Late oligocene and miocene faulting and sedimentation, and evolution of the southern Rio Grande rift, New Mexico, USA

SEDIMENTARY GEOLOGY ELSEVIER

Sedimentary Geology92 (1994) 79-96

Late Oligocene and Miocene faulting and sedimentation, and evolution of the southern Rio Grande rift, New Mexico, USA Greg H. Mack, William R. Seager, John Kieling Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA

Received November 3, 1993; revised version accepted February 9, 1994

Abstract The distribution of nonmarine lithofacies, paleocurrents, and provenance data are used to define the evolution of late Oligocene and Miocene basins and complementary uplifts in the southern Rio Grande rift in the vicinity of Hatch, New Mexico, USA. The late Oligocene-middle Miocene Hayner Ranch Formation, which consists of a maximum of 1000 m of alluvial-fan, alluvial-fiat, and lacustrine-carbonate lithofacies, was deposited in a narrow (12 km), northwest-trending, northeast-tilted half graben, whose footwali was the Caballo Mountains block. Stratigraphic separation on the border faults of the Caballo Mountains block was approximately 1615 m. An additional 854 m of stratigraphic separation along the Caballo Mountains border faults occurred during deposition of the middle-late Miocene Rincon Valley Formation, which is composed of up to 610 m of alluvial-fan, alluvial-fiat, braided-fluvial, and gypsiferous playa lithofacies. Two new, north-trending fault blocks (Sierra de las Uvas and Dona Ana Mountains) and complementary west-northwest-tilted half graben also developed during Rincon Valley time, with approximately 549 m of stratigraphic separation along the border fault of the Sierra de las Uvas block. In latest Miocene and early Pliocene time, following deposition of the Rincon Valley Formation, movement continued along the border faults of the Caballo Mountains, Dona Ana Mountains, and Sierra de las Uvas blocks, and large parts of the Hayner Ranch and Rincon Valley basins were segmented into smaller fault blocks and basins by movement along new, largely north-trending faults. Analysis of the Hayner Ranch and Rincon Valley Formations, along with previous studies of the early Oligocene Bell Top Formation and late Pliocene-early Pleistocene Camp Rice Formation, indicate that the traditional two-stage model for development of the southern Rio Grande rift should be abandoned in favor of at least four episodes of block faulting beginning 35 Ma ago. With the exception of two northwest-trending border faults of the Caballo Mountains block that may be reactivated along Eocene compressional structures, the majority of border faults and complementary basins throughout the history of the southern Rio Grande rift were north-trending, which challenges the conventional idea of a clockwise change in stress through time.

1. Introduction There has been considerable recent research directed towards understanding sedimentation in extensional basins. Of particular interest is the role of basin symmetry on lithofacies distribution,

as well as how sedimentary basins are influenced by along-strike variations in strain, i.e. transfer or accommodation zones (Crossley, 1984; Leeder and Gawthorpe, 1987; Frostick and Reid, 1987; Rosendahl, 1987; Ebinger, 1989; Mack and Seager, 1990; Scholz et al., 1990; Morley et al., 1990;

0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00032-P

80

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

Leeder, 1993; Leeder and Jackson, 1993). Receiving less attention is the effect of temporal changes in strain distribution on the evolution of rift basins (e.g. Ori, 1989; Duebendorfer and Wallin, 1991; Leeder et al., 1991). Throughout the history of a continental rift some faults become inactive and other, new faults develop, a process that affects the history of complementary basins. Thus, in order to fully understand rift sedimentation, it is necessary to have a detailed chronology of fault activity and to determine the role that fault activity played in basin evolution. Over the past few years we have undertaken a project to define more clearly the tectonic evolution of the southern Rio Grande rift, based on a combination of geologic mapping and stratigraphic analysis. Stratigraphic studies are especially pertinent in an area of the southern Rio Grande rift within a radius of approximately 50 km of the town of Hatch, New Mexico (Fig. 1), because the entire sequence of syn-rift sedimentary and volcanic rocks is exposed and because most of the syn-rift units are evenly distributed geographically, resulting in a high degree of confidence in paleogeographic reconstruction. The goals of the stratigraphic study are: (1) to use sedimentologic and provenance data to reconstruct the location, size, and symmetry of rift basins; (2) to define basin-bounding uplifts and their associated border faults; (3) to estimate the amount of stratigraphic separation across principal border faults; and (4) to demonstrate how temporal changes in deformation influenced the evolution of rift basins. We previously reported on the youngest (Plio-Pleistocene; Mack and Seager, 1990; Mack and James, 1993) and oldest (lower Oligocene; Mack et al., 1994) parts of the syn-rift sedimentary sequence. In this paper, we interpret the remainder of the sedimentary sequence (upper Oligocene-Miocene), and combine all of the interpretations into a model for the evolution of uplifts and basins in the study area. The results of this study suggest that this part of the Rio Grande rift was characterized by at least four episodes of deformation resulting in largely north-trending uplifts and basins, an interpretation that warrants re-examination of the traditional two-stage model for the development of

the southern Rio Grande rift, as well as the idea of clockwise change in stress through time (Seager, 1975; Chapin and Seager, 1975; Seager et al., 1984).

