 to 2 m thick’helow

the Pedernal Chert as evidence for a the chert is exposed locally and

paleoerosion surface which had been exhibits many features characteristic

exhumed in the San Pedro Mountains. No of soil development. Figure 20

study directed specifically at the origin schematically represents the proposed

of thecherthas been published since that paleosol profile, observed on the

time, even though it is one of the thickest southwest flank of Cerro Pedernal ((ee

nonmarine chert horizons described in the Fig. 8) and the northwest wall of

literature (compare Blatt and others, Carro Valdez (Fig. 21). From botton

1980; Flach and others, 1969; Litchfield to top, the profile reveals a medium-

and Mabbutt, 1962; Peterson and Von der to coarse-grained conglomerate (I)

Borch, 1965; Watts, 1975,.1978). Suggested (parenthetic numerals refer to the

environments for the Pedernal Chert range ‘ zones in Fig. 20), which grades upward , from lake bed to a perched water table, to a 0.5-m-thick zone of similar ma-

though neither has been substantiated. terial in which feldspars are highly

Most hypotheses developed since the weathered (11). Zone I11 is marked by L original work have been informal, com- a laterally continuous, greenish-

municated between workers, and unpublished. colored, clayey sandstone as much as

0.4 m thick. Above I11 and directly Lithologies below the chert lies a siliceous-

The Pedernal member is not a single carbonate horizon (IV) that, in places,

continuous layer of silica, but consists contains chert nodules (for example,

of one to four discontinuous, mildly Fig. 22). Above the chert layer lies

undulating chert layers which lie within a thin (0- to 4-4 layer of calcium

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'Figure 20.. Schematic representation of paleosol profile

and related deposit3 preserved on the southwest flank of' Cerro

Pedernal. Parenthetic numberals refer to the paleosol zones

and are discussed in the text.

Figure 20 9ppears on the following frame.

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scale in meters

gray to tan, poorly indurated, medium-Iocdbrsegrained granite and volconic conglomerate 4

(1 n to white, punky carbonate (cal i c he ?)

block, white, yellow and/or wax- 3- colored chert; nodular at base, massive towards topj(Pederno1 Chert)

2- up

I greenish fineqmined sandy claystme

tan medium-to-coorse-grained conglomerate, feldspars weathered I

medium- to-coarse- grained pebble conglomerate

Figure 20.

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Figure 21. Contact at Cerro Valdez between Pedernal Qhert (above) and

gneiss, granite, and limestone conglomerate of the Lower Member. In

shadow is thin white siliceous limestone layer between members. Lines

on stick are 5 cm apart.

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Figure 22. Chert micronodule replacing calcium carbonate. Sample is from

white calcium-carbonate layer at base of Pedernal Chert on north wall of

Temolime -Canyon. Long dimension of photo equals 1.65 'mm.

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carbonate which is poorly preserved (V). Discussion

Chemistry Postulated models for deposition

Whole-rock chemical analyses of 14 of the Pedernal Chert fall into three

major oxides were pgrformed on each of categories: water t.8ble.t lacustrine,

four samples taken from zones I, 11, 111, and pedogenic. The model which relies

and IV. These data are presented in upon simple silica precipitation at the

Table 2. air-water interface above the wattr

The ratios of SiO /A1 0 Si02/Fe203, table presents difficulties. Perched 2 2 2' Si02/(A120,+Fe203), and (K20tNa Ot-CaO+ water tables require impermeablk -I 2 FlgO)/A1203 can be used as indirect .sediments below, and the porous and

weathering indicators. In general, these permeable nature of the lower and

ratios decrease in value with increased Pedernal members precludes this model.

