Geographic gradients in the formation of shell concentrations: Plio-pleistocene marine deposits, Gulf of California

Palaeogeography, Palaeoclimatology, Palaeoecology, 101 (1993): 1-25

l

Elsevier Science Publishers B.V., Amsterdam

Geographic gradients in the formation of shell concentrations: Plio-Pleistocene marine deposits, Gulf of California Keith H. Meldahl

Department o f Geology, Oberlin College, Oberlin, OH 44074, USA (Received June 16, 1992; revised and accepted December 12, 1992)

ABSTRACT Meldahl, K.H., 1993. Geographic gradients in the formation of shell concentrations: Plio-Pleistocene marine deposits, Gulf of California. Palaeogeogr., Palaeoclimatol., Palaeoecol., 101:1 25. Shell beds (coarse concentrations of invertebrate bioclastic material) are rich sources of paleontologic and stratigraphic information. Studies often emphasize the relationship between shell bed formation and bathymetric gradients, because many sedimentologic and ecologic factors that influence shell-concentrating processes vary with water depth. However, shell bed formation is also influenced by factors that are not related to water depth. Shell beds in Plio-Pleistocene marine sediments of the Gulf of California (Mexico) formed in less than 10-15 m water depth, but under varying conditions of coastal topographic relief and tidal range. Sampling of shell beds focused on two regions: the northern mainland coast and the southern peninsular coast. Shell beds in the northern mainland region (13 total) formed along a macrotidal, low-relief coastline, whereas shell beds in the southern peninsular region (27 total) formed along a microtidal, high-relief coastline. Shell beds represent community beds (11); storm beds (3); beach berm beds (6); tidal channel bed (1); or current~wave-winnowedbeds (19). The community beds represent biogenic concentrations; the other four types represent sedimentologic concentrations. Sedimentologic concentrations formed either through short-term concentrating events (storm beds), or long-term dynamic bypassing (beach berm beds, tidal channel beds, and current/wave-winnowed beds). Concentrations formed through dynamic bypassing are far more common (26 beds) than storm concentrations (3 beds). Of the 26 concentrations formed through dynamic bypassing, 10 cap major stratigraphic discontinuities, representing temporal breaks ranging from 103 to 10a years. The abundance and distribution of the five types of shell beds is significantly different between the two sampling areas. The northern mainland coast is dominated by allochthonous beach berm beds capping linear beach ridges. The formation of beach ridges in the north is favored by the wide shallow shelf (representing a large reservoir for beach shells), and sediment starved conditions resulting from the low topographic relief and periodic reductions in sediment supply from the Colorado River delta. Beach berm beds are absent along the southern peninsular coast, where coarse terrigenous sediment shed from adjacent highlands overwhelms shell input to the beach, and the narrow steep shelf supplies the beach with few shells. The southern peninsular coast is dominated by parautochthonous current/wave-winnowed beds and autochthonous community beds. Southern shell beds have undergone significantly less reworking, winnowing and transportation than northern shell beds. This is a function of both the relative weakness of tidal currents in the south, and the relatively higher rates of terrigenous sedimentation along the rugged southern coastline, which can bury shell concentrations relatively rapidly. Geographic variation in topographic relief and tidal range results in significant variation in the distribution and abundance of shell beds within these shallow marine sediments. These variables have a root tectonic cause (relief reflects tectonic activity; tidal range reflects basin configuration), underscoring the potential utility of fossil concentrations for reconstructing earth history.

Introduction T h e field o f t a p h o n o m y h a s e n j o y e d i n c r e a s i n g attention from paleontologists over the past two

Correspondence to: K.H. Meldahl, Department of Geology, Oberlin College, Oberlin, OH 44074, USA. 0031-0182/93/$06.00

d e c a d e s . T a p h o n o m i c studies h a v e by n o w b e e n a p p l i e d to d i v e r s e p r o b l e m s in s e d i m e n t o l o g y , paleoenvironmental reconstruction, paleoecology a n d p a l e o b i o l o g y ( f o r r e v i e w s see A l l i s o n a n d Briggs, 1991; D o n o v a n , 1991). O n e f o c u s o f interest h a s b e e n t h e o r i g i n fossil c o n c e n t r a t i o n s , especially shell c o n c e n t r a t i o n s o r " s h e l l b e d s " . Shell b e d s

© 1993 - - Elsevier Science Publishdrs B.V. All rights reserved.

2

(coarse concentrations of bioclastic material; Kidwell, 1991), are rich sources of paleontologic and stratigraphic information. An increasing number of studies have attempted to interpret the ways shell beds form in different environments under various depositional conditions (e.g. Aberhan and Ffirsich, 1991; Geary and Allmon, 1990; Kidwell, 1991 and references therein; Meldahl and Cutler, 1992; Miller et al., 1988; Norris, 1986). Many studies of shell beds emphasize the role of bathymetric variation in controlling the distribution and abundance of shell beds. Bathymetry clearly exerts an important control on shell bed formation, because many sedimentologic and ecologic factors that influence shell-concentrating processes vary with water depth (e.g. intensity of storms, waves and currents; rate and consistency of sedimentation; intensity of biogenic reworking; distribution and abundance of food). But there are many variables influencing shell bed formation that are not linked to bathymetry. This study explores the effects of topographic relief and tidal range on shell bed formation in Upper Pliocene to Pleistocene shallow marine deposits from the Gulf of California. In sum, this study represents a natural experiment in which bathymetry varies only within a narrow range (all the shell beds reported on here formed in littoral to sublittoral settings not exceeding 10-15 m depth), while coastal topographic relief and tidal range vary substantially. Sampling of PlioPleistocene outcrops was designed to maximize differences in these variables: Two regions are compared: the mainland coast of the northern Gulf and the peninsular coast of the southern Gulf (Table 1 and Fig. 1). Study area

The Gulf of California is over 1000 km long, spans 9 ° of latitude, and ranges from 90 to 210 km in width. It is an active trans-tensional rift basin linking the East Pacific Rise with the San Andreas fault system. Structurally the rift basin reaches well into southern California, though this northern portion has been largely filled in through sedimentation from the Colorado River. Magnetic anomaly patterns at the mouth of the

K.H. MELDAHL

Gulf indicate the northwestward translation of the Baja peninsula away from mainland Mexico was initiated between 5.0 and 6.0 m.y. ago (Moore and Curray, 1982). However, paleontological evidence demonstrates that an extensive Gulf was probably present by the Middle Miocene, 12-14 m.y. ago, indicating that a "proto-Gulf" existed for a considerable time before significant right-lateral translation occurred to form the Baja peninsula (Smith, 1991 and references therein). During the Middle to Late Miocene, marine waters extended into southern California, southwestern Arizona and western Sonora (Gastil et al., 1983; Smith, 1991). Smith (1989, 1991) provides updated Miocene-to-Holocene paleogeographic reconstructions, based on improved molluscan biogeographic and biostratigraphic data. Useful references to the geology of the Gulf, the Baja peninsula, and the mainland coast include Abbott (1989); Dauphin and Simoneit (1991); Frizzell (1984); Ortlieb (1987, 1991); Perez-Segura and Jacques-Ayala (1991); and Van Andel and Shor (1964). Plio-Pleistocene marine sediments

Upper Pliocene and Pleistocene shallow marine sediments are common along both coasts of the Gulf of California (Ortlieb, 1987, 1991), commonly unconformably onlapping igneous basement, alluvium, fluvial or other marine strata. Upper Pliocene units consist of fossiliferous shallow marine sandstones, gravels and conglomerates of the Marquer Formation in the area of Loreto and Isla Carmen (Durham, 1950), and the Infierno Formation in the Mulege area (McFall, 1968). Durham (1950) and Anderson (1950) described Plio-Pleistocene deposits in the central and southern Gulf, and Durham (1950) and Durham and Allison (1960) established a systematic and biostratigraphic framework based on the diverse molluscan fauna. Pleistocene sediments are more geographically widespread in the Gulf than Pliocene sediments. Particularly common are wave-cut benches, terraces, and shallow marine sandstones, gravels, conglomerates and coquinas, typically ranging from 1 to 5 m thick. Many of these have been

GEOGRAPHICGRADIENTSIN THE FORMATIONOF SHELLCONCENTRATIONS TABLE 1 Comparison of northern mainland and southern peninsular coasts of the Gulf of California Northern Mainland coast (Area A in Fig.l)

Southern Peninsular, coast (Area B in Fig.l)

Coastal topography

Low-relief, bordering gently sloping alluvial plains.

High-relief, bordering rugged plutonic and volcanic highlands and plateaus.

Bottom slope (Van Andel, 1964)

99% less than 5 °

75% less than 5° 24.5% 5°-30 ° 0.5% greater than 30°

Coastal sediment (Van Andel, 1964; field observations)

Fine to medium subarkose or quartzarenite; molluscan calcarenite and calcirudite; minor mud and silt.

Volcaniclastic conglomerate, gravel and coarse litharenite; algal calcarenite; molluscan and coral calcirudite.

Dominant sediment sources (Van Andel, 1964)

Colorado River; local molluscan shell debris.

Local drainages from adjacent highlands; local molluscan and coral shell debris.

Tidal range (Ortlieb, 1987; Thompson, 1968; Roden, 1964)

Macrotidal: 3.5 m mean range (5.0 m spring) at Punta Penasco; 5.5 m mean (8.0 m spring) at the Colorado River delta.

Microtidal: 0.5-1.0 m mean range; rarely exceeding 1.5 m spring range.

