Pleistocene shoreline ridges from tide-dominated and wave-dominated coasts: northern Gulf of California and western Baja California, Mexico

MARINE QEOLOQY INTERNATIONAL JOURNAL OF MARINE GEOLOGY, GEOCHEMISTRY ANO GEOPHYSICS

ELSEVIER

Marine Geology 123 (1995) 61-72

Pleistocene shoreline ridges from tide-dominated and wave-dominated coasts: northern Gulf of California and western Baja California, Mexico Keith H. Meldahl Department of Geology, Oberlin College, Oberlin, 0H44074, USA Received 26 July 1994; revision accepted 18 November 1994

Abstract

The arid climate of northwestern Mexico preserves Pleistocene beach deposits forming linear, shoreline-parallel ridges on the coastal plains of Sonora, northern Gulf of California, and the Vizcaino Peninsula of Baja California. Beach deposits represent both cheniers (northern Gulf) and beach ridges (Vizcaino Peninsula). They formed during Pleistocene sea-level highstands, most commonly during oxygen-isotope stage 5e ( ~ 125 ka). The ridges are 0.5 to 5.0 m thick, < 10 to > 100 m wide, and < 100 to > 1000 m long, coarse-grained with common low- to high-angle cross stratification, and are dominated by bivalve shells. Ridges from the tide-dominated northern Gulf of California and the wave-dominated western Vizcaino Peninsula differ in underlying lithology, size, profile, and shell fragmentation. Northern Gulf ridges progradationally overlie deltaic or marsh deposits, are smaller (median thickness 1.4 m; median width 25 m; median length 300 m), typically have a seaward-dipping upper surface and a linear crest on the landward side, and have low levels of fragmentation of large bivalve shells. In contrast, Vizcaino ridges disconformably overlie bedrock on wave-cut terraces, are larger (median thickness 3.3 m; median width 100 m; median length 2000 m), typically have a landward-dipping upper surface and a linear crest on the seaward side, and contain beds of extensively fragmented large bivalve shells. These differences are interpreted to reflect contrasting processes of ridge formation in the two regions. The ridges of the tide-dominated northern Gulf are true cheniers--transgressive beach deposits produced during episodes of clastic starvation associated with shifts in Colorado River delta deposition. They are modified primarily through episodic washover processes during high tides and storms. The ridges of the wave-dominated Vizcaino Peninsula are regressive beach ridges developed on highstand wave-cut terraces. They are modified primarily through continuous wave accretion to the beach face, and secondarily by washover processes.

1. Introduction

Quaternary shoreline deposits receive much attention due to their importance in interpreting sea-level changes and neotectonic uplift/subsidence rates (e.g. Chappell and Shackleton, 1986). These deposits are also significant for information about ancient littoral environments and processes. Cheniers and beach ridges are long, linear, ridgeforming deposits representing the locations of 0025-3227/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0025-3227(94)00124-3

ancient shorelines. They occur most commonly on low-relief coastal plains, standing with several meters of relief above their surroundings. These distinctive features have been studied in m a n y regions of the world ever since Russell and Howe (1935) first described them in coastal southwestern Louisiana (see Augustinus, 1989, for a review). They m a y occur singly or as multiple sets of ridges running roughly parallel to the shoreline, forming a chenier plain. Typically the ridges in a chenier

62

K.H. Meldahl/Marine Geology 123 (1995) 61-72

plain become younger seaward, recording successive positions of an advancing shoreline (Hoyt, 1969; LeBlanc, 1972). Cheniers and beach ridges are geomorphically similar, but Otvos and Price (1979) recognize an important genetic distinction. Cheniers are transgressive ridge-forming beach deposits of coarse sediment that progradationally overlie landward mudflat/marsh deposits. Beach ridges are regressive ridge-forming beach deposits of coarse sediment that progradationally overlie seaward shoreface deposits. The distinction between cheniers and beach ridges is thus based largely on underlying lithology. In this study, I also use the term "beach ridge" to describe regressive ridge-forming beach deposits that disconformably overlie bedrock on wave-cut terraces. Ridge-forming beach deposit is used in this paper as a descriptive term encompassing both cheniers and beach ridges, without any genetic implication. Nearly all published studies of cheniers report Holocene ages. Pleistocene occurrences are rare, and cheniers older than Pleistocene have not yet been described (Augustinus, 1989). This study reports on Pleistocene cheniers and beach ridges from the northern Gulf of California and Pacific coast of Baja California, and interprets their formation in view of clastic sediment supply and tide-dominated versus wave-dominated shoreline processes.

