eocene and oligocene otay-type waxy bentonites …clays.org/journal/archive/volume...

Clays and Clay Minerals, Vol. 47, No. I. 70-83, 1999.

EOCENE AND OLIGOCENE OTAY-TYPE WAXY BENTONITES OF SAN DIEGO COUNTY A N D BAJA CALIFORNIA: CHEMISTRY, MINERALOGY,

PETROLOGY AND PLATE TECTONIC IMPLICATIONS

RICHARD ~ : BERRY

Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

Abstract--Otay-type waxy bentonites of San Diego County are illite-smectite (I-S) with 85% dioctahedral smectite mixed with dioctahedral illite showing a Reichweite of 0. Primary or secondary waxy bentonite exposures are found in all Eocene and Oligocene formations of southwest San Diego County and western Baja California north of Ensenada. Primary waxy bentonites formed when hot volcanic ash fell into quiet marine or brackish coastal water. The transformation from glass to bentonite occurred within hours or days but consolidation of the bentonite into its waxy consistency took longer. Primary waxy bentonite consists of 95 wt% I-S with the remainder consisting of volcanic glass shards, sanidine fourling twins, hexagonal biotite crystals and amorphous manganese oxides and hydroxides. Secondary waxy bentonite is primary waxy bentonite that was mixed with nonvolcanic detritus either before consolidation or after consolidation and subsequent erosion. The hydrophobic character of primary waxy bentonite allows it to reflect, accurately, the chemistry and petrology of the original volcanic material. Chemical analysis of primary waxy bentonites shows that the original lava was subduction-related and exhibited petrologic variations nearly identical to those of the modern Cascades of the northern Pacific coast of the lower United States. Primary and secondary waxy bentonites as well as smectites derived from the weathering of volcanic ash that fell outside the waxy bentonite-producing environments are the previously unrecognized products of extensive Eocene and Oligocene subduction-related volcanic activity. Baja California exposures of waxy bentonite demonstrate pre-Pliocene subduction tectonics that gave way to rifting tectonics.

Key Words--Chemistry, Mineralogy, Otay Bentonite, Plate Tectonics, Volcanism, Waxy Bentonite.

I N T R O D U C T I O N

San Diego County ' s waxy bentonites are of interest for 3 reasons: 1) their relationship to ground failure and other civi l engineer ing foundation problems (E1- liott and Berry 1991); 2) their reflection o f chemist ry and l i thology of volcanic materials f rom which they altered; and 3) the increased understanding they pro- vide about the early Tertiary Plate Tectonic history of Southern California. This paper provides mineralogi- cal, geochemica l and geological information about the Otay bentonite, a Clay Minerals Society Source Clay.

The first San Diego County waxy bentonite came f rom the Otay Format ion when it was mined, be tween 1917 and 1957, for use as a c leaning and de-color iz ing agent (Cleveland 1960). In the past decade, Otay-type waxy bentonite has been reported in all Eocene and Ol igocene formations f rom Oceanside, California, to Punta Mezqui te near La Misidn de San Miguel , Baja California, Mexico. San Diego County sample localities are shown in Figure 1. The Punta Mezqui te locality is in the sea cliffs adjacent to the Baja California Toll Road, approximately 50 km south of Tijuana and 1 km south o f " H a l f w a y H o u s e " (Medio Camino).

Unti l recently, the age and stratigraphic relationships of Eocene and Ol igocene formations (particular- ly the Otay Formation) have been unclear. Cleveland (1960) considered the Otay waxy bentonite beds to be part of the Pl iocene San Diego Formation. Minch

(1967), Minch et al. (1970), Kennedy (1973) and Ar- t im and Pinckney (1973) contr ibuted incomplete or in- adequate stratigraphic interpretations concerning the Otay Formation. Kennedy and Tan (1977) suggested that the age o f Otay Format ion fell somewhere between the restricted but age-constrained Pl iocene San Diego Format ion and the age-constrained Eocene Mis- sion Valley Formation.

Berry (1986) used clay mineral and zeoli te miner- a logy to establish that the Otay Format ion was different f rom the Eocene and Miocene format ions with which it had previously and erroneously been corre- lated. Berry speculated that the Otay Format ion could be Ol igocene. At the same time, Demere (1986) in- dependent ly placed the Otay Format ion in the latter part of the Ol igocene, based on the first suite of fossils (vertebrate) to be found in the Otay Formation. Berry (1991) picked fourling twins o f sanidine (some still at tached to glass shards) f rom the Otay waxy bentonite. Obradovich (1991, personal communica t ion) obtained an Ar /Ar determined absolute date o f 28.86 Ma (late Ol igocene) f rom sanidine crystals extracted f rom Otay waxy bentonite col lected at the Chester Grade locality. Obradovich (1990, personal communica t ion) also obtained an Ar /Ar determined absolute date o f 42.18 Ma (late Eocene) for waxy bentonite beds contained in the Eocene Mission Valley Formation. The Eocene sample was col lected f rom the south side o f

Copyright �9 1999, The Clay Minerals Society 70

Vol. 47, No. 1, 1 9 9 9 Otay-type waxy bentonites: mineralogy, petrology and plate tectonics 71

Figure 1. Index map of southwestern San Diego County, California.

Interstate Highway 8, between the Baltimore Drive and Jackson Drive overpasses.