Surface salinity (Ortlieb, 1987; Roden, 1964; Hendrickson, 1974)

35.5-37.5%o; up to 3 9 ~ in shallow coastal waters in summer.

34.8-35.2%o

Surface water temperature (Ortlieb, 1987; Roden, 1964)

23.2°C mean annual at Puerto Penasco; ranges from 15°C (Jan) to 31°C (Aug) for most of the northern Gulf.

24.7°C mean annual at La Paz; ranges from 20°C (Feb) to 29°C (Sept) for most of the southern Gulf.

Precipitation (Ortlieb, 1987, Thompson, 1968; Ezcurra and Rodrigues, 1986)

5-10 cm/yr

20-25 cm/yr

Air temperature (Ortlieb, 1987; Roden, 1964)

20.3°C mean annual at Puerto Penasco; ranges from 14°C (Jan) to 34°C (July) for most of the northern Gulf.

24.0°C mean annual at La Paz; ranges from 16°C (Jan) to 30°C (July) for most of the southern Gulf.

correlated by Ortlieb (1987) with several Pleistocene interglacial highstands, based on uraniumseries and amino acid dating, and geomorphic position. The most common of these terraces occurs 6-9 m above present mean sea level and correlates with the peak of the Sangamon interglacial highstand, about 125,000 yr B.P. (oxygen isotope stage 5e), when mean sea level stood approximately 6 m above present. Detailed PlioPleistocene faunal lists can be found in Durham (1950), Ortlieb (1987) and Smith (1991 and references therein). The Plio-Pleistocene sediments reported on here were deposited in littoral or sublittoral environments at depths not exceeding 10-15 m, based on the following evidence:

--The deposits often unconformably onlap nonmarine rocks and are rarely more than 5 m thick. --Volcaniclastic marine conglomerates and gravels are common in the sections and can sometimes be traced laterally into non-marine alluvium over distances of several hundreds of meters. --Deposition at or near the paleo-shoreline is indicated by facies representing tidal channels, intertidal flats, and beaches. - - T h e ubiquity of the coral Porites californica in central and southern Gulf Plio-Pleistocene sediments indicates water depths of 2-15 m (Durham, 1950). - - M a n y fossil mollusc taxa have extant representatives living in littoral or shallow sublittoral environments.

4

K.H. MELDAHL

/

\

/ 110 °

115 ° California Arizona

San Diego

/

New

1

Mexico

Tijuana • Tucson

------- 30 ° .3 " ;.-..

Sonora

'~:~:

30 o.

ii:"

* Hermosillo

Chihuahua

.Guaymas ~:'.. °.."

B

-.---- 25 °

\ La Paz 0

100

200

300

400

500 kilometers

!,

'....".

~.

0°''n g0 .

~

Mazatlan 115 °

/

110 °

/

105 ©

I

Fig. 1. The two areas of measured Plio-Pleistocene sections compared in this study: northern mainland coastline (area A), and southern peninsular coastline (area B). See Appendix for sample locations and references.

- - T h e common trace fossils Ophiomorpha and Thalassinoides record burrowing activities of catlianassid shrimp that live today in intertidal or shallow subtidal environments in the Gulf.

Oceanographic and geornorphic data Tides: Mixed semidiurnal tides dominate in the Gulf, and tidal range varies from microtidal in the south to macrotidal in the north (Table 1;

GEOGRAPHICGRADIENTSIN THE FORMATIONOF SHELLCONCENTRATIONS

Fig. 2a). Spring range may exceed 10 m at the Colorado River delta (Roden, 1964; Thompson, 1968). Coupled with the low relief of the coastline in the northern Gulf, the extreme tides generate extensive intertidal systems. The tidal fiats at Bahia la Choya, near Puerto Penasco on the mainland coast of the northern Gulf (31.4°N), have received much attention as a natural laboratory for taphonomic and paleoenvironmental studies (e.g. Cutler, 1991; Flessa et al., 1993; Flessa, 1987; Ffirsich and Flessa, 1987, 1991; Meldahl, 1987, 1990a,b).

5

the Pacific Coastal Plain province (south of Guaymas). Both regions are characterized by wide, lowrelief basins separating isolated volcanic- and plutonic-cored mountain ranges. This results in a gentle coastal gradient and wide shelf along most of the mainland. In contrast, the peninsular (western) coast of the Gulf borders on high-relief, westwarddipping volcanic plateaus and granitic batholiths of the Peninsular Ranges province, resulting in a rugged coastline and steep narrow shelf (Fig. 2b,c).

Temperature and salinity Coastal topography Several physiographic provinces of the North American Cordillera bound the Gulf, and these exert an important influence on the character of the coastline and its depositional processes. The mainland (eastern) coast of the Gulf lies within the Sonoran Desert province (north of Guaymas) or

a

b

The Gulf lies in a desert belt between 23 ° and 32°N latitude and evaporation exceeds fresh water input throughout the region, particularly in the north (Ortlieb, 1987). Precipitation averages 20-25cm/yr in the southernmost Gulf, and decreases northward to 5-10 cm/yr in the deserts around the delta (Roden, 1964; Ezcurra and Rodri-

c

!NT TYPES Iluvium

olcanic atholithic/ metamorphic lixed sedimentary, volcanic, batholithic

/ /

East / west sediment

/

boundary

f

Direction of flow

Fig. 2. (a) Mean and spring tidal ranges (from Roden, 1964). Northern Gulf is macrotidal, southern Gulf is microtidal. (b) Distribution of rock-dominated versus sand-dominated coastline (from Ortlieb, 1987). Mainland coast is mostly low relief sandy coastline, peninsular coast is mostly high relief rocky coastline. (c) Sources and dispersion of terrigenous sediments (from Maluf, 1983, after Van Andel, 1964). The northern mainland coast is dominated by Colorado River derived sediment. The southern peninsular coast is dominated by sediment derived from local high relief volcanic or plutonic sources.

6

guez, 1986). Exchange with the Pacific maintains salinities in the southern Gulf near 35%oo,but the northern Gulf experiences elevated salinities: 35.5-37.5%o offshore, approaching 39%0 in shallo~v coastal areas. Temperatures of surface waters vary annually from 20°C to 29°C in the south; 15°C to 31°C in the north (Roden, 1964).

K.H. MELDAHL L~HOLOGY 60 ¸

50 ¸

~

4o

g ~ 30

Methods

Building on prior work (Meldahl and Cutler, 1992), I measured and described stratigraphic sections at 10 locations on the northern mainland coast (area A, Fig. 1) and 10 locations on the southern peninsular coast (area B, Fig. 1). Observations, collected on a cm x cm scale, included sediment grain size, sorting and composition, physical and biogenic sedimentary structures, and faunal composition. Shell bed data consist of the stratigraphic geometry of the beds (lateral relationships, contact relationships, thickness, internal stratification, ichnofabric); taphonomy (closepacking, orientation, bioclastic composition, shell size, articulation, fragmentation, abrasion, bioerosion and encrustation); and faunal composition. These methods follow those described by Kidwell (1991). Results and discussion

The following discussion is organized around a comparison of Plio-Pleistocene lithology and shell beds on the northern mainland coast versus the southern peninsular coast (referred to hereafter as simply "north" or "northern" and "south" or "southern"). Lithology: north versus south

Plio-Pleistocene deposits yielded a total of 61 m of measured section in the north and 103 m in the south. Figure 3 illustrates the relative contributions of various lithologies. The northern sections are dominated by fine- to medium-grained subarkose and quartzarenite. Mudstone and siltstone are also common, and coarse calcarenite (composed of comminuted molluscan debris) is present. Conglomerate is rare; litharenite and algal calcarenite

Fig. 3. Relative abundance of sediment types, expressed as a percentage of total stratigraphic thickness, in sections measured in Plio-Pleistocene deposits from the northern mainland coast (61 m of section) and southern peninsular coast (103 m of section). See text for discussion.

absent. In contrast, the southern sections are dominated by algal calcarenite and volcaniclastic conglomerate and litharenite. Mudstone, siltstone and quartzose or arkosic sandstone are uncommon. Shell beds (calcirudites) make up a greater fraction of stratigraphic thickness in the north than in the south. Geomorphic differences between the north and south have a clear effect on the composition of modern coastal sediments (Van Andel, 1964; see Fig. 2c), and Plio-Pleistocene lithologies closely resemble the modern sediment distribution patterns. In the north, due to extreme aridity and low relief, little terrigenous coastal sediment is derived locally. Instead, most is delivered to the coastline by wind or longshore drift from the Colorado River delta, and reworked by strong tidal currents, resulting in fairly mature sands (subarkose and quartzarenite; Thompson, 1968). Fecund mollusc populations produce substantial amounts of shell material, much of which is reworked and concentrated into shell beds or comminuted to form coarse calcarenite. In contrast, the high relief plutonic and volcanic rocks along the southern coast shed coarse debris

7

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

directly onto the shoreline via local drainages. Terrigenous coastal sediments are dominantly volcaniclastic conglomerate and coarse immature litharenite. In addition, calcareous algae are common in the south, producing substantial amounts of algal calcarenite (Van Andel, 1964).