1.1. Pleistocene shorelines of northwestern Mexico

Pleistocene shallow marine sediments are common on both the east and west coasts of the Gulf of California and along the Pacific coast of Baja California. These sediments commonly form deposits representing beach, tidal fiat, tidal channel, marsh and nearshore shelf environments (Meldahl, 1993; Meldahl and Cutler, 1992). The deposits are composed of sandstone, gravel, conglomerate and coquina, typically range 1 to 5 m thick, and commonly rest disconformably on underlying units. A comprehensive description, mapping and chronostratigraphic study of these deposits was conducted by Ortlieb (1987, 1991) for the purpose of determining locations and mag-

nitudes of neotectonic deformation along the coasts of Sonora and Baja California. Many of these shoreline deposits have been correlated by Ortlieb (1987, 1991) with several Pleistocene interglacial highstands, based on uranium-series and amino acid racemization dating, elevation, and degree of subaerial degradation. The most common of these Pleistocene shorelines usually occurs 5 to 9 m above present mean sea level, and correlates with the peak of the Sangamon interglacial highstand (~ 125 ka, oxygen-isotope stage 5e), when mean sea level stood a few meters above present (Chappell and Shackleton, 1986).

1.2. Study areas

This study compares Pleistocene shoreline deposits from two areas: the coast of Sonora in the northern Gulf of California (area A, Fig. 1) and the coast of the western Vizcaino Peninsula of Baja California (area B, Fig. 1). These regions differ distinctly in tidal range, wave fetch, and clastic sediment sources (Table 1). The northern Gulf coastline is macrotidal with restricted fetch, limited by the width of the Gulf. Northern Gulf tides are mixed semidiurnal with a mean range of 4.5 m and a mean spring range of 6.5 m in the study area (Thompson, 1968). Spring ranges may exceed 10 m at the head of the Gulf, which is occupied by the Colorado River delta. The Colorado River is the only permanent river entering the region, and it is the dominant clastic sediment source in the northern Gulf (Thompson, 1968). In contrast, the Vizcaino coastline is mesotidal, with a mean range of 1.6 m and a mean spring range of 2.2 m (Ortlieb, 1987). Wave fetch is limited only by the width of the Pacific Ocean, so this coast experiences the full brunt of large oceanic swells. No permanent rivers flow to the western Vizcaino coastline. Clastic input is limited to that delivered to the coast by intermittent streams eroding local bedrock sources, or moved from the shelf onshore by wave action. Both the northern Gulf and Vizcaino are similar in having low relief coastlines that adjoin wide shallow shelves. (The Vizcaino coastline becomes steep and rugged near the northwestern tip of the

KH. Meldahl/Marine Geology 123 (1995) 61-72

63

/ 115 ~

/

110°

O

California

Arizona

L

• SanDiego

Enseaad~Tijuana

"~.

• Tucson

.

i

----- 30 0

i

~!:i::

Sonora

,~e G

Peninsula de

u

Hermosillo

=

~

Vizcafno

B ---

25 °

0

100

200

300

115°

/

400

500 kilometers 110°

f

Fig. I. Location of study areas. (A) Sonoran coastline of the northern Gulf of California, a tide-dominated coast. (B) Coastline of the western Vizcaino Peninsula of Baja California, a wave-dominated coast.