2. Geologic and stratigraphic setting The study area encompasses approximately 2700 km 2, centered around the town of Hatch in south-central New Mexico (Fig. 1). The region is characterized by a series of north- and northwesttrending, normal-fault-bound block uplifts and complementary basins. Rocks of Precambrian, Paleozoic, Cretaceous, and Tertiary age are exposed in the uplifts, including Oligocene and Miocene syn-rift sedimentary and volcanic rocks. Displayed in Fig. 1 are stratigraphic columns for those block uplifts interpreted to have been active in late Oligocene and Miocene time. Descriptions of the stratigraphic units appear in Table 1. The syn-rift sedimentary rocks examined in this study belong to the late Oligocene and Miocene Hayner Ranch and Rincon Valley Formations (Fig. 2). The Hayner Ranch Formation has a maximum exposed thickness of approximately 1000 m at San Diego Mountain (Fig. 1; Seager et al., 1971). In the northern part of the study area the Hayner Ranch Formation conformably overlies the Thurman Formation, which consists primarily of reworked fallout tephra derived from the 27.4 Ma Mt. Withington caldera (W. Mclntosh, pers. commun., 1993). Reversal magnetostratigraphy suggests that the Thurman Formation is probably not younger than 27.0 Ma (W. Mclntosh, pers. commun., 1993), based on the revised Oligocene polarity time scale of Mclntosh et al. (1992). These values (27.4, 27.0 Ma) represent a lower limit for the age of the Hayner Ranch Formation. Throughout most of the study area, the Hayner Ranch Formation is conformably overlain by the Rincon Valley Formation, although the age of the contact is unknown. Locally in the Rincon Hills and Apache Graben, the Hayner RanchRincon Valley contact is an angular unconformity. The maximum exposed thickness of the Rincon Valley Formation is approximately 610 m in

G.H. Mack et aL / Sedimentary Geology 92 (1994) 79-96

the southern Caballo Mountains (Fig. 1). The age of the Rincon Valley Formation is partially constrained by a basalt flow, the Selden Basalt, exposed in the upper part of the Rincon Valley Formation in the southern part of the study area. It has been dated by the K - A r method at 9.6 Ma (Seager et al., 1984). The age of the top of the Rincon Valley Formation is not known, but according to Seager et al. (1984) the Rincon Valley Formation is probably not younger than 7.1 Ma, the age of a basalt flow that extruded onto a deeply eroded fault block that developed after deposition of the Rincon Valley Formation. Throughout the study area, the Rincon Valley Formation has an angular unconformable relationship with the overlying upper Pliocene-lower Pleistocene Camp Rice Formation (Fig. 2; Seager et al., 1982, 1987).

3. Methods Three types of data are presented in this study: lithofacies distribution, paleocurrents, and provenance. Each of the 43 outcrops of the Hayner Ranch Formation (n = 10) and Rincon Valley Formation (n = 33) were assigned to one or more lithofacies types, based on criteria in Table 2. The alluvial-fiat lithofacies, as used here, corresponds to the alluvial-fan sandflat and upslope part of the dry mudflat lithofacies of Hardie et al. (1978) and Hubert and Hyde (1982). Examples of the various lithofacies types are shown in Fig. 3. The geographic distribution of individual lithofacies or groups of lithofacies is shown in Figs. 4A and 6A. In addition, paleocurrent data from imbricated pebbles and cobbles were collected at most outcrops, following the strategy of Cavazza (1986) and Ingersoll and Cavazza (1991). The strike and dip of the AB-plane of ten clasts per bed were measured, concentrating on the coarsest clasts available in each outcrop and avoiding beds with foresets or beds interpreted to represent bar tops. The vector mean, representing the sediment transport direction, was calculated for each bed. Data sets were only used if they had a standard deviation of ~<40° or a range of values of ~<90°.

81

A total of 22 vector means for the Hayner Ranch Formation are shown in Fig. 4A and 92 vector means for the Rincon Valley Formation are shown in Fig. 6A. Gravel-sized clasts at each outcrop were also identified according to the formation from which they were derived (cf. Fig. 1) and were separated into four provenance types for the Hayner Ranch Formation and seven provenance types for the Rincon Valley Formation. In order to quantify the provenance types, clast counts were performed on 62 beds (Hayner Ranch = 25; Rincon Valley = 37), utilizing 300 clasts per bed. The results of the clast counts are displayed on triangular diagrams (Figs. 5 and 7) and the provenance types are shown by varying the type of paleocurrent arrows in Figs. 4A and 6A. Finally, stratigraphic separations were calculated for the principal border faults for the time periods corresponding to the end of Hayner Ranch deposition and the end of Rincon Valley deposition. Stratigraphic separations represent the cumulative thickness of strata exposed in the footwall of the fault, plus the thickness of Hayner Ranch and Rincon Valley basin-fill strata deposited on the hanging wall adjacent to the border fault. The stratigraphy exposed in a particular footwall is based on the provenance of clasts in Hayner Ranch and Rincon Valley strata derived from the footwall block in question.