17A Mollusc beds (31) • 6' 5"

=

Types of shell beds A total of 40 shell beds occur in the PlioPleistocene deposits; 13 in the north and 27 in the south. The 13 northern shell beds are all dominated by the bivalves Chione californiensis and/or C. gnidia. 18/27 southern shell beds are dominated by molluscs; 9/27 by corals. The taxonomic composition of the southern beds is generally more diverse and variable than in the north. Particularly common taxa in mollusc-dominated beds of the south are the bivalves Codakia distinguenda, Trachycardium sp., Dosinia ponderosa, Spondylus sp., Pecten vogdesi, Argopecten sp., Lyropecten subnodosus, several species of Ostrea, Chama, Chione and Glycymeris, and several species of the gastropods Conus, Oliva, and Turbo. The coral beds are dominated by Porites californica and/or Pocillopora robusta, and may also contain Astrangia sp. The coral beds usually correlate with the Sangamon interglacial highstand (125,000 yr B.P.). The northernmost Sangamon-age coral beds observed in this study occur at Punta el Bajo and Isla Coronado (26.1°N). Isolated Sangamon-age Porites are reported by Ortlieb (1987) as far north as Punta Contrabando (27.5°N) on the peninsular coast, and Puerto Libertad (29.9°N) on the mainland coast. Coral beds become more common and mollusc beds less common south of 26°N along the peninsular coast (Fig. 4). The 40 shell beds are interpreted to represent five types of beds: community beds; storm beds; beach berm beds," tidal channel bed; and current~wave-winnowed beds. These can be recognized and distinguished by distinctive stratigraphic and taphonomic characteristics (Table 2 and Fig. 5). The discussion below summarizes evidence for these interpretations.

Community beds Community beds (Table 2; Fig. 5a,b) are autochthonous mollusc or mixed coral/mollusc assem-

Coral beds (9)

2:

"34 32

~BmmmBmBB

4"

"30

3"

'28

1

iittLJiw ~oOO~0~0~o

~" :~S:: o'~:S'~ ~¢~ ~"

I:1

~==

'26

'~

'24

~

= 22

-" E..~ ~ ' ~

~" ~ ~'.~ ~ Northern mainland coast

Southern peninsular coast

Fig. 4. Distribution of mollusc-dominated versus coraldominated shell beds. Histograms are number of shell beds at each sample location. Points are degrees north latitude. Coral beds occur only south of 26°N. Coral beds become more c o m m o n while mollusc beds become less c o m m o n south of 26°N.

blages forming in shallow subtidal environments on sand or mixed rock/sand substrates. The beds occur as tabular or lenticular accumulations, 10-85 cm thick, extending laterally along outcrop from several meters (for the smallest mollusc beds) to more than one kilometer (for the most extensive coral beds). Many bivalves (10- 50 %) are preserved in life position, valve articulation is high, fragmentation and abrasion are low, bioerosion (clionid sponges, spionid polychaetes) is low to moderate, and encrustation (bryozoans, barnacles) is low. The beds are primarily matrix-supported, with median bioclast density 10% by volume, ranging from 3 to 40%. Internal stratification when present consists of small-scale planar bedding, but physical stratification is generally absent, obscured by extensive bioturbation. Thalassinoides burrows are common. Lower contacts of the beds may be gradational, sharp or even erosional where corals or hard-substrate molluscs have colonized an erosion surface. Upper contacts are sharp or gradational. Diversity varies greatly, from nearly monospecific assemblages to those containing

8

K.H. MELDAHL

{ 2 8"'~

"~

8

.~ ~.~

I

o

o

^

N

N

.o •~ " "

e~

0

O

i"

~1 0

e.O

~

0

_'T

O

~"

~x

~

0

.£ ~

-6 I

2

~

N

"2

7

.'

0

&

•=

= o

%

o

8

§ -

o,.

~o=~ o.~'~ ~.-

~ •.~ .~ ~ o~ ~ . .0 ~

I

0

=

8

~

~~

8

I=

0

O

&£~

8

8 ,4=

& Z~

~

""

9

G E O G R A P H I C G R A D I E N T S IN T H E F O R M A T I O N O F S H E L L C O N C E N T R A T I O N S

..i="

~

o

g

~

~_

I-

_~-~-

~

'.:

~

o

'~

K~

,,,,,!

-~~ o

~

o

~ .~, o= 0

o d

o.~

~

o ~ • r-

~=

..~

-~o A

~ ' m : . B ~ ~ ~:'~

:~ o

=~ ~

~o~

o

.2o

o

~-~

~'N~=~ m

~

~

._'=,

0

~,._

"~" ;"o -~ ¢~

0

~' ~ . ~

o~.~

8~

~ ~

o

I:::::

,~

Q

2

N ~

=8~ o

i

o~

~ ~ .~

.S_

's°~

~ ~ ~'.-

-d ©

o o

:~

-

.o <

o

~0 o 0 ~-~-o ~= .=. ~ "~,

o

0

'."

~. _o~

g ~ , - . ~ . ~ ' . ~ ,-

.

.

~

o

.~ .o r,"

<

i~

m~

o

10 dozens o f species, but usually one or two species dominate each bed. C o m m u n i t y beds are ecologically and taxonomically variable, occurring in four basic categories: (1) Shallow infaunal communities in sand and m u d substrates d o m i n a t e d by the bivalves Chione gnidia or C. californiensis (Fig. 5a). (2) Mixed shallow infaunal/epifaunal c o m m u n i ties on gravel and boulder substrates d o m i n a t e d by the bivalves Codakia distinguenda, Trachycardium sp., Dosinia ponderosa, or Glycymeris sp. (3) Epifaunal communities on sand and gravel substrates d o m i n a t e d by the bivalves Spondylus sp., Pecten vogdesi, Lyropecten bakeri, or Argopecten sp. (4) Epifaunal communities on boulder substrates d o m i n a t e d by the coral Porites californica or Pocillopora robusta (Fig. 5b).

Storm beds Increasing recognition o f the importance o f short-term events in stratigraphy has led to a greater appreciation for the role o f storms in forming shell-rich strata. Storm shell beds have been described and interpreted in the literature on the basis o f both sedimentologic and t a p h o n o m i c features (e.g. Aigner, 1985; Brenner et al., 1985;

K.H. MELDAHL A b e r h a n and Fiirsich, 1991; Kreisa and Bambach, 1982; Miller et al., 1988; M o r t o n , 1988; Norris, 1986). Storms m a y p r o d u c e shell beds by scouring the sea floor, exhuming shells, winnowing away sediment to produce shell lags, then burying the lags with sediment settling out o f suspension during the storm's waning stages (Aigner, 1985). Repeated storms m a y p r o d u c e a m a l g a m a t e d lags several tens o f c m thick (Norris, 1986). Rapid burial results in well-preserved assemblages that m a y contain individuals buried alive. Criteria for the recognition o f storm beds include a basal erosion surface, h u m m o c k y graded sands, decreasing shell density f r o m the base o f the bed upward, evidence o f live burial (articulated, closed and reoriented bivalves; whole delicate taxa like starfish or crabs), and m a n y shells exhibiting low levels o f abrasion, fragmentation, bioerosion and encrustation. As A b e r h a n and Fiirsich (1991) point out, orientation o f shells m a y not be indicative o f storm reworking: b o t t o m return flows tend to arrange valves concave-down, while initial deposition o f suspended valves produces a predominantly concave-up pattern. Storm beds in G u l f o f California Plio-Pleistocene sediments (Table 2; Fig. 5c,d) are fining-upward tabular or lenticular shell concentrations up to 30 cm thick, extending intermittently for 10-200 m

Fig. 5. Representative examples of the five types of shell beds recognized in this study. Community beds: (a) Community bed of Chione gnidia, north Bahia Adair. View is down onto bedding plane. Preservation is exclusively as external molds and steinkerns (note articulated steinkerns). (b) Community bed of Porites californica, north Ensenada E1 Coyote. The corals occur in life position encrusting volcanic cobbles and boulders. Storm beds: (c) Storm bed of Argopecten abietis, south Punta el Bajo. The bed contains several hundred scallops (two are shown), 90% with articulated, closed valves. Notice the Balanus barnacles on the upper valve of one scallop and the lower valve of the other. 67% of barnacle-encrusted scallops are encrusted only on the lower valve, suggesting overturning followed by rapid burial during a storm. (d) Storm bed at south Isla del Carmen, dominated by Pecten vogdesi, Spondylus sp., Argopecten sp., Lyropecten subnodosus, and Arca sp. High frequency of articulation, random orientations, and displaced hard-substrate Arca suggest storm-induced transportation and rapid burial. Tidal channel bed: (e) Shells occur densely packed for more than two meters up from the base of the tidal channel. Preservation is exclusively as external molds and steinkerns. The section shown records a transgressive sequence. The channel incises fluvial deposits along a disconformity, and is overlain gradationally by bioturbated lower intertidal sands with abundant Ophiomorpha burrows. Cliff is about 9 m high. Beach berm beds: (f) View to the northwest along strike of a Sangamon-age chenier at Marley Ridge. Chenier is formed nearly entirely, of heavily abraded Chione californiensis valves. (g) Cross-section through beach berm bed at Cruz Point, more than 2 m thick, formed nearly entirely of heavily abraded, nested and imbricated valves of Chione californiensis, forming steep landward-dipping crossbeds. (h) Beach berm bed dominated by Chione gnidia, north Bahia Adair. Valves are heavily abraded, and occur dominantly concave-up and nested (the typical orientation of valves deposited on the landward-prograding side of shell berms). Planar-bedded and low angle cross-bedded shell hash sands underlie the shell bed. Current~wave-winnowedbeds: (i) Bedding-plane view of current/wave-winnowed bed dominated by Dosinia sp., Cerro los Machos. Valves are disarticulated, heavily abraded and fragmented, and stacked edgewise between wave-rounded volcanic cobbles and boulders. The visible clionid sponge borings are highly abraded. (j) Current/wavewinnowed bed overlying wave-rounded volcanic cobble and boulder bed, Punta Prieta. Bioclast size decreases upward in bed, and shells are heavily abraded, fragmented and densely packed.