64

K.H. Meldahl/Marine Geology 123 (1995) 61-72

Table 1 Comparison of Sonoran coast of the northern Gulf of California and coast of the western Vizcaino Peninsula, Baja California Northern Gulf coast (area A in Fig. 1)

Western Vizcaino coast (area B in Fig. 1)

4.5 m 6.5 m

1.6 m 2.2 m

Wave fetch

~ 100-200 km (limitedby width of the Gulf of California)

~ 10,000-12,000 km (limited by width of the Pacific Ocean)

Dominant sediment sources (Van Andel, 1964; Thompson, 1968; field observations)

Colorado River delta; local biogenic (molluscan) sediment

local volcanic and sedimentaryoutcrops; local biogenic (molluscan) sediment

Bottom slope (Van Andel, 1964)

99% < 5°

99% < 5°

Coastal topography

low-relief, bordering gently sloping alluvial plains

low-relief, bordering gently sloping alluvial plains or bedrock surfaces

Tidal range (Ortlieb, 1987; Thompson, 1968) mean spring

peninsula, but this area is not included in the present study.) Both regions are arid, with precipitation ranging 5 to 10 cm/year (Ezcurra and Rodriguez, 1986; Ortlieb, 1987). Indeed, the arid regional climate of northwestern Mexico probably accounts for the excellent preservation of Pleistocene highstand shoreline deposits, which would likely have been largely eroded away under more humid conditions. On the Sonoran coast of the northern Gulf of California, shallow marine and deltaic sediments correlating with isotope stage 5e (-,~ 125 ka) are extensively exposed in wave-cut cliffs, or as ridgeforming beach deposits standing in relief above the surrounding desert floor (Ortlieb, 1987). These deposits have been uplifted by several meters and gently folded by neotectonic movements associated with the Cerro Prieto Fault, a major dextral strikeslip fault joining the Gulf of California to the San Andreas system (Colletta and Ortlieb, 1984). Meldahl and Cutler (1992) described how shell beds formed in response to neotectonic deformation in these deposits. On the western coast of the Vizcaino Peninsula, Pleistocene shoreline deposits occur as ridge-

forming beach deposits or as thin veneers of coarse shallow marine sediment. Both commonly lie disconformably on wave-cut bedrock terraces. These deposits correlate with oxygen-isotope stages 5, 7, 9 and 11. The most common and best preserved of these are stage 5e deposits, which lie between 12 and 18 m above sea level, indicating uplift of several meters since formation. Stage 5a or 5c shoreline deposits, when present, occur 5-8 m above sea level. Older highstand remnants occur at higher elevations (Ortlieb, 1987, 1991).

2. Methods

Building on prior work (Meldahl, 1993; Meldahl and Cutler, 1992), I measured and described stratigraphic sections in Pleistocene shallow marine deposits at 11 locations along the Sonoran coast of the northern Gulf of California and 13 locations along the coast of the western Vizcaino Peninsula of Baja California. Data consist of sediment grain size, sorting and composition, physical and biogenic sedimentary structures, and thickness and contact relationships of component beds. I

K.H. Meldahl/Marine Geology 123 (1995) 61-72

described, measured and profiled 10 ridge-forming beach deposits occurring along approximately 80 km of northern Gulf coastline and 10 occurring along approximately 150 km of Vizcaino coastline. Data collected from ridge deposits include stratigraphic geometry (thickness, width, length, lateral size changes, cross-sectional profile of ridges, internal stratification and underlying lithology), faunal composition, and shell taphonomy (closepacking, orientation, bioclastic composition, shell size, articulation, fragmentation, abrasion, bioerosion and encrustation).

3. Results and discussion

3.1. Lithology of Pleistocene marine deposits Fig. 2 compares the lithologic composition of Pleistocene shallow marine deposits from the two study areas. The northern Gulf deposits are dominated by fairly mature terrigenous sediments: fineto medium-grained quartzarenite and subarkose.