It is now clear that the Otay Formation is of late Oligocene age. The general stratigraphic relationships of all early Tertiary formations in the San Diego area are now well established by Lohmar et al. (1991) and Bottjer et al. (1991). The general stratigraphy and depositional history for early Tertiary waxy bentonites in the San Diego area were presented by Berry (1993). Details of the paleontology, stratigraphy and depositional environments of the Otay and Sweetwater formations have been published by Walsh and Demere (1991).

PRIMARY AND SECONDARY WAXY BENTONITE

Otay-type waxy bentonites of San Diego County and adjacent areas may be primary or secondary. Pri- mary waxy bentonites most closely resemble paraffin in softness, translucence, luster and hydrophobic prop- erties. Beyond this the resemblance to paraffin ceases. Unweathered pieces of primary waxy bentonite have been left in distilled water for up to a year without any indication of slaking or disaggregation. Primary waxy bentonite may be white to light gray. When col- ored, it ranges between light pastel hues of yellow, pink and green. It is this material, mined from the Otay formation, that is archived as a Clay Minerals Society Source Clay.

Primary and secondary waxy bentonite, although similar in appearance, are quite different chemically. Primary waxy bentonites result from the fall of (probably hot) volcanic ash directly into water that is at least brackish. The alteration takes place in a matter of hours or days and the resulting deposit of pure, unconsolidated bentonite must be left undisturbed for a longer time until it develops its coherent waxy character. Origin of primary waxy bentonite will be dis- cussed and documented later in this paper.

Secondary waxy bentonites of San Diego County are of 2 types. One type occurs as a pebble conglom- erate or gritstone where the rounded clasts consist of waxy bentonite, reworked from an older deposit. These have been observed at 2 localities: 1) inside a tunnel that was dug by drug smugglers and is located near the international border crossing on Otay Mesa, and 2) within the Rosarito Beach Formation at Punta Mezquite, Baja California, Mexico. At both localities the clasts are nearly equidimensional, have largest di- ameters which range from 1 or 2 mm to 1 cm and are surrounded by a matrix of locally derived detritus. Geologic relationships at both localities suggest that clasts were stream-carved from an earlier deposit of primary waxy bentonite and redeposited after trans- portation. The plasticity, elasticity and softness of primary waxy bentonite, a/ong with its hydrophobic property, allowed the clasts to be transported what is estimated to be a maximum of 20 km downstream.

72 Berry Clays and Clay Minerals

I

Secondary Waxy Otay Bentonite

16

13.6

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Degrees 2 0 �9 Degrees 2 0 �9

Figure 2. XRD traces showing differences in clay mineralogy between primary and secondary waxy bentonites from the Otay Formation. Peaks are labeled in Angstrom units.

Once carefully separated from their detrital matrix, reworked clasts are chemically and mineralogically in- distinguishable from primary waxy bentonite. Detrital clasts of waxy bentonite may be broken down into silt- sized particles (or finer) and upon physically mixing with other detritus may resemble the type of secondary waxy bentonite described in the following paragraph.

The other type of secondary waxy bentonite occurs as a homogenous mixture of bentonite and detrital material. There is a strong resemblance to primary waxy bentonite until one observes the included detrital sand and silt. X-ray diffraction (XRD) shows I-S found in primary waxy bentonite mixed with soil clays that are typical of the products of weathering of local rocks (Figure 2). When a deposit of primary waxy bentonite was not allowed to consolidate, it became mixed with fluvial detritus, resulting in material that took on char- acteristics of both primary waxy bentonite and detrital components; being waxy yet opaque and frequently showing earthy shades of brown. Secondary waxy

bentonite of this type has been observed in separate layers and also grading upward and/or downward into a primary waxy bentonite layer.

MINERALOGY OF PRIMARY WAXY BENTONITE

Neither "bentonite" in general nor "waxy bentonite" in particular is a rigorously defined mineralogic term. Cleveland (1960), referring to the Otay bentonite, calls it "a rock term for a clay material composed principally of clay minerals of the montmorillonite group." Moore and Reynolds (1997) say about bentonite that, " in the most commonly used sense, this [is] material, often with peculiar, soapy propert ies . . ." and, "As early as 1917, it had been identified as a product of alteration of volcanic ash to the clay mineral smectite." Moore and Reynolds (1997) also state that "monoclinic K-feldspar sanidine is common in bentonites." Grim (1968) observed that biotite, in ad- dition to sanidine, is commonly associated with ben-


09 Z

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1500

1000

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NEW MOD: .85 Dismect-2Gly + Dimica

~ - ~ R e i c h w = 0, Fe = 0.1, Mica K = .8

I Waxy Otay Bentonite from Chester / Grade Site, Glycoiated

1'0 15 20 25 30 35 40 45

DEGREES TWO T H E T A

Figure 3. XRD pattern of Otay Formation primary waxy bentonite (Chester Grade locality) compared with the XRD pattern for I-S (85% dioctahedral smectite, 15% dioctahedral mica, Reichweite of 0) created by NEWMOD�9 The volcanic glass XRD signature is seen at 20 ~ on the Otay bentonite trace.

tonite and that volcanic glass shards or "shard structures of the ash as pseudomorphs" usually can be detected.

Lerman (1979) observes "[There are] three types of volcanic glasses, two zeolites and three clay minerals that commonly occur in oceanic sediments. Volcanic glass is a source phase, at whose expense zeolites and/ or clays can form." Lerman cites rhyolitic volcanic glass, clinoptilolite and montmorillonite as a unique marine association. The transformation of glass shards to smectite takes place either during fall of volcanic ash into water or shortly after accumulation and is not related to weathering (Grim 1968).