4. Sedimentation of the Hayner Ranch Formation Sedimentologic and provenance data indicate that the Caballo Mountains fault block was active during deposition of the Hayner Ranch Formation (Fig. 4). Hayner Ranch strata in the northern part of the study area display a south-southwestward change from proximal to distal lithofacies, as well as paleoflow directed away from the Caballo Mountains fault block. Particularly diagnostic of derivation from the Caballo Mountains fault block are proximal fan deposits composed of boulders in excess of 1 m in diameter that are exposed directly adjacent to the northwestern part of the Palm fault. In the Rincon Hills, the lower part of the Hayner Ranch Formation contains

82

G.H. Mack et al, / Sedimentary Geology 92 (1994) 79-96

C

KEY Pliocene-Quaternary alluvium Miocene Rincon Valley Formation

Jornada del Muerto

Miocene Hayner Ranch Formation Pre-Miocene volcanic and sedimentary rocks

Point of Rocks

Cretaceous sedimentary rocks ~ "7 /-- f T "TL~

Paleozoic sedimentary rocks

I

N

! (

0

mi

~.,,, , ~ 0 km 4

3

f

Precambrian

basement

normal fault

l

NEW MEXICO

)

Dona Ana Mts

/

I TwrlT J

I Tbt3 I Fig. 1. Generalized geologic map of the southern Rio Grande rift near Hatch, New Mexico, adapted from Seager et al. (1982, 1987). Descriptions of the rock units shown in the stratigraphic columns appear in Table 1,

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

from three to four thin ( < 1 m) beds of ostracod wackestones and carbonate mudstones of lacustrine origin. Locally, the tops of the limestones display stromatolites (Fig. 3H). Hayner Ranch strata in the northern part of the study area display an unroofing sequence commensurate with derivation from the Caballo Mountains fault block. The lower part of the formation consists primarily of clasts of Uvas Basaltic Andesite (Tu) and Bell T o p ash-flow tufts (Tbt5, Tbt6). Andesite clasts of the Palm Park Formation (Tpp) progressively increase upsection, and clasts of Precambrian granite and gneiss and Paleozoic sedimentary rocks derived from conglomerates of the Eocene Love Ranch Formation (Tlr) a p p e a r in the u p p e r part of the formation. That the Precambrian and Paleozoic clasts are second-cycle in origin is supported by the fact that they are anomalously well rounded c o m p a r e d to associated first-cycle clasts of Uvas, Bell Top, and Palm Park. Clasts derived from the

Love Ranch Formation are especially diagnostic of a Caballo Mountains source (Fig. 5). The southern part of the Hayner Ranch basin is less well constrained because of few outcrops. However, the southern margin of the basin can be determined by outcrops in the Sierra de las Uvas and Selden Hills (Fig. 1). Exposed in the northern Sierra de las Uvas is a paleovalley infilled with about 30 m of boulder and cobble conglomerate of the Hayner Ranch Formation. This north-northeast-trending paleovalley is 200 m wide, exposed for a distance of 1 km, and is incised into gently north-northeasterly dipping lava flows of the Uvas Basaltic Andesite (Tu). Clasts within the paleovalley consist entirely of Uvas Basaltic Andesite (Tu), constituting the southwestern provenance type (Figs. 4A and 5). North-northeastward paleoflow within the paleovalley is suggested by north-northeastward decrease in grain size and one set of imbrication paleocurrent measurements. Also critical to the

Table 1 Generalized pre-Miocene stratigraphy of south-central New Mexico Age Stratigraphic unit(s) Symbol Oligocene

Rock types Basaltic andesite Conglomerate with Tfbr clasts Ash-flow tufts (Tbt2-5) and conglomerate with Tpp clasts (Tbts) Rhyolite of Dona Ana Mountains White rhyolite (Twr) of SE study area; gray rhyolite of Caballo Mts Red to maroon flow-banded rhyolite

Uvas Coyote Canyon Bell Top

Tu Tcc Tbt

Dona Ana rhyolite Rhyolite intrusives

Tdar Twr/Tr

Flow-banded rhyolite

Tfbr

Eocene

Palm Park Love Ranch

Tpp TIr

Andesitic breccias and lava flows Conglomerate with clasts of Precambrian basement and Paleozoic sed. rocks

Cretaceous

Dakota

Kd

Quartz sandstone

Permian

Yeso

Py

Abo/Abo-Hueco

Pa/Pah

Dark gray limestone, dolomite, and quartz sandstone Red siltstone (Pa); limestone (Pah), locally green marble in Dona Aria Mts

Panther Seep

IPps

Magdalena

IPm

CambrianMississippian

Lower and middle Paleozoic

Pzlm

Limestone and dolomite; minor sandstone and shale

Precambrian

Basement

PC

Granitic and metamorphic rocks

Pennsylvanian

83

Laminated limestone locally green marble in Dona Ana Mts Limestone

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

84

t

Pleistocene -

-

1.6

late 3.4

Pliocene

Ranch Formation displays a northeastward change to more distal lithofacies and predominantly northeastward paleocurrents. In the Selden Hills, Hayner Ranch strata are composed primarily of flow-banded rhyolite clasts (Tfbr) and were derived from the south-central source terrane (Figs. 4A and 5). At San Diego Mountain, Hayner Ranch conglomerates consist of a mixture of Palm Park (Tpp), flow-banded rhyolite (Tfbr), and Uvas (Tu) clasts and were derived from the southcentral and southeastern source regions (Figs. 4A and 5).