[] NorthernGulf •

Vizca~o Peninsula

mudstone & siltstone

q ~ m

& subarkose

65

Mudstone and siltstone are common. About 25% of the sections are made up of biogenic sediments: molluscan calcirudite (shell beds) and calcarenite (shell hash). The Vizcaino deposits are dominated by biogenic sediments--molluscan calcirndite and calcarenite comprise more than 50% of the sections. The terrigenous component of the sediment is less mineralogically mature than in the northern Gulf. It is dominated by volcanic litharenite and sublitharenite, while quartzarenite and subarkose are relatively minor, and mud is rare. These lithologic patterns reflect the geologic settings of the two study areas. In both regions fecund mollusc populations on the shelf are a source for substantial amounts of molluscan sediment. In the northern Gulf most terrigenous coastal sediment originates from the Colorado River delta, a source of abundant fine clastics and mature sands, and is delivered along the coastline by longshore drift or wind (Thompson, 1968). In contrast, terrigenous sediment in the Vizcaino area originates from local sources, in particular Tertiary volcanics of the Sierra Santa Clara range, Mesozoic volcanics and ophiolites of the Sierra San Jose de Castro range, and outcrops of deepsea fan/turbidite sediments of the Cretaceous Valle Formation. These nearby sources generate fairly mineralogically immature sands with rare mud. The ephemeral streams here deliver little terrigenous sediment to the shoreline. Pleistocene shallow marine sediments of the Vizcaino are thus dominantly molluscan in origin.

3.2. Stratigraphy of Pleistocene beach deposits

litharenite& sublitharenlte

conglo~tc

¢aleirudite & enlcarenite (shell beds & shell hash) 0

10

20

30

40

50

60" 70

% total stratigraphic thickness Fig. 2. Lithologic composition of Pleistocene marine strata, expressed as a percentage of total stratigraphic thickness in measured sections. Data from 61 m of measured section from 10 localities in the northern Gulf (area A, Fig. 1), and from 57 m of measured section from 13 localities in the Vizcaino Peninsula (area B, Fig. 1).

In both study areas, beach deposits form linear, shoreline-parallel ridges that stand with several meters of relief above the surrounding desert floor. Ridges most commonly occur alone, less commonly in a set of two or three subparallel ridges. They range 0.5 to 5.0 m thick, < 10 to > 100 m wide, and < 100 to > 1000 m long. Bedding consists of layers of terrigenous and molluscan sands alternating with layers of whole shells and large shell fragments. Both shells and shell fragments are heavily abraded and worn. Shells and sand commonly form prominent high-angle, landward dipping cross beds (Fig. 3a). Seaward these may

K.H. Meldahl/Marine Geology 123 (1995) 61-72

66

oo i

e~

K.H. Meldahl/Marine Geology 123 (1995) 61-72

grade laterally into low-angle, seaward dipping cross beds. Densely packed shell-supported fabrics are common (Fig. 3a and b). Where shell density is low, whole valves usually occur in the concavedown (hydrodynamically stable) orientation. Where shell density is high, valves are commonly stacked and nested together in concave-up orientations (Fig. 3b). This pattern is most commonly observed within the high-angle landward dipping crossbeds. Where shell density is particularly high, valves are commonly nested in oblique or even edgewise orientations. Variable direction low and high-angle cross stratification is common (Fig. 3c). Bioturbation is rare, limited to small irregular vertical burrows. Where beach deposits have transgressed landward over fine-grained tidal fiat or marsh deposits, the basal contact may be deformed by pillow load structures (Fig. 3d). Where beach deposits lie disconformably on wave-cut bedrock

67

terraces, rounded bedrock cobbles and boulders commonly occur at the base. 3.3. Northern Gulf versus Vizcaino beach deposits 3.3.1. Description Pleistocene beach deposits from both study areas form linear, shoreline parallel ridges, but these differ distinctly in their underlying lithology, size, cross-sectional profile, taxonomic composition, and degree of large shell fragmentation (Table 2; Fig. 4). Ridges of the wave-dominated Vizcaino typically disconformably overlie wave-cut bedrock terraces. Ridges of the tide-dominated northern Gulf typically overlie fine-grained mudflat, marsh or deltaic deposits. Vizcaino ridges are typically several times thicker, wider and longer than those of the northern Gulf (Table 2). Ridges commonly have an cross-sectional profile that differs between