Mineralogy of the waxy bentonites of San Diego County is entirely consistent with what has been described in the geologic literature (cited above). The primary waxy bentonites consist almost entirely of I - S (mixed-layered illite-smectite). Mineralogic variations are very small, not only when comparing analyses of primary waxy bentonite from various exposures within the same formation but also when dealing with primary waxy bentonites from different formations. No primary waxy bentonite has been observed with less than 95 wt% I-S. Mineralogy of the I-S itself varies little from exposure to exposure, formation to formation, rarely departing more than 1 or 2 percent- age points from 85% dioctahedral smectite and 15% dioctahedral mica with a Reichweite of 0, as measured by comparison with the computer program NEW- MOD�9 developed by Reynolds (Figure 3).

The remaining portion (<5 wt%) of all primary waxy bentonites contain volcanic glass shards as the most abundant constituent. Individual shards are ob- servable by petrographic microscope in the < 100> 200 mesh (0.149-0.074 mm) size fraction. No primary waxy bentonite has been observed for which the entire sample did not pass 100 mesh. No glass shard material has been observed (optically) in fractions <200 mesh. Pseudomorphs of glass shards with largest dimensions of 1.0 mm have been observed in thin sections of primary waxy bentonite. The sections were made in the laboratory of Dr. Warren Huff at the University of Cin- cinnati. The shards exhibit varying amounts of devit- rification to opal A and opal CT. Evidence of this can be seen both on XRD patterns of primary waxy bentonite (Figure 3) and from the indices of refraction of shards. Shards showing the greatest degree of devit- rification show indices of refraction that range from 1.46 to 1.48, which is between the indices of refraction for opal A and opal CT (1.43-1.46) and the index of rhyolite glass (1.48-1.50, according to Wahlstrom 1955 and Ehlers 1987). A few, minimally devitrified, shards were extracted from the Otay and Mission Val- ley formations which have indices of refraction typical of a rhyolitic composition. Minimally devitrified shards from the Scripps Formation exhibit refraction indices of 1.51-1.53, typical of glass with andesitic composition.

The < 100> 200 mesh (0.149-0.074 mm) size fractions of all primary waxy bentonites contain individual

74 Clays and Clay Minerals Berry

Table 1. Otay waxy bentonite exchange cations.

Berry Post Madsen Newman

BEC--meq/100 g Layer charge Dominant exchangeable cation wt% CaO--CMS Source Clay wt% CaO--Otay Sanitary Land Fill wt% CaO--Chester Grade Locality wt% CaO--Telegraph Canyon Road Locality

129 - - 128 - - - - 0.46 - - 0.86 Na Mg Mg Na - - 1 . 3 7 - - - -

- - 0.79 - - - - - - 0.92 - - - - - - 1 . 4 0 - - - -

Analyses performed in the laboratories of Richard Berry--San Diego State University, James Post--Sacramento State University, Fritz Madsen--Swiss Federal Institute of Technology. Newman data were taken from Newman and Brown (1987).

crystals of sanidine (sometimes as fouding twins) and euhedral crystals of biotite. On rare occasions these minerals are still attached to glass shards. Normally, there is not enough sanidine or biotite to be detected on XRD patterns. Samples were taken from 12 waxy bentonite exposures in the Otay Formation and ana- lyzed by members of a San Diego State University graduate class in "Geology of Clays". X-ray analyses detected trace amounts of clinoptilolite or heulandite in samples from 3 of these localities. Cleveland (1960), referring to the Otay bentonite, noted, "The material is speckled with small disseminated grains of manganese oxide . . . . " Berry (1991), referring to manganese oxide, notes that a "speckling with amorphous, crumbly opaque, black grains is common to all of the waxy bentonites, regardless of age or sample locality".

Primary waxy bentonite lenses, regardless of formation, are essentially free of detrital, terrigenous con- stituents. Secondary waxy bentonites are mixed to varying degrees with detrital material. For this reason, chemical analyses of secondary waxy bentonites (with the exception of carefully extracted clasts of primary waxy bentonite) are of little use in working out the geologic history of the waxy bentonites.

CATION EXCHANGE CAPACITY (CEC) OF OTAY WAXY BENTONITE

The Otay bentonite has long been considered a " C a " bentonite although the author has been unable to document the basis for this widely held belief. Ex- changeable cation analyses were performed on samples of primary Otay waxy bentonite from the Chester Grade, Otay Sanitary Landfill and Telegraph Canyon Road localities. Analyses were performed at 3 different laboratories utilizing X-ray fluorescence (XRF; Post), ammonium acetate extraction (Berry) and ammonium SCN extraction (Madsen) (Table 1). One analysis yielded a BEC of 128 meq/100 g with Na as the dominant cation and another 129 meq/100 g with Mg as the dominant cation. A third analysis showed a layer charge of 0.46 with Mg the dominant exchangeable cation which may be compared with a layer charge of 0.86 and Na as a dominant cation published

by Newman and Brown (1987) for a composite sample of Otay bentonite source clay (Clay Minerals Society). The 2 layer charges compare well because the smaller one was based on one-half the formula weight of smectite of the larger one.