F 1.6

C a m p Rice 3.1

early 5.3

late 11.2 - -

Miocene

middle 16.6 - -

early --

9.6

23.7

late Oligocene

30.0 early

- -

Rincon Valley ? Hayner Ranch

28.5

Bell T o p 33.s 35.0 I

36.6

Interpretation

s Thurman N U v a s 28.0 ~. 27.4

I

35.7 I

Fig. 2. Stratigraphy of syn-rift sedimentary and volcanic rocks, south-central New Mexico. Numbers are radiometric ages in millions of years of volcanic rocks from Seager et al. (1984) and Mclntosh et al. (1991).

interpretation of the southern margin of the Hayner Ranch basin is an outcrop along the northwestern margin of the Selden Hills, where the Hayner Ranch Formation displays a southward onlap onto the Palm Park Formation over a distance of approximately 3 km (Fig. 1; Seager et al., 1971). Between the northern Selden Hills and San Diego Mountain (Figs. 1 and 4A) the Hayner

The Hayner Ranch basin is interpreted as a narrow (12 km), northwest-trending, northeasttilted half graben whose footwall was the Caballo Mountains block (Fig. 4B). Asymmetry of the basin is supported by relatively narrow lithofacies belts adjacent to the footwalI, the presence of basin-axis lacustrine limestones within a few kilometers of the footwall block, and wider-spaced lithofacies belts along the southern margin of the basin (cf. Leeder and Gawthorpe, 1987). There is no evidence for syn-depositional faulting along the southern margin of the basin and it is interpreted to be a northeastward-tilted hanging wall. The northwestern and southeastern extent of the Hayner Ranch basin cannot be determined, because of lack of outcrops and subsurface data. Stratigraphic separation across the border faults of the Caballo Mountains block during

Table 2 Criteria used for recognition of lithofacies Lithofacies

Bedding

Rock types

Ave. size of gravel

Matrixsupported cgl

Trough crossbeds

Pedogenic carbonate

Dominant process

Proximal fan Midfan Distal fan Braided fluvial Alluvial flat

poor; thick medium thin medium thin to medium medium

cgl cgl > ss cgl ffi ss > mdst cgl = ss > mdst mdst > ss > cgl

boulder cobble pebble pebble granule

common rare absent absent absent

rare common rare commom rare

rare rare present present present

mdst, gyps, carbonate

N/A

absent

rare

rare

debris flow channel flow sheetflood channel flow susp. fallout sheetflood susp. fallout evap.

Lacustrine

c g l f c o n g l o m e r a t e ; s s = s a n d s t o n e ; mdst = m u d s t o n e ; g y p s = g y p s u m ; susp. fallout=deposition from suspension; evap.= evaporation.

Fig. 3. Photographs of lithofacies recognized in this study. (A) Proximal alluvial-fan lithofacies composed of debris-flow conglomerates, Rincon Valley Formation, Cedar Hills; man is 1.75 m. (B) Channelized midfan conglomerates and coarse sandstones, Rincon Valley Formation, Selden Hills; man is 1.75 m. (C) Thin-bedded distal fan lithofacies, Rincon Valley Formation, Cedar Hills. (D) Distal fan pebble conglomerates and sandstones, Rincon Valley Formation, Rincon Hills; Jacob's staff is 1.5 m. (E) Braided stream lithofacies consisting of trough cross-bedded and plane-bedded pebble conglomerate and sandstone, Rincon Valley Formation, Cedar Hills; Jacob's staff is 1.5 m. (F) Alluvial-flat iithofacies composed of red mudstones and thin beds of sandstone, Rincon Valley Formation, Rincon Hills; Jacob's staff is 1.5 m. (G) Badlands topography developed on playa lithofacies of the Rincon Valley Formation in the Rincon Hills; ledges are beds of selenite gypsum and recessed beds are red mudstone. (H) Stromatolites in a lacustrine limestone of the Hayner Ranch Formation, Rincon Hills; hammer is 25 cm long.

i

g~

pc.

I

Tu

Southwestern

~ ~

South- ~

~

~

lacustrine

distal fan 4alluvial fiat

Mixed

South-Central

Southwestern

Caballo Mts

Central '~ | Southeastern I Tu I Tfbr West Selden Tpp Hills fault

,

Nat

_ti

2 /

~j

distal fan ÷ lacustrine

--

Paleocurrents and Provenance types

B

'~*e~.

%

-%

/ ~ , Caballo Mrs \~'~'~ block •

Fig. 4. (A) Sedimentologic and provenance data for the Hayner Ranch Formation (see Table 1 for description of stratigraphic units shown in stratigraphic columns); arrows depict paleoflow direction and provenance type. (B) Paleogeographic reconstruction of the study area during deposition of the Hayner Ranch Formation, based on data in (A).

A

0 mi 3 I ,t, } , I, 0 km $

t

distal fan

TIr

Tpp

OO

I

88

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

Hayner Ranch time can be estimated by adding the cumulative thickness of stratigraphic units exposed in the footwall to the maximum thickness of Hayner Ranch strata exposed near the footwall block. Provenance data indicate that the oldest stratigraphic unit exposed in the Caballo Mountains by the end of Hayner Ranch deposition was the Eocene Love Ranch Formation (Tlr), and the thickness of the Hayner Ranch Formation in the Rincon Hills is approximately 457 m. These data, along with the exposed thickness of the Love Ranch (Tlr), Palm Park (Tpp), Bell Top (Tbt), and Uvas (Tu) Formations, indicate that stratigraphic separation across the Palm, Red House Mountain, and Caballo faults was approximately 1615 m by the end of Hayner Ranch deposition. Because footwall-derived strata of the Hayner Ranch Formation fine upward, it is likely that much of the fault movement took place early in the history of the formation.