Table 2 Comparison of Pleistocene ridge-forming beach deposits from Sonoran coast of the northern Gulf of California and coast of the western Vizcaino Peninsula, Baja California. Data based on 20 ridge deposits (10 from each region) measured and described along approximately 80 km of coastline in the northern Gulf and 150 km of coastline in the western Vizcaino Northern Gulf coast (area A in Fig. 1)

Western Vizcaino coast (area B in Fig. 1)

1.4 m 0.4-2.3 m

3.3 m 0.5-5.0 m

25 m 10-50 m

100m 50-500m

300 m 100 m - > 1 km

2000 m

seaward 1°-3 ° on landward side of ridge

landward 1°-3 ° on seaward side of ridge

Underlying lithology

older Pleistocene mudflat, marsh and deltaic deposits

wave-cut terraces in Tertiary or Mesozoic bedrock

Taxonomic composition

95%->99% Chione californiensis or Chione

> 99% Tivela stultorum

Thickness median range

Width median range

Length median range

100 m - > 30 km

Cross-sectionalprofile (see also Fig. 4) slope of upper surface position of ridge crest

gnidia Fragmentation of large shells (see also Fig. 5)

nearly always low

commonly very high; shell fragments form "flat pebble" calcirndites

68

K.H. Meldahl/Marine Geology 123 (1995) 61-72

0

~

" tli~ii"

,~

0 ~ ,4D ~

~

~

~ ~,

~.:~

~ "~ =

~ "~

.~ ~'~

~'~

~,r ~ •

~

m

. .

~

~ ~ ~.~ ~ ~.

K.H. Meldahl/Marine Geology 123 (1995) 61-72

the two areas. Vizcaino ridges commonly have a broad flat upper surface that dips landward, and a linear crest on the seaward side of the ridge. Northern Gulf ridges commonly have the opposite profile, with a broad fiat upper surface that dips seaward, and a linear crest on the landward side of the ridge (compare Fig. 4a and b). Individual ridges in both regions are nearly always dominated by disarticulated valves of one bivalve species: Tivela stultorum (Vizcaino ridges), or Chione gnidia or Chione californiensis (northern Gulf ridges). These species vary in shell morphology, but are all large and robust. The degree of valve fragmentation of these species differs distinctly between the study areas. T. stultorum valves in Vizcaino ridges occur both whole and extensively comminuted, with large shell fragments forming distinctive "flatpebble" calcirudite beds. Chione spp. valves in northern Gulf ridges are usually preserved whole with only minor chipping around the edges (compare Fig. 4c and d).

3.3.2. Interpretation Differences in underlying lithology, size, profile, and shell fragmentation of ridge-forming beach deposits in the two study areas reflect different genetic processes. Northern Gulf ridge-forming beach deposits are true cheniers. They are transgressive beach ridges whose initiation is related to episodes of clastic starvation. Their growth and modification are dominated by episodic washover during spring tides or storms. Vizcaino ridgeforming beach deposits are regressive beach ridges whose initiation is related to episodes of terrace incision by waves. Their growth and modification are dominated by wave accretion of shells and sand to the beach face, occasionally punctuated by episodic ridge washover during storms. Cheniers of the northern Gulf form during periods of clastic starvation associated with shifts in the Colorado River delta (Fig. 5). Under starved conditions, breaking waves winnow fines and rework and concentrate shells and coarse sediment at the strand, forming a ridge. The pro-

69 PROGRADATION

level. . . . . . . . . .

REWORKING

3~

PROGRADATION

..............

vertical exaggeration ~100x Fig. 5. Idealized development of northern Gulf cheniers (modified from Hoyt, 1969, fig. 4, p. 327). Step 1: Depositional progradation of the shoreline due to clastic input from the Colorado River delta. Step 2: Reworking, shoreline retreat and chenier formation following reduction in clastic input. Fines are winnowed, sand and shells are concentrated on the beach by wave action, and deposited periodically on the landward side of the chenier by washover processes, causing landward transgression over adjacent marsh deposits (see also Fig. 3d). Step 3: Renewed clastic input causes the shoreline to prograde again, abandoning the chenier.