These data are significant for 2 reasons. First, they show that, contrary to popular belief, waxy Otay bentonite is not a Ca-bentonite. Post ran analyses of samples of Otay bentonite from several localities, showing the Ca content to be variable (Table 1). In most cases where analyses are available, Ca is the least abundant exchangeable cation and in no known case is Ca the dominant cation. Secondly, because there is no con- sistently dominant exchangeable cation in primary Otay waxy bentonite, the waxy property is not related to the type of exchangeable cation.

ENVIRONMENT OF DEPOSITION OF PRIMARY WAXY BENTONITE

Primary waxy bentonites result from the fall of volcanic ash directly into salt water. This assertion is based partly on literature citations (Cleveland 1960; Grim 1968; Lerman 1979; Moore and Reynolds 1997) and partly on the fact that the author has not observed primary waxy bentonite associated with riverine deposits. Walsh and Demere (1991) document a riverine environment of deposition for most, if not all, of the easternmost exposures of the Otay Formation. Al- though there are lenses of waxy bentonite associated with land fossils in these eastern outcrops, they are of secondary waxy bentonite reworked from older, perhaps Eocene, primary waxy bentonite. No land fossils have been found associated with the beds of primary waxy bentonite located to the west of the fossil localities described by Walsh and Demere (1991).

The environment of deposition for primary waxy bentonite is very close to that first suggested by Cleve- land (1960) when he says, "The ash appears to have fallen both on land and in the sea. The ash deposited directly in the sea may be represented in the Otay area by the principal clay bed which is relatively free of clastic impurities." By "sea" , Cleveland meant quiet, shallow, near-shore environments. Based on a more recent understanding of San Diego County early Ter-


tiary stratigraphy, Cleveland's concept of "sea" may be broadened to include estuarial, deltaic and coastal lagoon environments.

Cleveland was also the first to recognize that some waxy bentonite beds were of a different origin than those which resulted from a fall of ash directly into saline water. He says, on the same page noted supra, "The other bentonite beds . . . are considerably con- taminated, which indicates that the parent ash was transported by streams prior to disposition in the sea." These are the type of beds referred to in this paper as "secondary waxy bentonite."

The thickness of primary waxy bentonite beds is highly variable, usually the result of an undulating up- per boundary. Thickness variability in extreme is il- lustrated by the approximately 1-m flame structures in the primary bentonite beds at the Telegraph Canyon Road exposure of the Otay Formation. The beds of primary waxy bentonite diminish to almost zero thickness adjacent to the flames. The weight of overburden was responsible for upward intrusion of the flames. Both the overburden and the bentonite were very poorly consolidated at the time the flames developed. The ash must have altered to bentonite in a matter of days, certainly before deposition of the overlying material. The primary bentonite must have been partially consolidated or the waterborne overburden would have mixed with the bentonite, resulting in secondary waxy bentonite or sweeping the material away completely, rather than generating flame structures. A bed of ash- derived bentonite when allowed to rest, undisturbed, consolidated to become primary waxy bentonite. In- sight provided by flame structures about the physical consistency of the primary bentonite before it became waxy adds weight to the idea of the environment of deposition being quiet, shallow, coastal marine rather than riverine. Cleveland (1960) describes the "high grade" waxy bentonite beds that were being mined until 1957 at Otay Mesa as underlying an area of 1.3 square miles, an area of quiet deposition more consistent with a coastal marine rather than a riverine environment.

A less definitive line of evidence in support of primary waxy bentonite forming in water of marine or estuarial salinity is that although it is possible, chemically, to alter ash to bentonite in fresh water, the chemical path is simpler when salt water is involved. This is the basis for Grim's (1968) comment: "In order for bentonite to form, it is probably necessary for the ash to fall in water" and further, "Since much bentonite is associated with marine formations, it seems certain that the alteration can take place in sea water . . . . Whether or not the alteration can also take place in even more saline waters or in fresh water is not known definitely."

Discussion of the environment of deposition of primary waxy bentonite has been focused on the Otay

Formation because it is more extensively exposed for study and contains a greater abundance and greater variety of waxy bentonite lenses and beds than do formations of Eocene age. Exposures of Eocene waxy bentonites are sufficiently abundant to show that evidence of their environments of deposition point to environments identical with those for the Otay waxy bentonites (primary and secondary). Primary and secondary waxy bentonite is found in the Eocene Mission Valley Formation where it laps onto Cretaceous intru- sive igneous rocks at the Interstate 8 Grossmont Sum- mit locality. Waxy bentonite in this easternmost exposure of Eocene sediments of the area lends indirect evidence of 1 potential source for reworked waxy bentonites in easternmost exposures of the Oligocene Otay Formation.

RARE EARTH ELEMENT (REE) LINKS BETWEEN VOLCANISM AND WAXY

BENTONITE

The aquaphobic character of primary waxy bentonite has protected it from significant postdepositional chemical changes. Comparison of analyses of primary waxy bentonite and of shards extracted from the same sample showed remarkable similarity. This is particu- larly true for REEs. Analyses of both shards and bentonite could only be done for the Otay and Mission Valley formations' primary waxy bentonites. It took more than 25 kg of primary waxy bentonite to produce less than 1 g of minimally devitrified shards, including a few crystals of sanidine and biotite. No other formations contained primary waxy bentonite in suffi- cient quantity at any one exposure to yield the necessary weight of shards. Shard samples for REE analysis were divided into 2 representative aliquots. Con- tamination by detrital material and/or lack of homogenization of the shard samples would result in significantly different analytical results. Figures 4 and 5 show that both for the Eocene Mission Valley For- mation and the Oligocene Otay Formation the 2 aliquots yielded essentially identical results, confirming that errors owing to poor sample homogenization and detrital contamination were minimal. REE plots for the primary waxy bentonite from which the shards were extracted show depleted but essentially parallel config- urations with plots for their shards (Figures 4 and 5). REE depletion in the bentonite is largely a function of the large amount of water taken on by the bentonite during its formation. Loss on ignition (LOI) values of 25% or more are common. If the shards fell into sea water that was subject to evaporation, such as in a lagoon, the evaporite contribution could also cause an overall REE depletion. Neither the existence of a depletion nor the causes of the depletion are troublesome because the shape of the REE distribution curve re- mains the same from shards to bentonite.