Valley conglomerates primarily consist of angular to subangular, first-cycle clasts of red siltstone of the Permian Abo Formation, limestone of the Pennsylvanian Magdalena Group, and Tertiary gray rhyolite, the latter occurring as small intrusions in Permo-Pennsylvanian strata (Figs. 6A and 7). Rincon Valley strata exposed at San Diego Mountain were also derived from the Caballo Mountains fault block, as indicated by the south-

A

Tu + Tpp + Tbt _~.~/~

ff'/j

x~

Hayner Ranch Formation

5. Sedimentation of the Rincon Valley Formation The Caballo Mountains fault block continued to be an important source of detrital sediment during deposition of the Rincon Valley Formation. Although no proximal fan lithofacies are exposed directly adjacent to the Caballo Mountains border faults, Rincon Valley strata become more distal toward the south-southwest, lithofacies belts parallel the Palm fault, and imbrication data indicate paleoflow directly away from the Caballo Mountains fault block. In the Rincon Valley Formation is a well-exposed playa lithofacies, composed of red mudstones, some of which have isolated displacive crystals of gypsum, and thin ( < 30 cm) beds of selenite (Fig. 3G). The presence of gypsum indicates a relatively dry paleoclimate during deposition of the Rincon Valley Formation. Basal Rincon Valley conglomerates derived from the Caballo Mountains fault block are similar in composition to upper Hayner Ranch conglomerates in that both contain clasts of Uvas (Tu), Bell Top (Tbt), Palm Park (Tpp), and well-rounded Precambrian and Paleozoic clasts reworked from the Love Ranch Formation (Tlr). However, middle to upper beds of Rincon

Tlr

B

Tpp

Tfbr Tu

Southernprovenance:

Tfbr

Q Southwestern • Sotlth-central [] mixed

Fig. 5. Composition of the Hayner Ranch Formation, based on clast counts of conglomerates.Tu = Uvas Basaltic Andesite clasts; Tpp = Palm Park andesite clasts; Tbt = Bell Top ashflow tuff clasts; Tlr = Love Ranch limestone, siltstone, granite, and gneiss clasts; Tfbr = flow-banded rhyolite clasts.

89

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

southwestward paleocurrent data, but display a difference in provenance c o m p a r e d to other detritus derived from the Caballo Mountains. In addition to clasts of Tertiary rocks, Rincon Valley

~'~'~'%~

conglomerates exposed at San Diego contain sandstone clasts derived from ceous Dakota Formation (Kd) and clasts derived from the Permian Yeso

Caballo Mts

Red t House

Tpp Tir Pa IPm

lVltn fault

t distal~

~

ran . \

.,

,~l

~ ~

\'+7~ ~ v~./.

~.~.~ ~¢

playa

~

~

~

Jornada del Muerto Tu Tbt6 Tbt5 Tpp TIr Kd

~

Paleocurrents and Provenance Types Caballo Mts Jornada del Muerto Dona Ana Mts South-Central Southwestern Sierra de las Uvas Mixed

\\

\ playa ' , \ distal fan

+

Sieruaadse ia$ Yl TU Tbto Tbt5 Tbt~ Tb

eli/

I /

~

\

~

m ~

/ ~

distal fan

.+

8]luvial

fiat

braided / / ~ ~\ / midfa /~k'~ ( " " ~ ~ ) ~ ~,~

braided ~^

IBI le-/ t .

---v>l>

-~

-,L.:/ /T i \ "I

> I>

\ \

alluvialflat

A

Mountain the Cretalimestone Formation

/

I I ]

//

Tcu¢ ~. i . . . . .~ v , Tt"br, Twr Tbt3 3oum-ueniral Tbt3 Southwestern Tpp

Dona Aria Mrs Twr, Tdar Tpp

~

Pah

IPps

~=l N

O mi ' i Ii i I O km

i

3 t s II

Fig. 6. (A) Sedimentologic and provenance data for the Rincon Valley Formation (see Table 1 for description of stratigraphic units shown in stratigraphic columns); arrows depict paleoflow direction and the provenance type. (B) Paleogeographic reconstruction of the study area during deposition of the Rincon Valley Formation, based on data in (A).

90

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

inferred

o o~6e~o .%~t

.southern Rincon Valley

Dona Ana Mrs block N 0 [

0

B

mi I

[i

3 iI

km

1

II

5

Fig. 6 (continued).

(Py). Both the Dakota and Yeso Formations are only exposed along the eastern dip slope of the Caballo Mountains in an area referred to as the Jornada del Muerto (Figs. 1 and 6A).

