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Chapter 76. Depositional paleoecology of Miocene ostracodes in the Monterey
major source of, and reservoir for, hydrocarbons. Many fossil groups are documented in the literature on the Monterey Formation, including foraminifers, calcareous nannoplankton, siliceous
microfossils, macroinvertebrates, fishes, and marine mammals, but not ostracodes. This may be attributed to the rarity of ostracodes in the predominantly low-oxygen bathyal deposits which
characterize the Monterey Formation. If significant concentrations of marine ostracodes were to be
encountered in this formation, they would most likely be found in those basal units which were
deposited when the basins first subsided and incorporated displaced neritic sediments. This is often
seen in the stratigraphic succession as a rapid and conformable transition from inner-neritic
sands to deep-water mudstones. The occurrence of ostracodes in the Oligo-Miocene lower Rincon
Formation of coastal central California(Finger, 1983a, b) exemplifies this phenomenon. A somewhat
analogous situation encountered in the middle Miocene Monterey Formation of southern California is the subject of the present investigation. The purpose of this report is to determine the depositional paleoecology of these ostracodes.
1-Location of Laguna Hills sample localities referred to in this study.
L H - I LH-2
2-CSA81R bluff section along Moulton Parkway (Stadum, 19821, with demarcation of LH-1 to -4 samples analyzed in this study.
SHELL FRAGMENTS IN SANDSTONE
PECTEN AND GASTROPOD FOSSILS
3-CSA81P columnar section of the Monterey Formation as previously exposed in sewer trenches near
sample locality MAR-254 (Stadum, 1982).
The ostracodcs were collected in Laguna Hills, Orange County, in the Saddleback Valley region
of southern California (Text-fig. 1). They occur in a basal sequence of the Monterey Formation
which includes an unusual limestone deposit referred to as the "Pecten Reef" by local residents
because it contains dinner-plate sized scallops; this limestone is actually a discontinuous series of
small patch reefs interfingering with and covered by clastic sediments bearing abundant serpulids,
oysters, bryozoans, button sand dollars, and pectens (Stadum, 1982, 1984; see Text-figs. 2-4).
Because the area was targeted for extensive housing development, paleonotologists from the
Los Angeles County Museum of Natural History and salvage teams from the Natural History
Museum of Orange County acquired extensive collections of fossils from this region during the
1972-1982 interval. As is so often the case with California fossil localities, the majority of collecting
sites have since been obliterated by suburban sprawl.
PLIOCENE NIGUEL FORMATION
MID-PLIOCENE CAPISTRANO FM.
MIOCENE MONTEREY FORMATION
PHOSPHATE COBBLE HORIZON
VlTRlC ASH HORIZON
DARK ORGANIC MUOSTONE
SERPULID LIMESTONE HORIZON
MIOCENE SAN ONOFRE BRECCIA
Tm-FIG. &Generalized stratigraphic columnar section (Stadum, 1982) for Laguna Hills.
Miocene Ostracodes in Monterey Formation. California 1105
One of the lithologic units in the Laguna Hills area is a series of greenish-grey mudstone and
unconsolidated calcareous sand turbidites. Stadum (1984, p. 87) noted that the sand is extremely
rich in foraminifers, ostracodes, shell fragments, shark teeth, and fish bones and otoliths. Mr.
Mark A. Roeder, one of the consultants involved in the collections, salvaged an entire lens of this
sand (approximately 4 m3) and some of the surrounding greenish-grey mudstone exposed on the
surface of a graded area in the property immediately north of the intersection of Moulton and
Alicia Parkways (Text-fig. 1; MAR-254). This material is equivalent to Stadum’s (1982, 1984) locality CSA81N. After processing these sediments for fish teeth and otoliths, Mr. Roeder kindly
presented me with a washed residue that was extremely rich in foraminifersand ostracodes, and an
ample supply of unprocessed material from both lithologies. On a subsequent visit to the area in
December, 1983, G. L. Armstrong and I collected seven samples (LH-1 to -7) from two nearby
exposures (see Text-fig. 1) to supplement this study. Four sandstone samples (LH-1 to -4) were
obtained from what remained of Stadum’s (1982) CSA81R section along Moulton Parkway (Textfig 2), even though there was no evidence in the field to suggest the presence of calcareous microfossils. The other three samples are slightly younger mudstones (LH-5 to -7) collected from six stratigraphic metres of section that had been exposed along the southeastern side of Alicia Parkway
prior to grading. Abundant foraminifers were visible without magnification in two of these mudstones.