76

Table 2. noted.)

Berry Clays and Clay Minerals

Description of waxy bentonite sample localities. (All sites are accessible for collection of material unless otherwise

Name Location

Oligocene-Otay formation localities Chester Grade

Drug Tunnel

Otay Sanitary Land Fill

Telegraph Canyon Road

Eocene localities Friars Formation-Rancho Bernardo

Mission Valley Formation-Interstate 8

Mission Valley Formation-Grossmont Summit

Oceanside, Santiago Formation

Scripps Formation

Mexican (Baja California) locality Punta Mezquite

At the intersection of Otay ValIey Road and Heritage Road, Otay Mesa, near Brown Field (Airport), San Diego County, CA (Stop #5, 1993 Clay Minerals Society Mid-meeting Field Trip #2).

From inside tunnel which extends south from near Custom House Pla- za to the Mexican Border (approximately 1 km west of Otay international border crossing, San Diego, CA). U.S. Army Corps of En- gineers has responsibility for access and use of the tunnel.

Deepest of southerly excavation at the sanitary landfill located near the intersection of Nirvana Avenue and Energy Way, Chula Vista, CA (north of Otay Valley Road). San Diego County has responsibility for access to this site. Availability for sampling will cease when the excavation is back-filled with trash (Stop #4, 1993 Clay Minerals Society Mid-meeting Field Trip #2).

A series of road cuts, north and south sides of Telegraph Canyon Road, extending for approximately 1 km west of intersection with Otay Lakes Road, Chula Vista, CA (Stop #6, 1993 Clay Minerals Society Mid-meeting Field Trip #2).

Near New Hope Church remediation site. Collection site is now bur- ied.

Now obscured by a buttress fill on the south side of Interstate High- way 8 between the Baltimore Avenue and Jackson Road overpasses, just west of the Interstate 8, Highway 125 interchange at the Gross- mont Summit.

Several exposures now partially obscured by landscaping vegetation. Most accessible site is immediately north of Interstate Highway 8 and immediately east of Grossmont Center Drive where Grossmont Center Drive underpasses I-8.

Near Mission and Canyon roads. Now covered by a housing development.

In sea cliffs below La Jolla Farms Road. Sample site has been partially excavated and the remainder covered during landslide remediation.

In sea cliffs, west of the Tijuana to Ensenada Toll Road, approximately 50 km south of Tijuana and 10 km north of La Misi6n de San Miguel. Exactly 1 km south of Halfway House (Medio Camino).

P r ima ry waxy ben ton i t e s f rom several fo rmat ions and f rom dif ferent beds in the same fo rma t ion were ana lyzed for the i r R E E compos i t ion . Resul ts o f these analyses are p lo t ted on F igure 6. T he shapes of all R E E plots indica te a s t rong uppe r crusta l character , espec ia l ly wi th regard to deple t ion of heavy R E E s (HREEs ) and the s t rong Eu anom a l y (Taylor and M c L e n n a n 1985). The data are wha t one would expec t for subduc t ion- re la ted vo lcan ic mater ials . T he pos i t ive Ce a n o m a l y that occurs in some samples is in teres t ing because it is cons ide red by some ( inc lud ing W h i t e et al. 1985) to reflect preferent ia l s caveng ing o f Ce 4§ f rom seawater. This adds suppor t to the idea tha t the vo lcan ic shards a l tered to ben ton i t e in seawater. The whi te Otay waxy ben ton i tes show pos i t ive Ce anom- alies whereas the p ink ones do not. Perhaps the presence o f Fe (as ind ica ted by the p ink color) impedes the s caveng ing of Ce f r o m seawater. It is no t c lear w hy whi te p r ima ry waxy ben ton i t e s f rom the Ches t e r

Grade and Te legraph C a n y o n R o a d local i t ies of the Otay Fo rma t ion show an increase for the ve ry heav ies t H R E E s (Tm, Yb and Lu).

R E E plots for the Ol igocene Otay F o r m a t i o n and for the E o c e n e San t i ago and Mis s ion Valley fo rma- t ions are wha t one would expec t for waxy ben ton i t e a l tered f rom subduc t ion- re la t ed rhyol i t ic , daci t ic and andes i t ic vo lcan ic mater ia ls . Waxy ben ton i t e f rom the Eocene Friars F o r m a t i o n shows less dep le t ion of the H R E E s than is the case for the o ther format ions . This represen ts a more near ly basa l t i c compos i t ion , pe rhaps re la ted to m i x i n g of basa l t wi th more sil icic l ava during the subduc t ion process .