Two new fault blocks developed in the southern part of the study area during Rincon Valley deposition. Uplift of the Sierra de las Uvas along the Ward Tank fault is indicated by the presence

G.H. M a c k et al. / Sedimentary Geology 92 (1994) 7 9 - 9 6

of coarse, proximal fanglomerates directly adjacent to the fault. Some of these conglomerates indicate eastward paleocurrents (Fig. 6A). Furthermore, conglomerates adjacent to the Ward Tank fault are composed of clasts of Uvas Basaltic Andesite (Tu) and clasts of Bell Top ash-flow tufts (Tbt3, Tbt4, Tbt5, Tbt6), one of which (Tbt4) is only exposed in the Sierra de las Uvas (Figs. 1 and 7). Stratigraphic separation on the Ward

Tbt4,5,6 \

ff

X

/i"

Caballo Mts

I <> DonaAnaMts I n South-centraland / Southwestern

~,

Rincon Valley

Io

[ O Sierra de Ins Uvas

\

L.~ Mixed

Formay

o% Tpp + Tbt3 + Tfbr

o

Ps + Twr + Tdar Tbt3

"/(

_/

Xm Tpp

J

\,

Dona Ann Tfbr Mrs Fig. 7. Composition of the Rincon Valley Formation, based on clast counts of conglomerates. Tu = Uvas Basaltic Andesite clasts; T b t 4 , 5 , 6 = Bell Top ash-flow tuff clasts; Tpp = Palm Park andesite clasts; Tbt3 = Bell Top ash-flow tuff 3 clasts; Tfbr = flow-banded rhyolite clasts; Ps = Paleozoic sedimentary clasts; Twr = white rhyolite clasts; Tdar = Dona Aria rhyolite clasts. Conglomerates derived from the Jornada del Muerto are not shown on the diagram.

91

Tank fault apparently diminished northward, because proximal fanglomerates deposited adjacent to the northern end of the fault contain almost exclusively Uvas Basaltic Andesite clasts (Tu). The Dona Ana Mountains fault block was also uplifted during Rincon Valley deposition, an interpretation supported by paleocurrents directed westerly away from the Dona Aria Mountains, by a west-northwestward change from midfan to distal fan lithofacies, and by a distinctive provenance. Conglomerates derived from the Dona Ana Mountains consist of clasts of Paleozoic limestone (Pah, IPps) and red siltstone (Pah), andesite of the Palm Park Formation (Tpp), and Tertiary rhyolite (Twr, Tdar), one of which (Tdar) is unique to the Dona Ana Mountains. Among the Paleozoic clasts, those of the Panther Seep Formation (IPps) are especially diagnostic, because within the study area, this formation is only exposed in the Dona Ana Mountains. Moreover, within the Dona Ana Mountains the Panther Seep Formation locally has been contact metamorphosed to green marble, and these marbleized clasts are present in the Rincon Valley Formation. Absent in conglomerates derived from the Dona Ana Mountains fault block are clasts of Bell Top ash-flow tuffs (Tbt) and flow-banded rhyolite (Tfbr) (Fig. 7). Provenance and paleocurrent data also suggest that flow-banded rhyolite domes were important source rocks in the southern part of the study area during deposition of the Rincon Valley Formation and that a drainage divide trending north across the crest of the domes may have resulted in separate southwestern and south-central source areas (Fig. 6). Conglomerates derived from the southwestern source area display northwestward paleocurrents and are composed primarily of flow-banded rhyolite clasts (Fig. 7). In contrast, conglomerates derived from the south-central source area contain a large percentage of Palm Park andesite clasts (Tpp), in addition to clasts of flow-banded rhyolite (Tfbr) and ash-flow tuff 3 of the Bell Top Formation (Tbt3) (Fig. 7). Throughout much of the central and northern part of the basin, Rincon Valley conglomerates are composed of a mixture of two or more provenance types (Figs. 6 and 7).

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

92

Interpretation

Gawthorpe, 1987). The presence of Permian Abo (Pa) and Pennsylvanian Magdalena (IPm) clasts in footwall-derived conglomerates, along with 610 m of Rincon Valley strata exposed south of the Palm fault, suggest an additional 854 m of stratigraphic separation across the Caballo Mountains border faults in Rincon Valley time. Asymmetry of the southern Rincon Valley basin is clearly evident in the asyrmnetrical distribution of lithofacies. Sediment derived from the Sierra de las Uvas footwall is restricted to a narrow belt immediately adjacent to the border fault. Furthermore, axial-fluvial lithofacies of mixed provenance are also positioned within a kilometer or less of the footwall. In contrast,

Deposition of the Rincon Valley Formation took place in two half grabens, the northern of which had as its footwall the Caballo Mountains block and the southern of which had as its footwall the Sierra de las Uvas block (Fig. 6B). Asymmetry of the northern basin is not well displayed by lithofacies patterns, because there are few outcrops of strata derived from the hanging wall. However, a northeastward tilt of the northern basin is suggested by the close proximity of the basin-axis playa lithofacies to the footwall block and by the relatively narrow lithofacies belts of footwall-derived sediment (cf. Leeder and

Ranch Fm X" ~ ( l a tB.e Hayner Olig.-mid Miocene)

A. Bell Top Fm

--7"

"

(early Oligocene)

it

,

/

/'>,×"

%~o.

, / ,/'/ /,// //~ 2,

/

"-~.