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

11

Fig. 5a-e. laterally along outcrop. The original nature of the upper and low contacts of the beds has been obscured by bioturbation, though they currently appear gradational. The beds are primarily matrixsupported with minor bioclast-support. Disarticu-

lated valves are commonly nested and oriented preferentially concave-up, with secondary oblique and perpendicular orientations. Shell density is low, typically 1 5% and not exceeding 15% by volume. Fragmentation ranges from low to high,

12

K.H. MELDAHL

Fig. 5 (continued) f-j. For explanation see p. 10. and abrasion, bioerosion and encrustation are uniformly low. Articulated bivalves are displaced from life position. Due to pervasive bioturbation and physical

reworking in these shallow water deposits, sedmentary evidence of storms is generally obscured or obliterated. The interpretation of a storm origin is thus best assessed using taphonomic criteria.

13

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

Taphonomic data provide compelling evidence for the effects of storms in three instances. 1. South Punta el Bajo: The lowest shell bed in the Plio-Pleistocene section at South Punta el Bajo (Fig. 5c) contains several hundred Argopecten abietis, 90% of which are articulated and closed with the commissure subparallel to bedding. About 5% of the articulated A. abietis have adult Balanus (1-2 cm high) encrusting one or both valve exteriors. Of these articulated, barnacle-encrusted scallops, 2% have barnacles on both upper and lower valves; 31% have barnacles only on the upper valve; and 67% have barnacles only on the lower valve (Fig. 5c). Barnacles on the lower valves are in a position untenable for life (their apertures would have been forced into the sediment), implying overturning of the scallops after the barnacles grew to adulthood on the upper valve surfaces. Absence of barnacles on the presently upwardoriented surface of the majority of the scallops suggests the scallops were buried rapidly after overturning, before the newly-exposed valve could become encrusted. The evidence suggests overturning followed immediately by burial due to rapid sedimentation in the waning stages of a storm. Reorientation of the scallops was not random; the majority (67%) were flipped over. The size and number of barnacles on most of the scallops suggests the increasing weight of growing barnacles on the upper surface might have made the scallops gravitationally more stable in the overturned position, perhaps accounting for the fact that the majority of scallops are encrusted only on the lower valve. 2. South Isla del Carmen: The lowest shell bed in the Plio-Pleistocene section at South Isla del Carmen (Fig. 5d) contains abundant articulated bivalves, including the scallops Pecten vogdesi (10% articulated), Spondylus sp. (50% articulated), Argopecten sp. (50% articulated), Lyropecten subnodosus, and the epibyssate Arca sp. (100% articulated). The bivalves exhibit random orientations, and articulated valves are closed. The bed is heavily burrowed, about 30cm thick, matrix-supported, containing 5-15% bioclasts by volume, with shell orientations ranging from random to nested. Shells are mostly well-preserved and exhibit low degrees of abrasion and moderate levels of bioerosion and

encrustation. Several Argopecten are preserved with barnacles only on the underside, implying overturning followed by rapid burial (as discussed above). Area, which are epibyssate hard substrate dwellers, occur unattached to any hard substrate: suitable rocks are not present in the bed, and Area are not attached to other shells. Uprooting of Arca from adjacent rocky habitats (present about 200 m from the bed) during a storm is the most likely explanation. 3. North Bahia Choya: Aberhan and Ffirsich (1991) argue for a storm origin for shell beds in Upper Pleistocene deposits on the north side of Bahia la Choya. Abrasion, fragmentation, bioerosion and encrustation are low, and articulated bivalves are closed and displaced from life position. These taphonomic features reflect a short exposure time on the sea floor for many specimens, with exhumation and rapid burial of live animals. The percentage of articulated valves is low (<5%) compared to the storm beds described above, and is biased toward large individuals. Wave traction or burrowing organisms may have acted to separate valves of smaller individuals after burial, leaving larger ones intact.

Tidal channel bed One shell bed representing a basal tidal channel lag occurs in the northern Gulf. This laterally continuous bed ranges from 50 to 250 cm thick, the channel incising more than 2 m into underlying strata (Table 2; Fig. 5e). Aragonitic fauna are preserved almost exclusively as external molds and steinkerns, while calcitic fauna are preserved intact. The bed is dominated by Chione californiensis, and also contains Trachycardium, Tagelus, Tellina, Pholas, Ostrea, Melongena, Murex, Pecten, scallops and crab claws. The bed is bioclast-supported, with valves dominantly concave-down or randomly oriented, forming medium- to large-scale highangle cross-stratification. Bioturbation is absent except for burrows at the base of the channel. Shell abundance is high (60-70% by volume). Articulated valves are absent, fragmentation is high, abrasion is moderate, bioerosion (by spionid polychaetes and boring bivalves) is moderate to high, and encrustation is absent to low. The bed

14

formed from a large migrating tidal channel draining a tidal estuary. Active tidal channels in the northern Gulf accumulate thick shell lags as the channels migrate laterally, eroding adjacent intertidal deposits and winnowing away finer sediment fractions. Shells are also contributed by autochthonous fauna dying in the channel. Chione californiensis, Tagelus, Melongena, and Murex are common inhabitants of modern tidal channels or tidal estuaries in the area. Dead shells in tidal channels are typically highly degraded through corrosion and bioerosion by algae and other endoliths, such as spionid polychaetes. Where shell density is low, valves in channels are mostly concave-down, while random orientations dominate where shells are densely packed.

Beach berm beds Beach berm beds (Table 2; Fig. 5f-h) cap linear, arcuate beach ridges (cheniers) along the coast in the northern Gulf. Like tidal channel beds, beach berm beds do not occur in the south. The beach ridges are tens of meters wide, hundreds of meters long, and may have more than 3 m vertical relief. Shell concentrations form the capping units of the beach ridges, and these beach berm beds are the thickest of the shell beds studied; median thickness is 140 cm, range 40-230 cm. The beds are overwhelmingly dominated by disarticulated valves of either Chione gnidia or Chione californiensis. The shells form prominent high-angle cross-beds dipping landward (Fig. 5g), which may grade laterally over several meters into low-angle cross-beds dipping seaward. The cross-beds consist of layers of shell-hash sands alternating with layers of whole valves and large shell fragments. The beds are bioclast-supported, with shell abundance ranging from 50 to 90% by volume. Bioturbation is limited to rare, small, irregular burrows. Typically the majority of valves are oriented concave-up and nested within one another (Fig. 5h). Where shell density is particularly high, valves are commonly nested in oblique or even edgewise orientations, and are commonly imbricated. Shells are heavily abraded and worn, and fragmentation ranges from low to high. The shells rarely retain encrusters (a few barnacles are sometimes present), but are

K.H. MELDAHL

commonly bioeroded (by clionid sponges, spionid polychaetes and boring bivalves). Abrasion around the openings of endolith borings indicates that the borings were acquired first, before the shells were abraded, Individual beds pinch and swell and may pinch out laterally over a few tens of meters, though coquina-capped ridges are often hundreds of meters long. Where the beds overly fine-grained strata the lower contact may be deformed by balland-pillow loading structures. Beach berm shell beds represent the 125,000 yr B.P. shoreline recognized by Ortlieb (1987). The taphonomic and stratigraphic features of these beds are similar to shell accumulations forming the crests of modern berms and beach ridges in the northern Gulf. Modern berm shell accumulations commonly display steep landward-dipping cross beds on the landward side of the berm, which grade laterally into low angle seaward-dipping cross beds on the seaward side of the berm (Frey and Howard, 1988). Berm shell accumulations appear to prograde landward during high tides and storms, when the berm is crested by waves and shells are dumped on the back side of the berm to form landward-dipping cross beds (Thompson, 1968). The valves tend to settle concave-up and nest together, often in an imbricated arrangement, as they spill down onto the toe of the landward-prograding berm. Berm shell accumulations may be hundreds of meters to tens of kilometers long, and up to a few meters thick (Thompson, 1968). Shells accumulating in modern berm deposits vary in taxonomic composition from nearly monospecific to very diverse. Beaches become a reservoir for shells transported there from a variety of environments (yielding a high diversity assemblage), but strong hydrodynamic sorting can segregate taxa by shell morphology (yielding a low diversity or monospecific assemblage). Modern beach berms in the field area are often dominated by a narrow size range of heavily abraded whole valves of robust-shelled species, attesting to strong hydrodynamic sorting. Shells on modern berms are nearly always highly abraded, and fragmentation ranges from low to high. Shells may retain borings acquired from previous environments, and the borings may be

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

abraded at their openings. Encrusters are rarely retained on beach shells; most epibionts are not hardy enough to withstand the rigors of the swash zone. The low angle cross-bedded and planarbedded shell hash sands associated with the berm shell beds, and near-absence of preserved burrows, are characteristic of beach stratification.