Ana lyse s for R E E s in clasts ex t rac ted f rom a th ick depos i t of a Pebb le C o n g l o m e r a t e Fac ies of the Otay Fo rma t ion (exposed on ly wi th in the Drug Tunnel ) are s h o w n in F igure 7. R E E s in clasts ext rac ted f rom a lens of waxy ben ton i t e granules f rom the M i o c e n e Ro- sari to B e a c h Fo rma t ion exposed in sea cliffs at Pun ta


Figure 4. Chondrite normalized abundances of REE for Oligocene Otay formation waxy bentonite and glass shards (Chester Grade locality).

Mezquite, Baja California, Mexico, are also plotted in Figure 7. The shape of the REE plot for the Otay For- mation clasts is very nearly identical with that for white Otay primary waxy bentonite at the Telegraph Canyon Road locality, whereas the REE plot for the Rosarito Beach Formation clasts is very nearly identical with that for white Otay primary waxy bentonite at the Chester Grade locality. This indicates that re- working clasts of hydrophobic primary waxy bentonite in a stream environment does not damage the integrity of REE data. In the case of the reworked Rosarito Beach Formation waxy bentonite, the REE data further substantiate that the granules were reworked from an

older source that formed under conditions of subduction, unlike the coastal, basalt-dominated, rift-basin environment of deposition of the Rosarito Beach For- mation. Preliminary information about the primary bentonite source of the Rosarito Beach Formation granules has been presented by Berry and Ledesma (1997) and Ledesma and Berry (1997).

WAXY BENTONITE AND TERTIARY VOLCANIC HISTORY

REE analyses demonstrate the integrity of using the results of chemical analyses of aquaphobic primary waxy bentonites to reflect, accurately, the chemistry of

Figure 5. Chondrite normalized abundances of REE for Eocene Mission Valley formation waxy bentonite and glass shards (Interstate 8 locality).


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~eide - Eocene Pink Waxy Bentonite

White Waxy Otay Bentonite

Friars Fro., Rancho Bernardo - Eocene Green Waxy Bentonit~

I I I I I I I 1 I I I I

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RARE EARTH ELEMENTS

Figure 6. Chondrite normalized abundances of REE for white and pink Oligocene Otay waxy bentonite (Telegraph Canyon Road locality), Eocene Santiago Formation pink waxy bentonite (Oceanside locality), and Eocene Friars Formation green waxy bentonite (Rancho Bernardo locality).

the original volcanic ash. Analytical data obtained by XRF and inductive coupled plasma-mass spectrometry (ICP-MS) techniques from 6 primary waxy bentonites, and from clean granules extracted from secondary waxy bentonite at the Punta Mezquite locality, were subjected to standard igneous interpretation techniques, using NEWPET (see "Acknowledgments"). A set of data taken from a Clay Minerals Society Source Clay composite of Otay bentonite samples (Newman and Brown 1987), was supplemented with REE data from a similar composite of Otay bentonite samples (Mermut 1994, personal communication). The corn- posited data (labeled "Source Clay- -Newman") were included in the NEWPET interpretation procedures.

The data were plotted on an AFM diagram (where A = A1203, F = FeO, and M = MgO) (Figure 8) to show variations in major element composition. The Oligocene samples plot in a group. Two of the Eocene samples are similar to but slightly more aluminous than the Oligocene samples. The Eocene Friars For- mation plots well away from all other samples. This is consistent with REE data and possibly reflects an influence by basaltic magma.

Figure 9 shows a plot after Winchester and Floyd (1977, Figure 2). All samples (excluding the "New- man" composite sample) fall within the Rhyodacite/ Dacite field. There is a slight trend of increasing silica from Eocene to Oligocene. The Punta Mezquite sample plots with the Eocene primary waxy bentonites, adding to the body of evidence that the Miocene granules originated in a pre-Miocene (Eocene?) deposit of primary waxy bentonite.

Figure 10 shows another Winchester and Floyd (1977, Figure 6) plot which spreads the data out. All samples, except the Friars Formation, plot in Rhyod-

acite/Dacite, TrachyAndesite and Trachyte fields. The Friars Formation plots in the SubAlkaline Basalt Field, correlating well with the AFM plot and REE data. All the Oligocene samples plot with a slightly higher Zr/ TiO2 ratio than do the Eocene samples. The Punta Mezquite sample continues to show a chemistry more closely associated with Eocene than Oligocene sam- pies.

Figure 11 shows a totally different plot of the data after Middlemost (1985, Figure 3.3.3). In this treat- ment of the data, alkali and alkaline earth elements are taken into consideration. This was done to evaluate the effect of postdepositional smectite base exchange on the data. All data (with the exception of Source C l a y - - Newman) plot as the same or very nearly the same volcanic lithologies as they did for the Winchester and Floyd plots. The trend of increasing silica from Eocene to Oligocene is less well established in the Middlemost plot compared to the Winchester plots, but the Punta Mezquite sample continues to associate with the Eo- cene samples.

Waxy bentonites can portray the Tertiary volcanic history of Southern California and Baja California because their compositions closely reflect the composition of the lavas from which they were derived. Lavas that erupted along the coast of Southern California and Baja California during Eocene and Oligocene epochs were subduction-related; confirming with analytical data what has only been hypothesized previously.