cess is self-reinforcing because the growing ridge forms a barrier which further captures coarse material. Once initiated, the ridge may transgress landward over adjacent fine-grained marsh or mudflat deposits by washover processes when the ridge is crested during high tides or storms. If clastic input is renewed, the shoreline may prograde, leading to the isolation of the ridge from the shore by a band of tidal mudfiats. Modern cheniers in the northern Gulf of California today appear to be forming as a result of human-induced sediment starvation. Upstream dams and irrigation have reduced the Colorado River's flow to the northern Gulf to insignificant levels for more than 50 years. At present the encroaching shoreline reworks the sediment starved tidal fiats, building a shell ridge that is being driven landward over adjacent marsh deposits (Thompson, 1968).

70

K.H. Meldahl/Marine Geology 123 (1995) 61-72

Alternating episodes of clastic input and starvation may produce multiple shoreline-parallel sets of ridges, forming a chenier plain, with younger cheniers occurring seaward of older ones. The classic Mississippi River chenier plain formed during episodes of clastic starvation associated with shifting mud supply from the Mississippi delta (Gould and McFarlan, 1959; Penland and Suter, 1989). Holocene chenier plains exist in two locations in the Gulf of California region: in the northern Gulf west of the Colorado River delta (Thompson, 1968; Kowalewski et al., 1994), 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 Holocene cheniers west of the Colorado River delta formed in association with shifts in delta mud deposition over the past several thousand years (Thompson, 1968). The taphonomy and chronology of these cheniers are analyzed by Kowalewski et al. (1994). Ridge-forming beach deposits of the Vizcaino Peninsula are not true cheniers because they do not transgress over landward facies. They are beach ridges lying disconformably on wave-cut bedrock terraces. Their formation is not intimately linked to episodes of clastic starvation, like cheniers. The absence of a major clastic source (comparable to the Colorado River) in the Vizcaino region means the conditions of alternating clastic input and starvation important in chenier formation do not exist here. In the Vizcaino, beach ridge formation is linked to episodes of sea-level stillstand that produce terrace incision, coupled with high biogenic shell production on the adjacent shelf (Fig. 6). Shells provide the material to build up beach ridges in the absence of a major clastic

I

~

~

~

~

~

leve 1. . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

BEACH DEPOSITION 3

vertical exaggeration ~ 100x

Fig. 6. Idealized development of western Vizcaino Peninsula beach ridges. Step 1: Pleistocene high sea-level stillstand. Step 2: Formation of a wave-cut terrace, paved with bedrock clasts. Step 3: Waveaccretion of sand and shells builds a beach ridge. The processis enhancedby highbiogenicshellproduction on the shelf.

TIDE-DOMINATEDCOAST(Northern Gulf of California)

--

NLW - Smallridges with top dipping seaward ("tidal profile"). - Ridgegrowthand modificationdominatedby episodicwashover processesduringhightides and storms.

WAVIB-DOMINA~COAST(Vlzcalno Peninsula)

:

P

MHW

source.

Beach deposits of the northern Gulf and the Vizcaino reflect the distinctly different oceanographic conditions under which they formed (Fig. 7). The ridges of the tide-dominated northern Gulf are modified primarily through ridge cresting during spring tides or storm surges (Thompson, 1968). They are inactive much of the rest of the time, because they lie above the reach of waves throughout most of the tidal cycle. When water

- Largeridgeswithtop dil~ing landward("swellprofile"). - Ridgegrowthand modificationdominatedby wave accretion to beach face; episodic washover processes secondary in importance. Fig. 7. Interpretation of differences in size and cross-sectional profile of Pleistocene beach deposits from the tide-dominated northern Gulf of California and the wave-dominated Vizcaino Peninsula. Size of arrows proportional to importance of growth processes, either landward episodic washover or seaward accretion to the beach face. MHW=mean high water. MLW=

mean low water. See text for further discussion.