Current/wave-winnowed beds Current/wave-winnowed beds (Table 2; Fig. 5i,j) are the most common type of shell bed observed in this study. They are sheet-like or tabular shell concentrations, 12-80 cm thick, which extend laterally from 10 m to more than 1 km. The lower contact is either sharp or rapidly gradational, rarely erosional, while the upper contact is usually gradational. Internal stratification consists of planar or low-angle cross bedding, though bioturbation often obscures it. Shell density averages 15-20% by volume, but ranges from 1 to 70%. Bioclast-support is the rule, though matrix-support is not uncommon. The majority of shells are in concordant orientations with valves concavedown, though oblique, nested and edgewisestacked arrangements are common where shell density is high. Articulated valves are absent or very rare, and abrasion and fragmentation are very high, with many shells and fragments extensively rounded and worn. Bioerosion (by clionid sponges, spionid polychaetes and boring bivalves) is usually high, and encrustation (barnacles, bryozoans, calcareous algae) is low. Species diversity is usually high. Current/wave-winnowed beds represent highly time-averaged shell accumulations formed by extensive reworking under conditions of sediment bypassing and high wave and current energy. Norris (1986) uses the term "current-winnowed bed" to describe similar shell beds in the Pliocene Purisima Formation of California, and he attributes their formation to the Combined effects of background reworking and storm currents. Storms may have played a role in initially concentrating shells in current/wave-winnowed beds, but if so background reworking has obliterated the storm "signature" (see discussion of storm beds above). Current/wave-winnowed beds form

15

through the winnowing and segregation of sand and shells by tidal currents and waves. The sand is reworked into wave or current ripples, and shells are abandoned as a lag in ripple troughs (Brenner et al., 1985; Norris, 1986). Repeated reworking extensively abrades shells and shell fragments. Due to prolonged exposure on the sea floor, shells are commonly invaded by boring endobionts. While prolonged exposure would be expected to result in frequent encrustation as well, shells in current/wave-winnowed beds exhibit low levels of encrustation. It appears that abrasive reworking of shells in current/wave-winnowed beds has a more adverse effect on shell epibionts than their more sheltered endobiont counterparts.

Stratigraphic significance of shell beds The presence of a shell bed in the stratigraphic record reflects some type of shell-concentrating process. As Kidwell (1988, 1989, 1991) has demonstrated, the formation of shell concentrations requires net shell input to be high relative to net sediment input. Kidwell et al. (1986) recognize three major shell concentrating processes: diagenetic, biogenic, and sedimentologic. None of the 40 shell beds studied here represent diagenetic concentrations: the Plio-Pleistocene sediments are weakly lithified and concentration through pressure solution or compaction is not evident. Biogenic concentrations are represented by the community beds (11/40 beds), which formed through the gregarious nature of molluscs or corals. The rest of the shell beds represent sedimentologic concentrations (29/40 beds). Storm beds, beach berm beds, tidal channel beds, and current/wave-winnowed beds all formed as a result of the differential hydraulic behavior of shells versus other sediment fractions under various depositional conditions (Fig. 6a). Sedimentologic concentrations represent either short-term event concentrations (storm beds), or longer-term concentrations formed through dynamic bypassing (beach berm beds, tidal channel beds, and current/wave-winnowed beds). Dynamic bypassing (Kidwell, 1989) refers to the alternating erosion, deposition and strong winnowing produced by the passage of sedimentary bedforms across the sea floor under conditions of low net

16

K.H.

S T R A T I G R A P H I C CONTEXT OF S H E L L BED F O R M A T I O N

M E L D A H L

[] Northern Gulf [ •

Southern Gulf

(a) Biogenie vs sedimentologic concentrations

I

Biogenic concentrations Sodimentologic concentrations

/

0

5

10

15

20

25

30

15

20

25

30

(b) Formation of sedimentologic concentrations Storm events

Dynamic bypassing: beds not capping major stratigraphicdiscontinuities Dynamic bypassing: beds cap major stratigraphicdiscontinuities

1

0

5

10

(e) Types of stratigraphic discontinuities capped by sedimentologic concentrations Nonconformities

"T

Angular unconformities

z

~)

Disconformities

H dg un

f ,i

0

5

. . . .

!

. . . .

10

|

15

. . . .

|

. . . .

20

!

. . . .

25

|

30

N u m b e r of shell beds Fig. 6. Stratigraphic context of Gulf of California Plio-Pleistocene shell beds. (a) Shell beds represent either biogenic concentrations (I 1/40 beds) or sedimentologic concentrations (29/40 beds). (b) Sedimentologic concentrations formed either in association with short-term storm events (3/29 beds) or in association with longer-term dynamicbypassing (26/29 beds). 10 of the 26 sedimentologic concentrations formed through dynamic bypassing cap major stratigraphic discontinuities. (c) The stratigraphic discontinuities capped by shell beds, in order of increasing levels of erosion and temporal gap, are hardgrounds, disconformities, angular unconformities and nonconformities. They represent temporal breaks ranging from 103 to 108 yr. See text for discussion.

sedimentation. In this study, dynamic bypassing in beach berm, tidal channel, a n d current/wavewinnowed beds is indicated by a close association between shell beds and increased matrix grain size, and the occurrence of current/wave-winnowed beds in the middle of fining-upward or coarseningupward intervals, in which the shells represent

particles that are hydrodynamic intermediates between smaller and larger terrigenous grain sizes. As shown in Fig. 6b, sedimentologic concentrations formed through dynamic bypassing are far more c o m m o n (26/29 beds) than those formed through storm events (3/29 beds). If storm processes played a role in initially concentrating shells,

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

background reworking under conditions of dynamic bypassing has overprinted and obliterated the storm "signature" in all but three cases. These circumstances should be expected in shell beds formed in such shallow water. The 26 beds formed through dynamic bypassing occur either in the absence of major stratigraphic discontinuities (16/26 beds), or they occur as caps on major stratigraphic discontinuities (10/26 beds; Fig. 6b). Indeed, the first sediments to be deposited on discontinuities in the Plio-Pleistocene sections are very often shell beds; a situation reported several times by Kidwell (1988, 1989, 1991). Stratigraphic discontinuities are represented by hardgrounds, disconformities, angular unconformities, or nonconformities (Fig. 6c). They represent temporal breaks ranging from 10 3 t o 10 8 years. Examples of discontinuity-capping shell beds are shown in Fig. 7 and discussed further below.

Hardgrounds Shell beds cap two carbonate hardgrounds formed of cemented calcalgal sands in the section at north Punta el Bajo. The hardground surfaces are locally bored by lithophagid bivalves and exhibit irregular, karstic surfaces with up to 40 cm of relief (Fig. 7a). The capping shell beds are heavily fragmented and abraded current/wavewinnowed tags. The hardgrounds represent temporal breaks of perhaps 103-104 yr.

Disconformities Disconformities in the Plio-Pleistocene sections are commonly capped by shell beds (Fig. 7b). The disconformities formed during Pleistocene low sea level stands, and are cut on alluvial, fluvial and shallow marine units ranging from Upper Pliocene to Lower Pleistocene age. The shell beds are either current/wave-winnowed shell lags or hardsubstrate community beds deposited during the Sangamon highstand. Thus these erosion surfaces represent temporal breaks of 104-106 years.

Angular unconformities Shell beds capping angular unconformities are current/wave-winnowed shell lags or hardsubstrate community beds (Fig. 7c). These include the current/wave-winnowed shell lags called

17

"unconformity beds" by Meldahl and Cutler (1992) in the northern Gulf. Like the disconformities, the angular unconformities are cut on alluvial, fluvial and shallow marine units ranging from Upper Pliocene to Lower Pleistocene in age, while the capping shell beds are of Sangamon age. Thus these erosion surfaces represent temporal breaks of 104-106 yr.

Nonconformities Two nonconformities on Cretaceous granites or Tertiary volcanics were observed, both capped by current/wave-winnowed shell beds of Sangamon age (Fig. 7d). These erosion surfaces represent temporal breaks of 107-108 yr. Distribution of shell beds: north versus south

Types of beds Community beds, storm beds, beach berm beds, tidal channel beds and current/wave-winnowed beds have significantly different distributions between the northern and southern Gulf (Fig. 8). All five types of beds are present in the north. Beach berm beds are the most common (6/13), followed by current/wave-winnowed beds (4/13). Community beds, storm beds, and tidal channel beds are relatively rare (1 each). In the south, current/wave-winnowed beds are most common (15/27), followed by community beds (10/27), while storm beds are rare (2/27), and beach berm beds and tidal channel beds are absent. A G-test of independence (Sokal and Rohlf, 1981) comparing the number of community beds, current/wavewinnowed beds, and beach berm beds in the north versus the south indicates a highly significance dependence of bed type on geographic region (p<0.001). (Storm beds and tidal channel beds are too rare to fulfill the assumptions of the G-test and were thus not included.)