;weathering (Birkeland, 1974). Table 3 If one assumes that the siliceous-

illustrates a general decrease in these carbonate horizon (zone IV) represents

values through zones I, 11, and 111. an impermeable sedimentary layer, f Zone IV apparently is silica enriched. there remains the problem of space:

Jenny (1941) devised a relation called 8 9 vplume of conglomerate must be removed

the "leaching factor'' which compares and/or replaced by an equal volume of

three. parameters between weathered and chert. A 'simple water-table model

nonweathered, rocks : cannot explain this, whereas soil-

(weathered forming proceSsescan (see below). (K20 + Na20)/Si02 rock)

Leaching . . __ ~ Factor = (K20 + Na20)/Si0 Lacustrine siliceous deposits have 2 (parent rock) been described in arid regions of

Comparison of the leaching factors of Australia by Carlisle (1978), Litch-

zones 11, 111, and IV (Table 4) shows a field and Mabbutt (1962), Mabbutt -

significant decrease in value through the (1977), and Peterson and Von der Borch

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TABLE 2. WHOLE-ROCK CHmISTRY FROM PALEOSOL PROFILE BELOW PEDERNAL CHERT. ROW hTJMERALS REFER TO FIGURE 20

(1) ~ (11) (111) (IV) MEV 1-26a MEV 2-58 MEV 2-57 NEV 2-56

sio2 57.49 72.41 70.78 94.41

8.82 11.49 11.52 1.26 A1203 /

Fe203 1.75 2.70 2.31 0.20 FeO - - *N.D.

MgO 0.48 1.23 1.36 0.257

CaO 14.20 1.45 1.68 0.29

Na20 2.20 2.43 2.22 0 .l58

2.14 2.60 2.60 0.24 K2° H20 (+) 11.30 2.67 3.35 1.55

H20- 0.97 2.75 3.42 0.67

Ti02 0.20 0.36 0.43 0.06

0.06 0.06 0.07

SrO 0.018 0.020 0.027 0.004

-sog <0.1 <0.1 c0.s

TOTAL 99.66 100 * 20 99.79 99.10

*N.D.: Not Determined

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TABLE 3. SILICA:ALUMINA, SIL1Ch:IRON-SESQUIOXIDE, SILICA:SESQUIOXIDES, AND BASES:ALUMINA RATIOS FROM THE PALEOSOL PROFILE BELOW PEDERNAL CHERT -(SEE FIG. 20)

I I1 i11 IV

(MEV-1-26a) (MEV-2758) (MEV-2-57) (MEV-2-56)

SiO2/AI2O3 6.52 6.30 6.14 74.93

SIO2/Fe2O3 32.85 26.82 30.64 472.0 sio,

A1 0 +Fe203 5.44 5.10 5.12 64.66 23 K2 O+Na2 O+

CaOtMgO 2.16 0.67 0.68 0.75 Al,j03

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TABLE 4. COMPARISON OF LEACHING FACTORS motr THE PALEOSOL BELOW THE PEDERNAL CHERT (SEE FIG. 20)

Leaching Factor

1171 0.92

III/I 0.90

IV/I 0.56

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(1965). These models of silica depositior 1978; Peterson and Von der Boreh,

(silcrete) rely upon annual evaporation 1965); this is in marked contrast

of salt lakes to concentrate silica in 'with the 0.5- to 2.5-m thickness of Y the water, thereby causing its precipita- the Pedernal Chert.

tion. Peterson and Von der Borch (1465) The presence of a weathered soil

also noted the effect on this process of profile below the major chert layer

biologically induced changes in the pH of provides the basis for an internally

lake wates during alternating seasons consistent model. The aridity of the

of high and low rates of photosynthesis. time of deposition of the Abiquiu is

The silica is deposited as a gel in these suggested by several lines of evidence,

lakes and gradually 'recrystallizes to LeMone and Johnson (1969) have described

chalcedony. Collapse and slump structures flora from DoGa ha County (southern

may develop in the silica gel before it New Mexico) which sugges.t arid climate.

is lithified. Such structures can be The paleobotanical work of Axelrod

recognized following lithification and Bailey (1976) and the mainmalian

because of common laminations resulting fossil materials described from the

from seasonal deposition (Peterson and upper member of the Abiquiu Formation

Von der Borch, 1965); Pedernal Chert is and the Chama-El Ritq member of the

nodular at its base and massive through- Tesuque FormaKion, as well as the

out, lacking any trace of laminations eolian cross-bedding of the Ojo. Caliente

and containing numerous enclosed clasts. member, also suggest an arid climate

The base of the chert is commonly undula- (Galusha and Blick, 1971).