Primary waxy bentonites were altered from rhyolitic, dacitic, andesitic and subalkaline basaltic lavas that fell as pyroclastic material into seawater or brackish estuarial environments. The aquaphobic character of the primary waxy bentonite prohibits significant postdepositional chemical change. Samples gathered from

Vol. 47, No. 1, 1999 Otay-type waxy bentonites: mineralogy, petrology and plate tectonics 79

IJJ 100 (.9 Z < c~ Z

m ,< a LLI N

< 10

r~ O Z LLI I--

a Z O "1- O

REWORKED WAXY BENTONITE

I

1 l 1 I I I l I I t l I l

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

R A R E E A R T H E L E M E N T S

Figure 7. Chondri te normalized abundances of REE for clasts picked from deposits of reworked primary waxy bentonite at the Drug Tunnel and Punta Mezqui te localities.

A A F M

M i o c e n e 0 Punta Mezquite

O l i g o c e n e �9 Otay, Chester Grade

Otay. Telegraph Canyon (white)

�9 Otay, Telegraph Canyon (pink)

[ ] Source Clay (Newman)

E o c e n e �9 Mission Valley Fm,

(Grossmont Summit) �9 Santiago Fro.- O c e a n s i d e

V Friars Fro. Rancho B e m a r d o

F M

Figure 8. A F M plot of waxy bentonites f rom San Diego County and northern Paja California, Mexico, where A A120:,, F = FeO, and M = MgO.


~ ~ - - ~ 3 ~ ~ _ WinchesterMiocene (fig &6)FI~

Phonolite O Punta Mezquite Corn/Pant Oligocene

1 _- �9 Otay, Chester Grade /k Otay, Telegraph Canyon

'~. (white) ~'*,,, �9 Otay, Telegraph Canyon

Rhyolite (pink) ~ " ~ [ ] Source Clay (Newman)

Trachyte " " ' - , Eocene �9 Misslon Valley Fro.

O_ TM .1 Rhyodaci te /Daci te " . . . . . . . . ~ . . . ~ �9 - (Grossmont Summit) N~ �9 Santiago Fm.- Oceanside

" ~ - ~ . �9 V Fdars Fm. Rancho " " ~ . . Bernardo TrachyAnd . . . .

Andesite

. . . . . . . . j .01 -- .............. sssssS ~1

Andesite/Basalt ,,,,

. . . . . . . . . . . . . . . . . . . . . . . AIk-Bas Bsn/Nph -

SubAIkaline Basalt .001 .I t ~ l _ . . J . . . 2 _ _ I _ t _ ~ _ _ ~ _ _ ~

,01 .1 1 10 Nb/Y

Figure 9. SiO 2 vs. Zr/TiO2 of waxy bentonites from San Diego County and Northern Baja California, Mexico (after Win- chester and Floyd 1977, Figure 2).

widely spread geographic localities show better cor- relation between silica richness and geologic age of the sample than between silica richness and sample collection locality. The data suggest that subduction- related volcanic eruptions became more silica-rich from Eocene to Oligocene.

The Friars Formation source lava was basaltic and may represent the same conditions ascribed by Gard- ner et al. (1995) to basaltic lava phases in his study of Mount St. Helens. The flux of basalt may have been high during the eruption of volcanic glass that altered to form the Friars Formation waxy bentonite. Re- worked waxy bentonites whose pure clasts can be separated cleanly from matrix reflect the geologic age and formation from which the primary waxy bentonite was eroded. Miocene Punta Mezquite (Rosarito Beach For- mation) secondary waxy bentonite was derived from a subduction-related, pre-Miocene (Eocene?) source rather than from the rift-related basalts that were erupt- ing during deposition of the Rosarito Beach Forma- tion.

SUMMARY OF CONCLUSIONS

Waxy bentonites are found in all Eocene and Oli- gocene formations of western San Diego County, Cal- ifornia, and northwestern Baja California, Mexico. Waxy bentonites may be primary or secondary. Pri-

mary waxy bentonites (including the Clay Minerals Society Source Clay, Otay bentonite) formed when subduction-generated volcanic ash fell into quiet, shallow, coastal marine environments. Volcanic ash altered to poorly consolidated primary waxy bentonite in a matter of hours or days. It took longer for the bentonite to consolidate into its waxy form. Primary waxy bentonites consist of 95 wt% I-S, which is 85% dioctahedral smectite and 15% dioctahedral mica with a Reichweite of 0. The remaining 5% is primarily volcanic glass shards with minor amounts of sanidine, biotite and manganous oxides. Contrary to popular belief, primary Otay waxy bentonite is not a Ca-bentonite. Dominant exchange cations are either Na or Mg.

If the primary waxy bentonite depositional environment was disturbed, the result was generation of secondary waxy bentonite or destruction of the bentonite deposit. "Secondary waxy bentonite" is primary waxy bentonite that has been mixed, prior to consolidation, with waterborne detrital materials. Consolidated waxy bentonites may erode, be mixed with other detrital sediments and be redeposited as secondary waxy bentonite. Although primary and secondary waxy bentonites bear a superficial physical resemblance, they are dis- tinctly different, chemically. Primary waxy bentonites are aquaphobic and therefore accurately reflect the chemical composition of the volcanic material from