Degree of autochthony In Fig. 9, the classification of shell beds follows the scheme of Kidwell et al. (1986). Autochthonous fossil assemblages are those which preserve the living community fairly well intact, as evidenced by many individuals preserved in life position in their original habitat with relatively little rework-

18

K.H.MELDAHL

Fig. 7. Examples of sedimentologic concentrations capping stratigraphic discontinuities. (a) Current/wave-winnowed molluscan shell bed overlying hardground (north Punta el Bajo, southern peninsular coast). Shell bed overlies and infills a pitted, karstic surface on lithified algal calcarenite. Lithophagid bivalve borings occur along the contact elsewhere. Dashed lines highlight contact. Shells in the lower unit are preserved as molds, possibly reflecting leaching during subaerial exposure. Shells in the upper unit are preserved intact, Bioclast size decreases upward from the base of the bed. (b) Current/wave-winnowed of Porites fragments and assorted molluscs overlying a disconformity on alluvial cobbles and boulders (south Ensenada E1 Coyote, southern peninsular coast). Shells are concentrated in the 20-30 cm immediately above the contact. About 4 m of section are shown. (c) Two current/wave-winnowed beds dominated by Chione californiensis truncate deformed shallow marine strata at gentle angular unconformities (Playa Arenosa, northern mainland coast). The beds, highlighted by dashed lines, are 5-15 cm thick. The upper shell bed is fiat-lying and truncates the deformed strata below containing the lower shell bed. The lower shell bed is deformed, and truncates at an angular unconformity strata that are even more deformed. These beds are discussed in detail in Meldahl and Cutler (1992). Cliff is about 6 m high. (d) Current/wave-winnowed bed of heavily abraded, rounded Porites fragments nonconformably overlying Cretaceous granite (south Bahia Pulmo, southern peninsular coast). ing a n d n o t r a n s p o r t a t i o n . These are represented here by the c o m m u n i t y beds. Allochthonous fossil assemblages consist o f fossils that have u n d e r g o n e significant t r a n s p o r t a t i o n so that taxa are n o t preserved in their original habitat. These are represented here by beach b e r m beds a n d s t o r m beds. Taxa in beach b e r m beds have been t r a n s p o r t e d a n d mixed from a variety

o f intertidal a n d shallow shelf habitats. Some storm beds c o n t a i n h a r d substrate taxa that have been t r a n s p o r t e d a n d buried alive several h u n d r e d s of meters from the nearest a p p r o p r i a t e attachm e n t sites. Parautochthonous fossil assemblages occur between the above two extremes. T h e y have u n d e r g o n e some t e m p o r a l a n d spatial mixing so

GEOGRAPHICGRADIENTSIN THEFORMATIONOF SHELLCONCENTRATIONS 20.

DISTRIBUTIONOF SHELLBEDS

[] m ,~ 15' • .~ [] []

Communitybeds Stormbeds Beachbermbeds Tidalchannelbeds dbeds Current]wave-winnowe

~

~-~ 10'

0

- Northern Gulf (13)

SouthernGulf (27)

Fig. 8. Distribution and abundance of shell beds, northern Gulf versus southern Gulf. The northern Gulf is dominated by beach berm beds, while the southern Gulf is dominated by current/ wave-winnowed beds and community beds. Differences between north and south are highly significant (p<0.001; G-test of independence). See text for discussion.

2o

DEGREE OF AUTOCHTHONY • Autochthonous [] Parautochthonous

~¢~ , 10~155 =[] a Allochthonous ~Z z

Northern Gulf (13)

SouthernGulf (27)

Fig. 9. Abundance of autochthonous, parautochthonous and allochthonous shell beds, northern Gulf versus southern Gulf. The northern Gulf is dominated by allochthonous and parautochthonous beds, while the southern gulf is dominated by parautochthonous and autochthonous beds. Differences between north and south are highly significant (p < 0.01; G-test of independence). The differences are interpreted to reflect regional differences in sediment supply and tidal current reworking. See text for discussion.

that the assemblage is taphonomically altered and highly time-averaged, but the mixing is on a local scale and the fossils are preserved in their original life environment. These are represented here by current/wave-winnowed beds and tidal channel beds. The formation of current/wave-winnowed shell lags in modern lower intertidal and shallow subtidal environments in the Gulf involves reworking and time-averaging of shells, but typically not

19

transportation out of the life habitat (Ffirsich and Flessa, 1987; Meldahl, 1990b). Channelized tidal currents can transport shells considerable distances (Fiirsich and Flessa, 1987), but the parautochthonous classification for tidal channel beds is justified on the grounds that the majority of taxa observed occur alive in modern tidal channels. A simple comparison of the degree of autochthony between the north and south is made using the following autochthony index (AI):

AI= (a + (b/2))/(a + b + c); where

a = number of autochthonous beds b = number of parautochthonous beds c = number of allochthonous beds.

AI equals 0.0 if all beds are allochthonous; 0.5 if there are equal numbers of autochthonous, parautochthonous and allochthonous beds; and 1.0 if all beds are autochthonous. The degree of autochthony is much lower in the northern Gulf than in the southern Gulf: AI(north)=0.19; AI(south) = 0.65. The distribution of shell beds according to degree of autochthony is summarized in Fig. 9. A G-test of independence comparing the number of autochthonous, parautochthonous and allochthonous beds in the north versus south indicates a highly significant dependence of bed type on geographic region (p <0.01). Packing density The degree of close-packing of shells in the shell beds varies widely (Fig. 10). The majority of northern shell beds are bioclast-supported, whereas the majority of southern shell beds are matrixsupported. The differences outlined above in the distribution and abundance of shell beds, the degree of shell bed autochthony, and packing density are interpreted to reflect differences in sediment supply and shell supply, tidal current reworking and shell bed taxonomic composition between the northern and southern Gulf, as discussed below.

Sediment supply and shell supply: north versus south Most northern shell beds are beach berm shell beds capping linear beach ridges (cheniers). Beach

K.H.MELDAHL

20

P A C K I N G DENSITY 30' • Bioclast-supportedI

25 [] Matrix-supported ~"~ 20 , ~

"~

I0: 5 0

NorthernGulf SouthernGulf (13) (27)

Fig. 10. Packing density of shell beds, northern Gulf versus southern Gulf. Northern beds are mostly bioclast-supported, southern beds are mostly matrix-supported. The differencesare interpreted to reflect regional differences in sediment supply and tidal current reworking. See text for discussion.

ridges built of mollusc shells have been described on many coasts (e.g. Alberzart and Wilkinson, 1990; Hoyt, 1969; LeBlanc, 1972). Shell beach ridges form when breaking waves winnow sediment and concentrate shells at the strand under conditions of low net terrigenous sediment supply. A wide, shallow shelf enhances the process by acting as a large reservoir for shells. The ridge-building process is self-reinforcing because the growing ridge forms a barrier to transport which further captures shells (Thompson, 1958). If sedimentation is renewed and the shoreline progrades, the ridge may be abandoned, and a new ridge may form seaward of the previous one. Multiple, shorelineparallel sets of beach ridges (chenier plains) may form in this manner, the ridges becoming younger toward the ocean. New ridges may form in as little as 12-20 yr (Curray et al., 1969). Holocene chenier plains are presentin two 10cations in the Gulf: on the extensive tidal fiats north of San Felipe west of the Colorado. Rive~ delta (Thompson, 1968) and on the coast of Nayarit on the mainland south of the Gulf (Curray et al., 1969). Both of these chenier plains formed by coastal progradation after Holocene sea level had nearly reached its present position about 5000 yr B.P. The beach ridges studied here are all of Sangamon age (Ortlieb, 1987), and they do not form chenier plains, but occur alone or in association with one or two other ridges.

Major factors that favor shell beach ridge formation are low wave energy, reduced terrigenous sediment supply, and low coastal relief (Hoyt, 1969; LeBlanc, 1972). Tidal range does not appear to be very important; shell beach ridges form on both microtidal coasts (such as in southwestern Louisiana) and macrotidal coasts (such as the western Colorado River delta; Thompson, 1968). Differences in wave energy do not explain the absence of shell beach ridges in the southern Gulf. Coastal wave energy throughout the Gulf tends to be low, because wave fetch for the prevailing winds is limited (Maluf, 1983), and the strength of waves approaching the coast is diminished by numerous islands (in the southern Gulf) and the shallowness of the bottom (in the northern Gulf). Coastal topographic relief and style of sedimentation in the north and south are significantly different (Table 1). These differences appear to exert an important control on beach ridge formation. The extreme aridity and low relief of the northern coastline conspire to yield little terrigenous sediment locally. The dominant sediment source in the north is the Colorado River delta, and delta sediment is dispersed along the coast by wind, tidal currents and longshore drift (Van Andel, 1964; Thompson, 1968). Although in the long term the Colorado River has supplied a tremendous quantity of sediment to the northern Gulf, periodic reductions in sediment supply to parts of the coastline, associated with shifts in the locus of delta deposition, could initiate beach ridge formation. Thompson (1968) attributes Holocene beach ridge formation in the northern Gulf to this depositional switching mechanism. Indeed, beach ridges are actively growing in.the northern Gulf today as a result of human-induced sediment starvation. Upstream dams and irrigation have effectively shut off the Colorado River's flow and sediment supply to its delta for more than 50 years. Currently the encroaching shoreline reworks the sediment-starved surface of the tidal flats, winnowing sediment and building shell ridges (Thompson, 1968). Radiocarbon dates on tidal flat shells confirms that Holocene sedimentation rates in the north are extremely low. Thirty dated bivalve shells collected from the surface of the tidal fiats at Bahia la Choya (31.4°N) have median calendar

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

ages of 483 yr (inner fiats) and 427 yr (tidal channel). Surficial shell accumulations are highly time-averaged, with shells ranging in age from living to 3569 yr B.P. (Flessa et al., 1993). In contrast, the southern Gulf coast is one of high relief and steep gradients--circumstances not favorable to formation of shell beach ridges. Abundant sediment is supplied to the shoreline through local drainages from adjacent volcanic and plutonic uplands (Van Andel, 1964), overwhelming the input of shells in the formation of beach sediments. Furthermore the steep gradient provides only a narrow shelf to act as a reservoir of shells. Hence there is a marked paucity of shells on modern beaches in the south relative to beaches in the north. In addition, higher sedimentation rates along the southern coast due to the presence of local terrigenous sources should favor the preservation of intact community beds through rapid burial.