Although abundant foraminifers were retrieved from four of the sandstones and two of the
mudstones, only the siltstones and sandstones of MAR-254, LH-1, LH-3, and LH-4 yielded ostracodes. All of these ostracode-bearing sediments are from the turbidite unit stratigraphically positioned above the reef complex (Text-fig. 4).
The silty sand of MAR-254 contains approximately 8000 foraminifers and 900 ostracode valves
per kg. Neither of these microfossils were recovered from the surrounding mudstone. Processing of
both MAR-254 lithologies for siliceous microfossils and calcareous nannofossils yielded negative
Most of the ostracodes in MAR-254 are preserved as complete carapaces which have undergone
slight dissolution. Preservation of complete carapaces has been enhanced by cementation of their
internal margins with secondary calcite. Although thr carapaces are not infilled, attempts to disarticulate them inevitably fragment the valves. Examination of marginal pore canals and muscle
scars has been hindered by these preservational features. Further study and documentation of the
microfauna is planned for a forthcoming publication.
The MAR-254 ostracode assemblage consists of 10 genera and 14 new species: Microcytherura
sp. (33 %), Aurila sp. A (27 %), Aurila sp. B (11%), Aurila sp. C (7 %), “Cytheretta” sp. (5 %), Loxoconcha (Loxoconcha) sp. (4 %), Ambostracon sp. (3 %), Loxoconcha (Palmoconcha) cf. L. (P.)
polugari Finger (3 %), Paracosta cf. P. huddlestoni Finger (3 %), Hamanella cf. H. implexa Finger
(2%), Neonesidea sp.? (< 1 %), Cytherelloidea sp. (< 1%), “Microcytherura” sp. (< 1%), and
Xestoleberis sp. (< 1 %).
Calcite cement made it particularly difficult to isolate clean and unbroken ostracodes from the
sandstones along Moulton Parkway. One specimen of Cytherella was obtained from LH-1,and
a few early instars of Aurila were retrieved from LH-3. LH-4 yielded a substantially richer assemblage which includes Aurila, Hermanites. Loxoconcha, Loxocorniculum, Neonesidea?, “Paijenborchella”, and Paracytheridea, but most of the specimens are juveniles and/or too poorly preserved
to warrant further study.
1106 K. L. FINGER
Most of the ostracode species recovered from Laguna Hills are illustrated as scanning electron
micrographs in Plates 1 and 2.
There are remarkable similarities between the Laguna Hills species of Loxoconcha (Palmoconcha), Hamanella, Paracosta, and “Paijenborchella”, and those described from the Oligo-Miocene
lower Rincon Formation approximately 250 km to the northwest (Finger, 1983a). The Laguna
Hills species of Paracosta is morphologically and chronostratigraphically intermediate between
P . huddlestoni Finger from the lower Rincon Formation and P . simiensis (LeRoy) from the upper
Pliocene part of the Fernando Formation and the lower Pleistocene Santa Barbara Formation
(see LeRoy, 1943; Cronin et al., 1983).
Precise age-dating of the Laguna Hills section relies on other fossil groups which have been
more extensively studied and correlated in the California Neogene. Stadum (1982) employed several
consultants to analyze the local micropaleontology: H. Haga for palynomorphs, E. D. Milow
for calcareous nannoplankton and siliceous microfossils, and A. D. Warren for foraminifers.
Recovery of microfossils from the limestone was practically nil, whereas the sandstone and mudstone yielded significant associations of dinocysts, gymnosperm and angiosperm pollen, calcareous
nannofossils, diatoms, silicoflagellates, ebridians, and foraminifers. The most diagnostic assemblages are those of the calcareous nannofossils, diatoms, and benthic foraminifers, each of
which is characteristic of the early middle Miocene, 14-16 Mya. The same age is implied by the
Luisian Stage benthic foraminifera1 assemblages isolated from the neritic sands and bathyal mudstones analyzed in the present study.