0

~ _ ~ / C . Rincon Valley Fm • " ~ , / (mid-late Miocene)

I

i~.

km

lO

post-Rincon Valley ~ / , ~ , < and Camp Rice Fm ~.,M.~ (latest Miocene-Recent)

H

! 0 i

km

|0 ..a

Fig. 8. Evolution of basins and uplifts of the southern Rio Grande rift in the vicinity of Hatch ( H ) , New Mexico. Lined pattern corresponds to uplift and blank areas represent no outcrop or subsurface data. Stippled pattern represents basin-fill sedimentary rocks, and dense stipples in part D corresponds to location where all four basins are superimposed. A G M = Aneestral Goodsight Mountains; BF = Bell Top footwall block; C M = Caballo Mountains; S U = Sierra de las Uvas; DA = Dona Ana Mountains; RH = Rincon Hills; SM = San Diego Mountain; C H = Cedar Hills; S H ~ Selden Hills; RM = Robledo Mountains.

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

detritus deposited on the hanging wall dip slope occupy a broad depositional belt, as predicted by the half graben model of Leeder and Gawthorpe (1987). Stratigraphic separation on the Sierra de las Uvas border fault (Ward Tank fault) is estimated at 549 m. No estimate of stratigraphic separation can be made for the border fault of the Dona Ana Mountains fault block (Jornada fault), because there are no data on the thickness of Rincon Valley strata deposited east of the border fault.

6. Discussion

Crustal extension in the southern Rio Grande rift is interpreted to have begun approximately 35 Ma ago with deposition of the Bell Top Formation in an eastward-tilted half graben (Fig. 2; Mack et al., 1994). The north-trending border faults responsible for the Bell Top half graben do not appear to have influenced the development of the border faults of the Caballo Mountains block, because they do not coincide geographically. However, two of the Bell Top faults determined to a large extent the source rocks exposed in the hanging wall of the Hayner Ranch half graben (Fig. 4A). The western Bell Top fault, shown as a dashed line in Fig. 4A, brought flowbanded rhyolite domes (Tfbr) to the surface, whereas the Palm Park Formation (Tpp) was exposed by movement on the eastern fault. These stratigraphic units were still exposed when the Hayner Ranch half graben developed and were responsible for along-strike variations in provenance of the hanging wall-derived detritus. Although the border faults of the Caballo Mountains footwall were not affected by earlier rift structures, the Palm fault and Caballo fault are located within and are parallel to a 20-kinwide zone of Eocene compressional deformation characterized by largely southwestward-dipping, basement-cored reverse and thrust faults (Seager and Mack, 1986; Seager et al., 1986). The possibility exists, then, that the northwest trend of border faults of the Caballo Mountains was controlled by this belt of northwest-trending Eocene deformation. However, the Caballo Mountains

93

border faults do not directly coincide with mapped Eocene compressional faults, except locally. Late Miocene time, corresponding to deposition of the Rincon Valley Formation, saw continued movement of the border faults of the Caballo Mountains block, as well as the creation of two new basin-bounding border faults in the southern part of the study area. Although this fault activity created two half grabens, sedimentation within the half grabens was not independent. Interaction between the two basins is best expressed by the playa lithofacies and by sediment dispersal along the hanging wall dip slope of the southern basin. The playa lithofacies, which was situated in the topographically lowest part of the region, extends beyond the southeastern end of the northern half graben into the northern end of the southern half graben. Furthermore, the hanging wall of the southern basin appears to have had a northnorthwestward dip. Indeed, detritus with a mixed southern provenance extends well beyond the northern limit of the Sierra de las Uvas footwall into the southern part of the northern half graben. There may have been two factors contributing to the interaction between the Rincon Valley half grabens. One factor may have been greater magnitude of displacement on the border faults of the northern half graben, as suggested by greater stratigraphic separation, resulting in a greater amount of subsidence and a topographically lower area in the northern basin, Drainage in the southern basin also may have been affected by structures developed during Hayner Ranch time. Northwestward drainage in the southern basin of the Rincon Valley Formation may have resulted from westward tilt of the southern half graben superimposed on an original northeastward tilt inherited from the Hayner Ranch half graben. Finally, northwestward paleodrainage on the hanging wall of the southern basin may reflect development of a ramp produced along the transfer zone between the Jornada and Ward Tank faults. The type of interaction between the Rincon Valley half grabens does not conform to the style of interaction predicted in transfer zones. The border fault systems of the Caballo Mountains and Sierra de las Uvas qualify as an "approach-