that most southern beds are matrix-supported while most northern beds are bioclast-supported (Fig. 10) supports the interpretation that southern beds have undergone relatively less reworking and winnowing. Taxonomic composition: north versus south

An additional factor increasing the degree of autochthony in the south is the fact corals begin to form shell beds south of 26°N latitude. Coral beds (dominated by Porites californica) make up 9 of the 27 southern shell beds. In general, the frequency of coral beds increases south of 26°N while the frequency of mollusc beds declines (Fig. 4). At the southernmost sample location (Bahia Pulmo; 23.4°N) all the sheli beds are made of corals. Styles of preservation differ between coral beds and mollusc beds in the southern Gulf (Scheme 1): Community beds

Tidal current reworking: north versus south

Tidal range varies dramatically between the macrotidal northern Gulf and the microtidal southern Gulf (Table 1). Tidal currents in the north are powerful: the sea floor of the northern Gulf is dominated by large, current-parallel tidal ridges and channels (Thompson, 1968); current velocities in intertidal channels commonly exceed 100 cm/s; and incoming tides move as rapid sheet flow across intertidal flats. The mobility of sediment in lower intertidal environments can within a few weeks completely bury and re-expose cement blocks measuring 20x20x40 cm (A. Cutler, pers. comm., 1990). The preservation of autochthonous shell beds in littoral or shallow sublittoral environments seems unlikely under such an onslaught of tidal reworking. The single community bed present in the northern Gulf represents a relatively deep water, sheltered environment. It is dominated by in situ Chione gnidia, a bivalve found today "in bays and offshore to depths of 33 m" (Keen, 1971, p. 188). In comparison, tidal currents along the southern coast are weak (Roden, 1964), and in conjunction with higher rates of terrigenous sedimentation, this may increase the probability of preserving benthic communities intact. The fact

21

Molluscs Corals

6 4

Storm beds

Current wave-winnowed beds 2 0

10 5

The large, robust and well-cemented corals tend to resist displacement and transportation better than molluscs. Thus, a greater proportion are preserved as autochthonous community beds. Nonetheless, even when only mollusc beds are considered, significant differences between the northern and southern Gulf persist. A G-test of independence comparing the number of autochthonous, parautochthonous and allochthonous mollusc beds in the north versus south indicates a significant dependence of bed type on geographic region, though the significance level (p<0.05) is lower than if coral beds are included in the comparison (p<0.01). Conclusions

(1) The 40 Plio-Pleistocene shell beds studied here represent community beds (11), storm beds (3), beach berm beds (6), tidal channel bed (1), or current/wave-winnowed beds (19). These five types of beds can be recognized and distinguished based on distinctive taphonomic and stratigraphic characteristics.

22

(2) Nearly all shell beds formed in water depths of less than 10-15 m, but under widely varying conditions of coastal topographic relief and tidal range. Shell beds of the northern mainland Gulf formed along a low-relief macrotidal coast, while shell beds of the southern peninsular Gulf formed along a high-relief microtidal coast. (3) The majority of Gulf shell beds are sedimentologic concentrations (29/40 beds); the remainder are biogenic concentrations (11/40 beds). The majority of sedimentologic concentrations (26/29 beds) formed under conditions of dynamic bypassing (winnowing and reworking under conditions of low net rates of sedimentation). Few sedimentologic concentrations preserve evidence of concentration through event (storm) processes (3/29 beds). 10 of the 29 sedimentologic concentrations cap major stratigraphic discontinuities--hardgrounds, disconformities, angular unconformities or nonconformities - representing temporal breaks ranging from 1 0 3 - 1 0 8 years. These observations accord well with the findings of Kidwell (1988, 1989, 1991). (4) There are significant differences in the distribution and abundance of the five types of shell beds between the northern and southern Gulf. Beach berm beds are the most common type of shell bed in the north, but are absent in the south. Current/wave-winnowed beds are the most common shell bed in the south, but are less common than beach berm beds in the north. Community beds are common in the south, but rare in the north. The degree of shell bed autochthony differs significantly between the north and south. Northern beds are dominantly allochthonous; southern beds are dominantly parautochthonous and autochthonous. Most northern beds are bioclast-supported, while most southern beds are matrix-supported. The taxonomic composition of the shell beds differs between north and south as well. Northern shell beds are dominated by molluscs, whereas southern shell beds are usually dominated by corals south of 26°N latitude. If coral beds are excluded from the sample, statistically significant differences between north and south still persist. (5) Differences in the distribution and abundance of the five types of shell beds, degree of autoch-

K.H. MELDAHL

thony, and packing density between north and south reflect variations in the relative supply of terrigenous sediment and shells (as controlled by topographic relief and shelf width), and variations in tidal current reworking. The formation of beach berm beds in the north is favored by the low coastal gradient, wide shelf, and periodic reductions in sediment supply from the Colorado River delta. The macrotidal setting and low net rates of sedimentation in the north conspire to rework and transport shells, resulting in a very low degree of shell bed autochthony. In the south, abundant coarse terrigenous debris shed off the high relief coastline typically overwhelms the input of shells in beach sediments, and the steep gradient provides only a narrow shelf from which beach shells can be derived. The presence of these local sources of terrigenous sediment in the south means accumulating sublittoral shell beds may become buried relatively rapidly. In conjunction with the relative weakness of tidal currents, this limits the degree of reworking shell concentrations experience, yielding a greater proportion of autochthonous or parautochthonous beds. (6) Geographic variation in topographic relief and tidal range results in significant variation in the distribution and abundance of shell beds in these shallow marine sediments. Ultimately these variables have a tectonic cause (relief reflects tectonic activity; tidal range reflects basin configuration). This connection between the formation of fossil concentrations and tectonic setting underscores the potential application of fossil concentrations to reconstructing geologic history.

Acknowledgments Field work was supported by a research stipend, a research and development grant, and equipment from Oberlin College and the W.M. Keck Foundation, for which I am grateful. Thanks to Alan Cutler for initiating a study of Pleistocene shell beds in the northern Gulf several years ago. Special thanks to Sarah Mesnick for assistance in the field and for showing me the beaches of Baja. Thanks to reviewers whose thoughtful comments greatly improved the manuscript.

GEOGRAPHIC

GRADIENTS

IN THE FORMATION

:~

~

:~

~

~" ~

~

~

:~,~

~ ~

23

OF SHELL CONCENTRATIONS

~

o

o

-

~,

;~

~

0

~

~

~

~

0

0

0

0

0

0

; ~

888

oI-~ oI-~

~ ~

~

Z

xO

-¢

~

O~

t¢~

t~

L¢~

,4

0

0a ell

I

0

;< Z

cr~

<

e~

.........

8

c~

~,~,~.~,~

~

~ ~ ~ ~ ~~.~.~

m

0

o,

0

0

.b- .i~~

~

24

References Abbott, P.L. (Editor), 1989. Geologic Studies in Baja California. Pac. Sect. SEPM Book, 63, 140 pp. Aberhan, M. and Fiirsich, F.T., 1991. Paleoecology and paleoenvironments of the Pleistocene deposits of Bahia la Choya (Gulf of California, Sonora, Mexico). In: F.T. F/irsich and K.W. Flessa (Editors), Ecology, Taphonomy, and Paleoecology of Recent and Pleistocene Molluscan Faunas of Bahia la Choya, northern Gulf of California. Zitteliana, 18: 135-164. Aigner, T., 1985. Storm Depositional Systems. Springer, Berlin, 174 pp. Albertzart, L.S. and Wilkinson, B.H., 1990. Barrier backbeach shell assemblages from the central Texas gulf coast. Palaios, 5: 346-355. Allison, P.A. and Briggs, D.E.G. (Editors), 1"991. Taphonomy: Releasing the Data Locked in the Fossil Record. Plenum Press, New York, 560 pp. Anderson, C.A., 1950. Geology of islands and neighboring land areas, Part I of the E.W. Scripts cruise of the Gulf of California. Geol. Soc. Am. Mem., 43, 52 pp. Ashby, J.R. and Minch, J.A., 1987. Stratigraphy and paleoecology of the Mulege embayment, Baja California Sur, Mexico. Cienc. Mar., 13(2): 89-112. Brenner, R.L., Swift, D.J.P. and Gaynor, G.C., 1985. Reevaluation of coquinoid sandstone depositional model, Upper Jurassic of central Wyoming and south-central Montana. Sedimentology, 32: 363-372. Curray, J.R., Emmel, F.J. and Crampton, P.J.S., 1969. Holocene history of a strand plain, lagoonal coast, Nayarit, Mexico. In: A.A. Castanares and F.B. Phleger (Editors), Simp. Lagunas Costeras. UNAM-UNESCO, pp. 63-100. Cutler, A.H., 1991. Processes of hardpart breakdown and models of stratigraphic disorder in shallow marine environments. Thesis. Univ. Arizona, Tucson, 256 pp (unpublished). Dauphin, J.P. and Simoneit, B.R.T. (Editors), 1991. The Gulf and Peninsular Province of the Californias. AAPG Mem., 47, 834 pp. Davis, O.K., Cutler, A.H., Meldahl, K.H., Palacios-Fest, M.R., Schreiber Jr., J.F., Lock, B.E., Williams, L.J., Lancaster, N., Shaw, C.A. and Sinitiere, S.M., 1990. Quaternary geology of Bahia Adair and the Gran Desierto Region. In: Deserts: Past and Future Evolution. IGCP, 252, 32 pp. Donovan, S.K. (Editor), 1991. The Process of Fossilization. Columbia Univ. Press, New York, 303 pp. Droser, M.L. and Bottjer, D.J., 1989. Ichnofabric of sandstones deposited in high-energy nearshore environments: measurement and utilization. Palaios, 4: 598-604. Durham, J.W., 1950. Megascopic paleontology and marine stratigraphy, Part II of the E.W. Scripts cruise of the Gulf of California. Geol. Soc. Am. Mem., 43, 216 pp. Durham, J.W. and Allison, E.C., 1960. The geologic history of Baja California and its marine faunas. Syst. Zool., 9: 47-91. Ekdale, A.A., 1987. Late Cenozoic rocks in the Puerto Penasco area. In: K.W. Flessa (Editor), Paleoecology and Taphonomy of Recent to Pleistocene Intertidal Deposits, Gulf of California. Paleontol. Soc. Spec. Publ., 2: 34-43. Ezcurra, E. and Rodrigues, V., 1986. Rainfall patterns in the Gran Desierto, Sonora, Mexico. J. Arid Environ., 10: 13-28.