A subtropical to warm-temperate biota inhabited coastal California during the early middle
Miocene. Stadum (1982, 1984) has determined the geologic and ecologic history of the Laguna
Hills section in considerable detail ;the following is a slightly modified version of her interpretation:
The depositional environment of the limestone reef was a submerging northward trending
strait with a steep escarpment to the west and a semi-arid low-relief plain to the east across which
a river meandered through a marsh or swamp. Submarine toes of debris fans extending from the
schistose highland to the west formed a substrate on which finer sediments accumulated and
serpulid worms flourished. Eventually, the limey tubes of these animals formed a series of welded
frameworks, or patch reefs. Oysters added to the growth of these reefs in the fluviomarine areas
subject to fluctuating salinities and turbid waters. Cheilostome bryozoans replaced this community wherever normal marine salinities and clear waters prevailed, as warm marine currents
PLATEl-Fig. 1 . Ambosrrucon sp., left lateral view of female carapace, sample no. MAR-254, ~ 7 5 Fig.
Microcytheruru sp., right lateral view of female carapace, sample no. MAR-254, x 73. Fig. 3. Cytherelloidea
sp., left valve of male carapace, sample no. MAR-254, x 60. Fig. 4. Humanella cf. H.implexu Finger, right
lateral view of female carapace, sample no. MAR-254, x 68. Fig. 5. “Microcytheruru” sp., left lateral view
of female carapace, sample no. MAR-254, x 77. Fig. 6. “Cytherettu” sp., left lateral view of male carapace,
sample no. MAR-254, X81. Fig. 7. Neonesideu sp.?, right lateral view of female? carapace, sample no.
MAR-254, x37. Fig. 8. Purucosru cf. P. huddlestoni Finger, left lateral view of female carapace, sample
no. MAR-254, X 69. Fig. 9. Loxoconchu (Loxoconchu) sp., right lateral view of male? carapace, sample no.
MAR-254, x 80. Fig. 10. Loxoconcha (PuImoconchu) cf. L. (P.)poluguri Finger, left lateral view of female
carapace, sample no. MAR-254, x 111.
Miocene Ostracodes in Monterey Formation, California 1109
from the south circulated through the embayment. Other benthic invertebrates patchily distributed among the reefs included echinoids, barnacles, bivalves, gastropods, limpets, and boring
sponges. As the embayment began to subside, nearshore sands and shell fragments blanketed the
reefs. Rapid subsidence of the basin to bathyal depths resulted in turbidity currents which spilled
out a series of silty sands and muds on the basin floor. The turbidite sands incorporated rich
assemblages of inner-neritic (0-50 m) to upper middle-bathyal (500-1 500 m) benthic foraminifers, with a predominance of neritic species, as determined from the upper-depth limits of benthic foraminifers designated and subsequently modified by Ingle (1980; 1985, unpubl. data). Downslope displacement of sediments was rapid enough to bury alive ostracodes derived from the neritic
zone, as evidenced by the predominance of complete carapaces in their assemblage (see Oertli,
1971; Finger, 1983a, b). These sands also incorporated calcareous nannoplankton and planktic
foraminifers which were being transported into the bay from the open ocean.
The mudstones higher in the section contain assemblages of foraminifers, palynomorphs, and
siliceous microfossils, as well as some of the earlier macrofauna. Benthic foraminifera1 species
indicate that the basin floor was still within upper middle-bathyal(500-1500 m) depths, but bathyal
species now predominate in the assemblage. Significant downslope displacement is evidenced by
the inclusion of inner-neritic to upper bathyal foraminifers, terrestrial palynomorphs, and remnants of the reef macrofauna. Planktic assemblages of foraminifers, diatoms, silicoflagellates, and
ebridians precipitated into these muds from overlying oceanic currents. Ostracodes are not present
in these bathyal sediments. The contrast between the sandstone and mudstone facies, which may
represent proximal and distal turbidites, respectively, suggests that the embayment continued
to subside and widen during this interval of time. Subsequent deposition of diatomites resulted
from increased upwelling during the middle to late Miocene cooling trend and preceded gradual
emergence of the area from the Pliocene to the Recent.
It is somewhat difficult to explain why ostracodes and foraminifers are more abundant in the
turbidite sands than in the earlier reef deposits. Perhaps this is a preservational factor, as many of
the earlier units are well-indurated with calcite cement.