K~H. Meldahl/Marine Geology 123 (1995) 61-72

levels crest the ridge, washover processes cause it to transgress landward. Shells and sand are dumped on the back side of the ridge as highangle landward dipping cross beds (Thompson, 1968). Valves settle in concave-up, nested orientations that reflect their initial deposition. Episodic washover events are short-lived, so valves are not reoriented into the more stable concave-down position. The dominance of episodic washover and landward transgression molds the ridge into a "tidal profile", with a seaward dipping upper surface and a linear crest on the landward side of the ridge (Fig. 7). The low wave energy of the northern Gulf, coupled with abandonment of ridges during most of the tidal cycle, means shells exhibit relatively low levels of fragmentation. The ridges of the wave-dominated Vizcaino experience rather different conditions. Here large waves are effective agents of beach sediment accretion, building up thick, wide ridges with a "swell profile"--a landward-dipping upper surface and a linear crest on the seaward side (Fig. 7) (see Komar, 1976, pp. 288-290 for a discussion of development of beach swell profiles). Wave accretion and the development of a swell profile results from swash/backwash velocity asymmetry. The swash typically occurs during a shorter period of time and reaches a higher velocity than the backwash (Komar, 1976, fig. l l - l l , p. 310). Additionally, the power of the backwash is reduced by percolation into the beachface. As a result, more wave power is expended in a unit amount of time on the swash phase, producing net movement of particles up the beach face. The crest of the ridge reflects how far up the beach face non-storm waves toss particles. Continuous pounding by large waves is manifested in the high levels of shell fragmentation observed in Vizcaino beach deposits. In addition to continuous non-storm wave action, Vizcaino ridges also experience inundation by Pacific storm waves. Storms are expected to both tear down ridges and wash over them. Washovers cause Vizcaino ridges to prograde landward episodically, and this process, as in the northern Gulf, is manifested in high-angle landward-dipping crossbeds. But the common preservation of swell profiles in Vizcaino ridges suggests

71

they are dominated by continuous non-storm wave accretion rather than by episodic storm washover. 4. Conclusions Ancient shoreline deposits are useful for investigating sea-level changes, vertical crustal movements, and sedimentary dynamics and processes in coastal zones. Pleistocene beach deposits along the Sonoran coast of the northern Gulf of California and the coast of the western Vizcaino Peninsula of Baja California form coarse-grained, linear, shoreline-parallel ridges that stand in relief above their surrounding coastal plains. They are 0.5 to 5.0 m thick, <10 to > 100 m wide, and < 100 to > 1000 m long, and are built of bivalve shells and coarse elastic and biogenic sand. They exhibit common low- to high-angle cross stratification, basal load structures where they have prograded over fine-grained deposits, or basal rounded bedrock clasts where they lie disconformably over bedrock. They formed during one of several Pleistocene highstands, most commonly during oxygen-isotope stage 5e ( ~ 125 ka). Ridges of the northern Gulf of California overlie fine-grained mudflat or marsh deposits, are smaller than Vizcaino ridges, commonly have a broad upper surface that dips seaward and a linear crest on the landward side of the ridge, and exhibit low levels of fragmentation of large bivalve shells. Ridges of the Vizcaino disconformably overlie wave-cut bedrock surfaces, are larger than northern Gulf ridges, commonly have a broad upper surface that dips landward with a linear crest on the seaward side of the ridge, and commonly contain extensively fragmented beds of large bivalve shells. These differences reflect different ridge-forming processes in the two regions. In the tide-dominated northern Gulf of California, beach deposits form cheniers--transgressive shell and sand ridges whose initiation is related to episodes of elastic starvation from the Colorado River delta, and whose growth and modification are dominated by episodic washover processes during spring tides or storms. In the wave-dominated Vizcaino Peninsula, beach deposits form beach ridges-regressive shell and sand ridges whose initiation is related to episodes of terrace incision, and whose

72

K.H. Meldahl/Marine Geology 123 (1995) 61-72

growth a n d modification are d o m i n a t e d by cont i n u o u s wave accretion o f coarse material to the beach face, occasionally p u n c t u a t e d by episodic washover d u r i n g storms. R i d g e - f o r m i n g beach deposits in b o t h regions reflect their e n v i r o n m e n t a l setting, i n d i c a t i n g the potential of these features as e n v i r o n m e n t a l indicators in coastal regions.