K.H. MELDAHL Flessa, K.W. (Editor), 1987. Paleoecology and Taphonomy of Recent to Pleistocene Intertidal Deposits, Gulf of California. Paleontol. Soc. Spec. Publ., 2, 237 pp. Flessa, K.W., Cutler, A.H. and Meldahl, K.H., 1993. Time and taphonomy: quantitative estimates of time-averaging and stratigraphic disorder in a shallow marine habitat. Paleobiology, 19, in press. Frey, R.W. and Howard, J.D., 1988. Beaches and beach-related facies, Holocene barrier islands of Georgia. Geol. Mag., 125: 621-640. Frizell, V.A. (Editor), 1984. Geology of the Baja California Peninsula. Pac. Sect. SEPM, Los Angeles, CA, 267 pp. Fiirsich, F.T. and Aberhan, M., 1990. Significance of timeaveraging for paleocommunity analysis. Lethaia, 23: 143-152. Fiirsich, F.T. and Flessa, K.W. (Editors), 1991. Ecology, taphonomy, and paleoecology of Recent and Pleistocene molluscan faunas of Bahia la Choya, northern Gulf of California. Zitteliana, 18, 180 pp. Fiirsich, F.T. and Flessa, K.W., 1987. Taphonomy of tidal flat molluscs in the northern Gulf of California: Paleoenvironmental analysis despite the perils of preservation. Palaios, 2: 543-559. Gastil, R.G., Minch, J. and Phillips, R.P., 1983. The geology and the ages of the islands. In: T.J. Case and M.L. Cody (Editors), Island Biogeography in the Sea of Cortez. Univ. California Press, Berkeley, pp. 13-25. Geary, D.H. and Allmon, W.D., 1990. Biological and physical contributions to the accumulation of strombid gastropods in a Pliocene shell bed. Palaios, 5: 259-272. Hertlein, L.G. and Emerson, W.K., 1956. Marine Pleistocene invertebrates from near Puerto Pefiasco, Sonora, Mexico. Trans. San Diego Soc. Nat. Hist., 12: 154-175. Hoyt, J.H., 1969. Chenier versus barrier, genetic and stratigraphic distinction. AAPG Bull., 53: 299-306. Keen, A.M., 1971. Sea Shells of Tropical West America. Stanford Univ. Press, Stanford, 2nd ed., 1064 pp. Kidwell, S.M., 1988. Taphonomic comparison of passive and active continental margins: Neogene shell beds of the Atlantic coastal plain and northern Gulf of California. Palaeogeogr., Palaeoclimatol., Palaeoecol., 63: 201-223. Kidwell, S.M., 1989. Stratigraphic condensation of marine transgressive records: Origin of major shell deposits in the Miocene of Maryland. J. Geol., 97: 1-24. Kidwell, S.M., 1991. The stratigraphy of shell concentrations. In: P.A. Allison and D.E.G. Briggs (Editors), Taphonomy: Releasing the Data Locked in the Fossil Record. Plenum Press, New York, pp. 212-290. Kidwell, S.M., Ftirsich, F.T. and Aigner, T., 1986. Conceptual framework for the analysis and classification of fossil concentrations. Palaios, 1: 228-238. Kreisa, R.D. and Bambach, R.K., 1982. The role of storm processes in generating shell beds in Paleozoic shelf environments. In: G. Einsele and A. Seilacher (Editors), Cyclic and Event Stratification. Springer, Berlin, pp. 200-207. LeBlanc, R.J., 1972. Geometry of sandstone reservoir bodies. In: T.D. Cook (Editor), Underground Waste Management and Environmental Implications. AA.PG Mem., 18:133-190. Maluf, L.Y., 1983. Physical Oceanography. In: T.J. Case and

GEOGRAPHIC GRADIENTS IN THE FORMATION OF SHELL CONCENTRATIONS

M.L. Cody (Editors), Island Biogeography in the Sea of Cortez. Univ. Calif. Press, Berkeley, pp. 26-45. McFall, C.C., 1968. Reconnaissance geology of the Concepcion Bay area, Baja California, Mexico. Stanford Univ. Publ. Geol. Sci. 10(5), 25 pp. (plus 1:70,000 geologic map). McLean, H., 1988. Reconnaissance geologic map of the Loreto and part of the San Javier quadrangles, Baja California Sur, Mexico. U.S.G.S. Map MF-2000, scale 1:50,000. Meldahl, K.H., 1987. Sedimentologic and taphonomic implications of biogenic stratification. Palaios, 2: 350-358. Meldahl, K.H., 1990a. Sampling, species abundance, and the stratigraphic signature of mass extinction: A test using Holocene tidal fiat molluscs. Geology, 18: 890-893. Metdahl, K.H., 1990b. Paleoenvironmental and stratigraphic implications of taphonomic processes: case studies from Recent and Pleistocene shallow marine environments. Thesis. Univ. Arizona, Tucson, AZ, 440 pp. (unpublished). Meldahl, K.H. and Cutler, A.H., 1992. Neotectonics and taphonomy: Pleistocene molluscan shell accumulations in the northern Gulf of California. Palaios, 7: 187-197. Miller, K.B., Brett, C.E. and Parsons, K.M., 1988. The paleoecologic significance of storm-generated disturbance within a Middle Devonian muddy epeiric sea. Palaios, 3: 35-52. Moore, D.G. and Curray, J.R., 1982. Objectives of drilling on young, passive continental margins: Application to the Gulf of California. Init. Rep. DSDP, 64: 27-33. Morton, R.A., 1988. Nearshore responses to great storms. Geol. Soc. Am. Spec. Pap., 229: 7-22. Norris, R.D., 1986. Taphonomic gradients in shelf fossil assemblages: Pliocene Purisima Formation, California. Palaios, 1: 256-270. Ortlieb, L., 1987. Neotectonique et variations du niveau marin au Quaternaire dans la region du Golfe de Californie, Mexique. Inst. Fr. Rech. Sci. Devel. Coop. Coll. Etud. Theses, 2 vols.: 779 and 442 pp. Ortlieb, L., 1991. Quaternary shorelines along the northeastern Gulf of California: geochronological data and neotectonic implications. In: E. Perez-Segura and C. Jacques-Ayala (Edi-

25

tors), Studies of Sonoran Geology. Geol. Soc. Am. Spec. Pap., 254: 95-120. Perez-Segura, E. and Jacques-Ayala, C. (Editors), 1991. Studies of Sonoran Geology. Geol. Soc. Am. Spec. Pap., 254, 130 pp. Roden, G.I., 1964. Oceanographic aspects of the Gulf of California. In: T.H. van Andel and G.G. Shor Jr. (Editors), Marine Geology of the Gulf of California. AAPG Mem., 3: 30-58. Sirkin, L., Padilla-Arredondo, G., Pedrin-Aviles, S., DiazRivera, E., Gaitan-Moran, J. and Stuckenrath, R., 1984. Quaternary marine deposits, raised marine terraces, and tectonism in Baja California Sur, Mexico: A report on research in progress. In: V. Malpica-Cruz, S. Celis-Gutierrez, J. Guerrero-Garcia and L. Ortlieb (Editors), Symp. Neotectonics and Sea Level Variations in the Gulf of California Area, Hermosillo, Sonora, April 1984. Univ. Nac. Auton. Mexico, Inst. Geol., Hermosillo, pp. 319-340. Smith, J.T., 1989. Contrasting megafaunal and sedimentary records from opposite ends of the Gulf of California: implications for interpreting its Tertiary history. In: P.L. Abbott (Editor), Geologic Studies in Baja California. Pac. Sect., SEPM Book, 63: 27-36. Smith, J.T., 1991. Cenozoic marine molluscs and paleogeography of the Gulf of California. In: J.P. Dauphin and B.R.T. Simoneit (Editors), The Gulf and Peninsular Province of the Californias. AAPG Mem., 47: 637-666. Sokal, R.R. and Rohlf, F.J., 1981. Biometry. Freeman, New York, 2nd ed., 859 pp. Thompson, R.W., 1968. Tidal fiat sedimentation on the Colorado River Delta, Northwestern Gulf of California. Geol. Soc. Am. Mem., 107, 131 pp. Van Andel, T.H., 1964. Recent marine sediments of Gulf of California. In: T.H. van Andel and G.G. Shor Jr. (Editors), Marine Geology of the Gulf of California. AAPG Mem., 3: 216-310. Van Andel, T.H. and Shor Jr. G.G. (Editors), 1964. Marine geology of the Gulf of California. AAPG Mem., 3, 408 pp.