80 I I I I I I I 1 I I i I I I I I I i I I I I I I I I I I I I I I I

W i n c h e s t e r & F l o y d 1 9 7 7 /

Rhyol i te / ( f ig 2) 75 _ / -- Miocene

Corn/Pant O Punta Mezquite Oligocene

�9 Otay, Chester Grade 70 - Rhyodacite/Dacite - /k Otay, Telegraph Canyon

(white) 6 v �9 Otay, Telegraph Canyon

65 - ,"~ 7 _ (pink) V ~ Trachyte [] Source Clay (Newman)

o~" [] ~ Eocene v _ �9 Mission Valley Fm. "~ 60 - Andes i te (Grossmont Summit) t",l .O �9 Santiago Fm.- Oceanside U') V Friars Fm. Rancho

55 -- Phonol i te Bernardo

S f

50 -- Sub-AB

45 A B / Bas/Trach/Neph

40 ~ ~ ~ ~ t ~ t ~ l ~ D ~ ~ ~ 1 t , , , , , L , I t I , , , , .001 .01 .1 1 10

Z r / ' r i o 2

Figure 10. Zr/TiO 2 vs. NbfY of waxy bentonites from San Diego County and northern Baja California, Mexico (after Wincheter and Floyd 1977, Figure 6).

which they were derived. Chemica l analyses demonstrate that waxy bentonites were der ived f rom a suite of rock types, typical of subduction-related volcanic activity. The data suggest a trend of increasing silica richness of the lava f rom Eocene to Ol igocene time.

PLATE T E C T O N I C I M P L I C A T I O N S

Southern California and Northern Baja California, Mexico , underwent a Tertiary transition f rom subduc- t ion-dominated to r i f t -dominated tectonics. Engebret- son et al. (1985) proposed that at 28 Ma, the approximately east-west trending boundary be tween subduction to the north and rifting to the south nearly coin- cided with the present day M e x i c a n - U S A border. Al though Engebretson 's proposal was based primari ly on computer reconstructions, it agrees wel l with geologic relationships in the international border region, especial ly those that developed during Eocene, Oli- gocene and Miocene time. Rift basin deposits of the Miocene Rosari to Beach Format ion are intercalated with r if t-generated basalt flows. Exposures of the Ro- sarito Beach Format ion are found south o f the international border, extending south nearly to Ensenada, Mexico , which is totally in conformity with the En- gebretson model. Secondary waxy bentonite clasts in the Rosari to Beach Format ion at Punta Mezqui te were reworked f rom a pre-Miocene (Eocene?) source. This could only be possible i f the boundary be tween sub-

duct ion and rifting activity were located more to the south prior to its location at 28 Ma.

The Ol igocene Otay waxy bentonites and older waxy bentonites f rom Eocene formations in San Diego County are ev idence of subduction north of Engebret- son's 28-Ma subduction/rif t ing boundary. Chemica l variations f rom one waxy bentonite to another are directly related to variations in volcanic chemist ry f rom one eruptive event to another. Eocene and Ol igocene variations in San Diego County are similar to those found among subduction-fed Holocene volcanos in the Cascade Mountains. It has been only a decade since the age o f the Otay Format ion was determined accurately and the relat ionship o f waxy bentonites to sub- duct ion-generated volcanic ash was established. Prior to 1985, the existence of a subduction zone along southern California and Baja California was in doubt because o f the perce ived lack o f geologic ev idence of its presence. Recogni t ion o f the plate tectonic signifi- cance of pr imary waxy bentonites was delayed because the bentonites are found in l imited and discon- t inuous deposits which may be misidentif ied as fault gouge (Elliott and Berry 1991).

A C K N O W L E D G M E N T S

The author thanks the following who contributed significantly to this paper. A. Mermut of the University of Saska- toon for REE data from the Clay Minerals Society Otay


/

11 12 14

+

0 ~ Q $ g ~

18 19 20 [] 22

17 k PA

o _L____I____J_ 42 50 60 70

SiO 2 (wl %)

Figure l I.

1 I I

5

m

15

23

80

Middlemost 1985 (fig 3.3.3)

Miocene 0 Punta Mezquite

Oligocene �9 Otay, Chester Grade /k Otay, Telegraph Canyon

(white) �9 Otay, Telegraph Canyon

(pink} [] Source Clay (Newman)

Eocene �9 Mission Valley Fro.

(Grossmont Summit) �9 Santiago Fro.- Oceanside

V Friars Fro, Rancho Sernardo

Na~O + KzO VS. S i O 2 of waxy bentonites from San Diego County and northern Baja California, Mexico (after Middlemost, 1985, Figure 3.3.3).

Source Clay. W. Huff of the University of Cincinnati for preparation of waxy bentonite thin sections. J. Obradovich of the U.S. Geological Survey Denver labs for argon-argon dating. GeoAnalytical Laboratory of Washington State University for excellent XRF and ICP-MS data from samples that were very small or excessively waxy. G. Girty, Department of Geolog- ical Sciences, San Diego State University, who helped with interpretation of REE data. J. Ledesma-Vazquez for valuable Mexican field and stratigraphic assistance. T. Foster for assistance with access to the "Drug Tunnel" and for drug tunnel sample preparation. U.S. Army Corps of Engineers for per- mission to explore and sample the "Drug Tunnel". Students in several Geology of Clays graduate classes at San Diego State University who were involved with some of the very earliest work on the Otay waxy bentonites. Members of the engineering geology community in San Diego for calling at- tention to many exposures of waxy bentonite that otherwise would have gone unnoticed. The plotting program NEWPET is available on the Internet at "'http://ftp.tu-clausthal.de/pub/ institute/inggeo/msdos/misc/np90107.txt".

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(Received 21 January 1998; accepted 4 August 1998; Ms'. 98-009)

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