tory and highly irregular, which is at Whole-rock chemical data (Table 2)

variance with the smooth, flat surface .show that as the SiO content increases 2 of a lake bed. The thickest single through the sampled profile, CaO

layer of lacustrine chert described to decreases from almost 15% to <0.5%.

date is 1 cm thick (Durand and Meyer, These data and the presence of a highly ,

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altered carbonate horizon capping the. horizon could develop above the hori-

chert strongly suggest secondary replace- zon. Such inversions have been seen

ment of calcrete by silica. This in Pleistocene fans in arid regions

process has been observed in Quaternary (S. G. Wells, 1979, personal commun.).

rocks by Flach and others (1969). The possibility of further clarifi-

Groundwater enriched in silica (possibly cation of the origin of the chert using

after percolation through volcaniclastic isotope geochemistry seems remote.

rocks) might provide the mechanism neces- Oxygen-isotope study of the chert it-

sary for such replacement. An equally self is i'bpossible without detailed

reasonable model for replacement is the knowledge of the prevailing ground-

lateral migration of siliceous ground water chemistry during the early

waters which saturated all of the Abiquiu Neogene. The chemistry of. present

section. Cooke and Warren (1973) cite ground-water conditions is much dif-

numerous examples of silica replacement ferent from the chemistry that existed

of calcrete (caliche) horizons in arid before extrusion of the Jemez volcanic

regions. (Fig. 22 shows calcium carbo- pile. I hate in the Abiquiu being replaced by PALEOGEOGRAPHY AND PALEOTECTONICS silica.) Thus, it appears that Pedernal Introduction Chert layers formed by a combination of

weathering and secondary replacement. Integration of the new data obtained 6 Locally, chert and carbonate (C horizon: from this study of the Abiquiu Forma-

rest on claystone (B horizon) (Fig. 20), tion with knowledge of Paleogene

a sequence that is inverted relative to paleotectonics and paleogeography per-

that seen in many paleosols. If these mits a synthesis of basin evolution

Abiquiu soils developed first under rela- from the enci of the,Laramide to early

tively moist conditions and then were Neogene. Figure 23 depicts paleogeog-

subjected to an arid environment, the C raphy (paleodrainage) based upon

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Lote Eocene Eorly Abiauiu Time r incipient deposition

4Albuquerque (from Baltz, 1978) a b

Pedernol Time Lot e A bio uiu' T i me

<-; r I-* inti en Jcmer vdlconism?ks

r. r. Alkuquerque Albuquerque 0

C d

Figure 232 Schematic paleogeographic diagrams showing sediment

dispersal patterns in north-central New Mexico from late Eocene through

early, middle, and late Abiquiu time. Arrows indicate paleocurrent

directions.

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stratigraphic,-sedimentologic, and petro- the relatively broad Brazos-Sangre de

graphic evidence. Cristo geanticline must have been eroded

to a broad rid?e, because &he lower Pre-Abiquiu Time portions of the Los Pinos and Conejos

In the late Eocene, tectonic elements Formations had been deposited upon it;

present in north-central New Mexico and Baltz (1978, p. 224) suggests that

south-central Colorado included the "the region was characterized by low-

Brazos-Sangre de Cristo geanticline hill and alluvial plain phygiography."

(uplift), Chama basin, Nacimiento uplift, A hiatus.of several million years

Gallina-Archuleta arch, and San Juan basin exists between deposition of the El

(Baltz, 1978). Figure 2 shows their Rito and Abiquiu Formations. An angu-

geographic relationships. To the south, lar unconformity of as much as SO"

a narrow basin, resting in an extension separates the two units, suggesting

of the Chama syncline, was filled with podt-Laramide tectonism. The oxidizing

clastic detritus of the Galisteo Forma- environment present during El Rito

tion (Baltz, 1978; Gorham, i979; Gorham time had vanished"'by e&rly Abiquiu

and Ingersoll, 1979). It is not cer- time. After deformation of the El Rito,

tain whether the south-southeast-trending a soil may have developed upon this

topographic trough was continuous from the erosional surface during a period of

Colorado border to the Galisteo basin. relative tectonic stability.