In addition to those found in the Laguna Hills area, significant limestone beds in the Monterey
Formation have been recorded from Palos Verdes Hills (Bramlette, 1946), west of Santa Barbara
near Naples (Bramlette, 1946), San Clemente Island (Susuki and Stadum, 1978; Vedder and Howell, 1976; Vedder and Moore, 1976), and Santa Catalina Island (Vedder and Howell, 1976; Vedder and Moore, 1976). Most of these occurrences are unrelated. On San Clemente Island, however,
Susuki and Stadum (1978) collected an ostracode-bearing loosely consolidated orange sandstone
(sample locality 6317) in a sequence similar to that of Laguna Hills. T. Susuki presented me with
some washed residue from this sample, which contains a shallow-marine ostracode assemblage
PLATE2-Fig. 1. Hermanites sp., external view of left valve, sample no. LH-4,
X 110. Fig. 2. Paracytherideasp.,
external view of right valve, sample no. LH-4, x 150. Fig. 3. Cytherella sp., external view of right valve,
sample no. LH-1, x90. Fig. 4. "Paijenborchellu" aff. "P." sp. Finger (1983a), external view of left valve,
sample no, LH-4, x 120. Fig. 5. Loxocorniculum sp., external view of left valve, sample no. LH-4, X 130. Fig.
6. Xestole beris sp., left lateral view of female carapace, sample no. MAR-254, X 110. Fig. 7. Aurila sp. B,
left lateral view of female carapace, sample no. MAR-254, x72. Fig. 8. Aurila sp. C , left lateral view of
female carapace,sample no. MAR-254, x 79. Fig. 9. Aurilu sp. A, left lateral view of female carapace, sample
no. MAR-254, x 74 (the pronounced anterior cardinal angle of the left valve is a variationseen in both sexes
of this species). Fig. 10. Aurilu sp. A, right lateral view of male carapace, sample no. MAR-254, x74.
1110 K. L. FINGER
dominated by species of Aurila and Ambostracon, and includes some of the same species recognized
in Laguna Hills. There can be little doubt that these assemblages are part of the same fauna.
The neritic ostracode assemblage recovered from Laguna Hills occurs in a turbidite sequence
which was deposited in an embayment as it subsided to upper middle-bathyal depths during the
early middle Miocene. Predominance of complete carapaces supports other evidence for this
mode of deposition. The turbidite unit is stratigraphically above a nearshore patch-reef complex
and below upper middle-bathyal mudstones. These superjacent mudstones, dominated by bathyal
foraminifers and devoid of ostracodes, are more typical of the lower Monterey Formation elsewhere.
In a previous study, paleontologic and lithostratigraphic analyses of the lower Rincon
Formation reveal much about the development of the Santa Barbara Embayment (Finger, 1983a, b).
Similarly, interpretation of the lower Monterey Formation in Laguna Hills provides a geologic
record of the early Santa Ana Embayment. Ostracodes complement other data useful in determining
the depositional histories of these basins. Further study of the basal Monterey Formation and
equivalent units could uncover additional ostracode assemblages of similar value in reconstructing
the evolution of the California Continental Borderland. Knowledge of such assemblages is critical
to our understanding of the ecology, phylogeny, and evolution of the East Pacific fauna. Another
piece in this very incomplete puzzle is anticipated from a more detailed documentation of the calcareous microfauna recovered from Laguna Hills and San Clemente Island.
I would like to thank a number of colleagues who have assisted me in this study: M. A. Roeder
brought the Laguna Hills ostracodes to my attention and provided the ostracode-rich sample; G. L.
Armstrong aided in fieldwork, sample picking, scanning electron microscopy, and drafting of the
figures; C. J. Ford processed the samples; E. M. Brouwers and R. C. Whatley reviewed my generic
assignments. Much of the information conveyed in this report was extracted from C. L. Stadum’s
excellent thesis on the subject area, and I appreciate the time she spent discussing it with me and
reviewing this manuscript. Chevron Oil Field Research Company and Chevron U.S.A., Inc. (Western Division) kindly granted publication permission. T. M. Cronin was most gracious in presenting this study for me at the Ninth Internatianal Symposium on Ostracoda.
1946. The Monterey Formation of California and the origin of its siliceous rocks. Prof. Pap. U.S.
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