94

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96

ing" (faults do not overlap), "conjugate" (faults dip in different directions), "divergent" (faults dip away from each other) transfer zone, following the scheme of Morley et al. (1990). This type of transfer zone is predicted to have a structurally and topographically high area between the basins, which limits interaction between them. This condition was apparently not met in the Rincon Valley basins, perhaps due to the influence of a prior strain event on subsequent basin development. Large parts of the Hayner Ranch and Rincon Valley basins were segmented into smaller fault blocks in latest Miocene or early Pliocene time, following emplacement of the 9.6 Ma Selden Basalt (Figs. 1, 2, and 8). Seager et al. (1984) constrained the period of major uplift on both new and old faults between 7.1 and 9.6 Ma in the vicinity of the Robledo Mountains, although movement has continued on many faults to the present. Major new fault blocks include the Robledo Mountains, San Diego Mountain, Cedar Hills, Rincon Hills, and Selden Hills (Fig. 1). Stratigraphic separation across border faults of the Robledo Mountains is approximately 1000 m, and San Diego Mountain rose more than 2300 m through the center of earlier Hayner Ranch and Rincon Valley basins. Uplift of the Caballo, Sierra de las Uvas, and Dona Ana ranges continued as the new fault blocks rose. Stratigraphic separation attributable to latest Miocene and Pliocene movement on border faults of the Caballo and Sierra de las Uvas uplifts is 640 m and 427 m, respectively. The sedimentary record of this latest episode of faulting is the Camp Rice Formation of late Pliocene and early Pleistocene age (Mack and Seager, 1990). Previous interpretations of rift evolution in south-central New Mexico have emphasized: (1) initiation of the rift approximately 28 to 27 Ma ago; (2) the concept of two stages of rift development (late Oligocene-Miocene and latest Miocene-Recent); and (3) change in orientation of extensional stress field between "early" (NE-SW) and "late" (E-W) rift stages (Chapin and Seager, 1975; Seager et al., 1984; Morgan et al., 1986; Keller et al., 1990). Our current understanding of the southern Rio Grande rift differs notably from

earlier concepts. We now believe that the southern part of the rift was initiated approximately 35 Ma ago and has evolved either continuously or by stepwise spurts of faulting up to the present (Fig. 8). Earliest rift basins were half grabens which now are almost entirely inverted, both structurally and topographically. However, some fault-block uplifts initiated sequentially between 35 and 7 Ma continue to be imposing features of the landscape. Major episodes of faulting began in early Oligocene, late Oligocene, middle Miocene, and latest Miocene to early Pliocene, each episode producing new fault blocks or new groups of fault blocks while disrupting older rift basins (Fig. 8). Only in a few places, where the various generations of basins have been superimposed, is there a continuous record of syn-rift sedimentary rocks (Fig. 8). Whether the deformation was continuous or episodic is still unclear, but current evidence indicates that the idea of two-stage rift evolution is oversimplified and misleading. Furthermore, as Fig. 8 suggests, there is little evidence to support deviation from general east-west extension for the last 35 Ma. Indeed, most fault blocks and their border faults are north-trending, with the exception of the northwest-trending Palm and Caballo faults, which may be reactivated along Laramide structures.

7. Conclusions

The southern Rio Grande rift near Hatch, New Mexico, USA, experienced at least four episodes of block faulting and basin development beginning 35 Ma ago, two of which are described here and correspond to deposition of the Hayner Ranch Formation (upper Oligocene-middle Miocene) and Rincon Valley Formation (middle-upper Miocene). Each episode resulted in a unique assemblage of uplifts and basins by the creation of new, largely north-trending faults a n d / o r by movement on faults initiated during previous episodes. Consequently, early basins were modified by subsequent fault-block development, and only one small area experienced continuous sedimentation. This interpretation illustrates the role of temporal changes in strain on the history of

G.H. Mack et al. / Sedimentary Geology 92 (1994) 79-96 extensional basins and challenges traditional ideas t h a t t h e s o u t h e r n R i o G r a n d e rift b e g a n 2 8 - 2 7 M a a g o , d e v e l o p e d in t w o stages, a n d e x p e r i e n c e d c l o c k w i s e r o t a t i o n o f stress f r o m n o r t h e a s t - s o u t h w e s t to e a s t - w e s t .

Acknowledgements T h i s s t u d y w a s f u n d e d , in p a r t , by t h e N e w Mexico Bureau of Mines and Mineral Resources a n d by t h e N e w M e x i c o G e o l o g i c a l S o c i e t y . M . R . Leeder and J.F. Hubert read an earlier version of t h e m a n u s c r i p t a n d m a d e s u g g e s t i o n s f o r its improvement.

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Seager, W.R., 1975. Cenozoic tectonic evolution of the Las Cruces area, New Mexico. N.M. Geol. Soc., Guideb., 26: 241-250. Seager, W.R. and Mack, G.H., 1986. Laramide paleotectonics of southern New Mexico. Am. Assoc. Pet. Geol., Mem. 41: 669-685. Seager, W.R., Hawley, J.W. and Clemons, R.E., 1971. Geology of San Diego Mountain area, Dona Ana county, New Mexico. N.M. Bur. Min. Miner. Resour., Bull., 97: 1-38. Seager, W.R., Clemons, R.E., Hawley, J.W. and Kelley, R.E., 1982. Geology of northwest part of Las Cruces l°X2 ° sheet, New Mexico. N.M. Bur. Min. Miner. Resour., Geol. Map 53.

Seager, W.R., Shafiqullah, M., Hawley, J.W. and Marvin, R.F., 1984. New K - A t dates from basalts and the evolution of the southern Rio Grande rift. Geol. Soc. Am. Bull., 95: 87-99. Seager, W.R., Mack, G.H., Raimonde, M.S. and Ryan, R.G., 1986. Laramide basement-cored uplift and basins in south-central New Mexico. N.M. Geol. Soc., Guideb., 37: 123-130. Seager, W.R., Hawley, J.W., Kottlowski, F.E. and Kelley, S.A., 1987. Geology of east half of Las Cruces and northeast El Paso °X2° sheets, New Mexico. N.M. Bur. Min. Miner. Resour., Geol. Map 57.