The presence of at least two distinct, Early Abiquiu Time temporally equivalent formations, Galisteo

and El Rito (Logsdon, 1981), within the The lower member of the Abiquiu

trough may suggest either low physio- Formation has many characteristics of

graphic relief (Baltz, 1978) or some type a large piedmont alluvial fan in which

of sediment barrier between the El Rito debris flows and braided streams act

and Galisteo basins. By early Oligocene, as the primary modes of deposition.

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The sediment is poorly sorted and contains Miocene boundary) for the initiation

both angular and well-rounded clasts, of lower member deposition.

which suggests reworking of debris-flow Middle Abiqniu Time deposits during short-lived periods of

high-intensity runoff (Bull, 1972). Near the top of the lower member,

Bedding is absent, as are definite example a lithologic and paleocurrent tran-

of cross-bedding or graded bedding. Clast sition exists, suggesting a major

range from silt- and sand-sized to large change in the sedPmentologic patterns.

cobbles, which may show imbrication. The The dominantly Precambrian crystalline

mean transport .direction for the entire nature of the clasts within the lower

lower member is 222”, which, when com- member grades vertically into sand-

bined with its Precambrian crystalline and cobble-sized‘ volcanic. lithic

provenance, indicates that the Brazos material. Paleocurrent directions

uplift provided the sediment... The basin indicate a shift to approximately 170°, depocenter probably was located near the possibly due to the initiation of the

northwestern part of the modern Jemez vol- Espacola bas.in. Soils were developed

canic center, on the basis of locations of on surfaces of nondeposition at one

the thickest lower member deposits (Fig. to ’four different stratigraphic levels.

3) The duration of the hiatuses which

The lack of radiometric dates from the these soils represent cannot be deter-

lower member prevents precise age determined, but the fact that there were

mination of the initiation of piedmont significant periods of nondeposition

development during Abiquiu time. Basalts during Pedernaf member time is clear.

within the Los Pinos Formation (26 m.y.) Calcium-carbonate-rich horizons with-

are probably slightly older than lower in these paleosols were replaced by

member sediments. This implies an age silica at unknown times. Fragments

of approximately 24 m.y. (Oligocene- of the chert are present in deposits

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on Blanca Mesa, where a layer of Pedernal known to have produced the Sunshine

Chert is absent. This indicates that Peak Tuff at 22.5 m.y. B.P. (Lipman

silica replacement occurred either during and others, 1978), which may have

Pedernal member time or shortly thereafter. been the source for airborne volcanic -

The majority of the sand-sized volcanic detritus. Older volcanic centers in

lithic detritus within the Pedernal member the San Juan field, as well as other

is vitric (glass shards). These materials localities, probably provided some of -I could not have survived kilometers of the far-traveled volcanic detritus.

aqueous transport and, therefore, repre- Late Abiquiu Time sent either air-fall deposits from distant

sources or materialaderived from local The upper member records a period

vents. In either case, these deposits were of southeastward transport of large

reworked into the detritus on the piedmont volumes of quartzose and volcaniclastic 'fi surface of the lower member. Pebbles and sand by braided streams. The lateral

cobbles of all types show good rounding migration of these depositional'environ-

and probably were transported from the ments gave rise. to alternating thick-

north for significant distances. These and thin-bedded', fine- and medium-

also may represent volcanic ejecta, but grained volcaniclastic sandstones. The

their rounding and sorting indicate much piedmont surface probably sloped

reworking. Lipman and others (1978) have gently into the developing Espafiola

presented evidence that vents which pro- basin. The moderately abundant un- .c duced rhyolitws of the Hinsdale Formation altered vitric volcanic lithic dstritus

in the San Juan volcanic field fall within and pyriboles suggest that airborne '

j. an age range of 23.0 to 21.9 m.y. This volcanic detritus was responsible for

age is close to that of the Pedernal mem- much of the volcanic input. The possi-

ber. The Lake City caldera of the ibestern bility of local vents, now covered by

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I Abiquiu and younger sediments, remains, CONCLUSIONS although no evidence for such vents exists