Acknowledgements C o m m e n t s a n d suggestions by Skip Davis, Bill T a n n e r a n d D o n n G o r s l i n e greatly i m p r o v e d the manuscript. Field w o r k was supported by f u n d s a n d e q u i p m e n t from O b e r l i n College a n d the Keck F o u n d a t i o n . The paper was written d u r i n g a research leave s u p p o r t e d b y a Keck F o u n d a t i o n fellowship g r a n t e d t h r o u g h O b e r l i n College. T h a n k s to D a v i d Bottjer a n d the D e p a r t m e n t o f E a r t h Sciences, University o f S o u t h e r n California, for a c c o m m o d a t i n g me d u r i n g leave. T h a n k s to A l a n Cutler for initiating collaborative work in the n o r t h e r n G u l f o f California, a n d to Sarah M e s n i c k for help enjoying sunsets o n the Vizcaino Peninsula.

References Augustinus, P.G.E.F., 1989. Cheniers and chenier plains: a general introduction. Mar. Geol., 90: 219-229. Chappell, J. and Shackleton, N.J., 1986. Oxygen isotopes and sea level. Nature, 324: 137-140. Colletta, B. and Ortlieb, L., 1984. Deformations of Middle and Late Pleistocene deltaic deposits at the mouth of the Rio Colorado, northwestern Gulf of California. In: V. Malpica-Cruz, S. Celis-Gutierrez, J. Guerrero-Garcia and L. Ortlieb (Editors), Neotectonics and Sea Level Variations in the Gulf of California Area. Symp. (Hermosillo, Sonora, April 1984.) Univ. Nac. Autonoma Mexico, Inst. Geol., Hermosillo, pp. 31-50.

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), Lagunas Costeras, un Simposio. UNAM-UNESCO, pp. 63-100. Ezcurra, E. and Rodrigues, V., 1986. Rainfall patterns in the Gran Desierto, Sonora, Mexico. J. Arid Environ., 10:13-28. Gould, H.R. and McFarlan, E., 1959. Geologic history of the chenier plain, southwestern Louisiana. Gulf Coast Assoc. Geol. Soc. Trans., 9: 261-270. Hoyt, J.H., 1969. Chenier versus barrier, genetic and stratigraphic distinction. AAPG Bull., 53: 299-306. Komar, P.D., 1976, Beach Processes and Sedimentation. Prentice-Hall, Englewood Cliffs, NJ, 429 pp. Kowalewski, M., Flessa, K.W. and Aggen, J.A., 1994. Taphofacies analysis of Recent shelly cheniers (beach ridges), northeastern Baja California, Mexico. Facies, in press. LeBlanc, R.J., 1972. Geometry of sandstone reservoir bodies. In: T.D. Cook (Editor), Underground Waste Management and Environmental Implications. AAPG Mem., 18: 133-190. 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. 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. Ortlieb, L., 1987. Neotectonique et variations du niveau matin au Quaternaire dans la region du Golfe de Californie, Mexique. Inst. Fr. Rech. Sci. Dev. Coop. Coll. Etudes Theses, 779 pp. and 442 pp. Ortlieb, L., 1991. Quaternary vertical movements along the coasts of Baja California and Sonora. In: J.P. Dauphin and B.R.T. Simoneit (Editors), The Gulf and Peninsular Province of the Californias. AAPG Mem., 47: 447480. Otvos, E.G. and Price, W.A., 1979. Problems of chenier genesis and terminology--an overview. Mar. Geol., 31: 252-263. Penland, S. and Suter, J.R., 1989. The geomorphology of the Mississippi River chenier plain. Mar. Geol., 90: 231-258. Russell, R.J. and Howe, H.V., 1935. Cheniers of southwestern Louisiana. Geogr. Rev., 25: 449461. Thompson, R.W., 1968. Tidal Flat 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.