Large-scale volcanism was occurring in the Radiometric and palynological ages

San Juan volcanic field during this time from the Abiquiu Formation itself

and may represent the source for this and from temporally granitic and

airborne detritus. (Also, see Nanley, gneissic conglomerates of the lower

1981, for a discussion of source areas member were depositcd starting about

for the closely related Los Pinos Forma- 24 m.y. B.P. This member is thickest

tion.) (90 m.) in the vicinity of the northern

Cut-and-fill channels within massive San Pedro Mountains and may thin to the

volcaniclastic sandstones suggest the northeast; paleocurrents suggest that

reworking of low-relief, piedmont-fan the sediments were derived primarily

deposits (Bull, 1972). Isolated cobbles from the northeast. The most likely

and boulders within these medium-grained depositional analog is a broad alluvial

rocks may be rounded volcanic bombs, fan surface (piedmont) developing

reworked pediment material, or reworked adjacent to an uplift, with the thick-

channel-)ag deposits; the evidence is est deposits in the medial region of

not conclusive. a the fan (Fig. 24). Significant re-

These deposits grade both vertically . working in the uppermost part of the

and laterally into younger deposits o'f lower membcr and intermixing of volcani- -( the Los Pinos Formation and the Santa Fe clastic detritus in this part of the

Group. These relationships may point section correspond roughly with volumi-

to a bolson-type environment in which nous volcanic activity in the San Juan

alluvial deposits from different sides volcanic field in south-central Colo-

of the basin coalesced. rado and possibly other volcanic fields.

Paleocurrent orientations indicate-

that sediment transport was southwesterly,

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sw EARLY ABIQUIU TIME NE

- Sangre de geonticline

# / PEDERNAL TIME

soil formatio

Espafiolo basin formaiion

LATE ABIQUIU TIME

Santa R Group and thin post-Abiquiu deposits

Figure 24. Two possible models €or Abiquiu basin evolution. See text for

discussion.

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but net Sediment accumulation was reduced detritus separated by as many as 4

5- I substantially. This might be explained b> silicified yiLrHorizons. Point-count b. ;fr -p,. I. incipient EspaGola basin development or f: data docum-&t&-i-q,$ncrease.. -- in the -- retrogradation (Fig. 24). In the vicinitJ ratio of volcaniclastic to Precambrian ., e". of the San Pedro Mountains'and the Cerro crystalline and Paleozoic sedimentary

Pedernal (previously the site of the rocks through middle Abiquiu time.

thickest accumulation of lower member Concurrently, cobble-sized material t conglomerates), major-oxide chemical anal) became less common, and sand-sized

sis and leaching-factor analysis (Birke- material changed from quartzofeld-

landk 1974) suggest the development of soi Spathic to lithic (volcaniclastic).

horizons and indicate a reduction in depo- $hese changes in lithology were accom-

sition at this time (Pedernal m$mber). panied by a simultaneous eastward shift

Sedimentation was locally sporadic, of the Abiquiu depocenter.

allowing formation and hurial of as many Nearly 200 m of volcbniclastic

as four soil profiles. sandstone were deposited dtT$ng late 2% Silicification of carbonate hokizons Abiqhiu time in broad, coalescing sheets

within these soil profiles ultimately which were dissected locally by 1- to

formed the Pedernal Chert. It is diffi- 3-m-deep channels. Sediments were

cult to establish the precise timing of derived primarily from the north.

siligification, but the presence of sedi- Because the Rio Grande rift is

ments containing chert fragments in a thought to have begun to form near

transitional zone (og Blanca Nesa) between the end of the OliGcene in this area

lower and upper members suggests that (Woodward, 1977), the history presented

silicification had begun in some &aces here places some temporal and tectonic

before the deposition of the upper member. constraints on its development. The

The Pedernal member contains as much , change in transport direction between

as 50 m of crystalline and volcaniclastic early Abiquiu time (southwestward)

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and late Abiquiu time (southeastward) celli, S. G. Wells, and L. A. Woodward.

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MANUSCRIPT RECEIVED BY THE SOCIETY

OCTOBER 1, 1981- YMUSCRIPT ACCEPTED

OCTOBER 1, 1981

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