The purpose of this field trip is to examine sedimentary environments, fauna, and Pleistocene deposits in the northeastern Gulf of California. The coastal area experiences tides of up to 8 meters in amplitude, the Gulf harbors a rich invertebrate fauna, especially molluscs, crustaceans, and echinoderms, and the arid climate of the region makes for a distinctive sedimentary regime. Adjacent fossiliferous Pleistocene deposits provide a glimpse into both ancient habitats and taphonomic processes. In short, the area is an excellent natural laboratory – a laboratory for the study of Recent animal-sediment relations, sedimentary processes, and taphonomy.

Pleistocene-Holocene sedimentary rocks, which outcrop along the northern Gulf of California coast in the vicinity of Puerto Peñasco, Sonora, have a very heterogeneous, mixed carbonateclastic composition. Carbonate grains are primarily molluscan, echinoderm, bryozoan and calcareous algal fragments. Terrigenous grains are primarily silicate minerals (e.g., quartz and feldspar) derived from three nearby igneous prominences (the basaltic Punta Peñasco and Cerro Prieto and the granitic Punta Pelicano).

Porosity of the sedimentary rocks, which commonly exceeds 50% in many units, may be intergranular and/or moldic. Cements in eolianite, precipitated within the vadose zone, are mainly of the meniscus spar type at grain contacts. Cements in beachrock, precipitated within the intertidal or shallow subtidal zone, include isopachous calcite spar and/or acicular aragonite.

A grain size analysis (including the carbonate fraction) was performed on sediment samples from each of the following environments of Bahia la Choya: tidal flat, spit, beach, and estero. The sediments of the tidal flat have an average grain size of 2.88 phi, range from very well sorted to moderately well sorted, and have a carbonate shell fraction ranging from 1 to 30%. The spit sediments have an average grain size of 0.96 phi, are moderately to poorly sorted, and have a carbonate shell fraction of approximately 50%. The beach sediments have an average grain size of 1.22 phi, range from poorly sorted to moderately well sorted, and have a carbonate shell fraction of approximately 48%. The sediments of the estero average 39% sand, 40% silt, and 20% clay. The carbonate fraction in the estero is approximately 25%. The noncarbonate fraction in all the environments is made up of feldspar, quartz, biotite, heavy minerals, and igneous rock fragments.

The major source for the sediment is the Pelican Point granite on the south side of the bay. Pleistocene rock in and around the bay provides a second source of sediment and volcanic rocks to the north may also contribute to the sediment.

I observed and measured the orientations of common physical sedimentary structures on the beach and spit, estero, tidal channel, and inner and outer tidal flats of Bahia la Choya. The spit is characterized by wind ripples and the beach typically displays swash marks. Mudcracks typify the high marsh portions of the estero while the meandering estero channel contains ripples and megaripples. The inner and outer tidal flats are dominated by small, ebb-oriented, straight-crested to undulatory current ripples. The current ripples are often superimposed on flood-oriented ridges and runnels of the outer flat. Rill marks, tool marks, and current crescents occur sporadically. Ebb-oriented megaripples, small current ripples, and primary current lineations occur in the tidal channel.

Ripple crests trend from N15E - N40E on the southern tidal flats to N10W - N20W on the northern flats. The crests roughly parallel the shoreline and are normal to tidal currents.

These keys are intended for use on dead shell material found on the sand flat, spit and estero of Bahia la Choya. They describe one hundred seventy molluscan species, three irregular echinoids and an inarticulate brachiopod. The taxonomic list on which the keys are based was compiled from shell material collected mainly during February and March, 1985. An attempt was made to make the keys as complete as possible. However, due to seasonal variations in shell abundances, not all shell material found at Bahia la Choya may be included in the key. Additionally, not all taxa found in the Pleistocene rock exposed around the bay are described here.

The keys are based on morphological similarites, not phylogenetic relationships. Descriptive terminology has been kept to a minimum, and a glossary and labeled sketches are included for those technical terms which were employed. The reader is advised to become familiar with the anatomical parts of shells before embarking on an identification.

External shell morphology can be altered by physical and biological destruction. In particular, fragile shell structures may be broken off, and shell sculpturing may be worn down. The absence of these sometimes critical features may lead to an incorrect identification. Additionally, bleaching of shells can alter or destroy the original colors. Therefore, shell color is not necessarily diagnostic and is mainly used as an additional character in the description of the species.

Species are described only within the keys. When the species name has been reached, phrases within the couplet contain additional descriptive information and often some ecological information.

Populations of the intertidal gastropod, Theodoxusluteofasciatus, exhibit spectacular variety in shell color and color pattern. Few significant differences in coloration occur among the individuals sampled from Bahia la Choya and Estero Morua. Furthermore, no obvious correlation exists between shell coloration and any observed environmental parameters.

Bioerosion–the active excavation of rock and shell material by organisms– is a significant degradational process in many marine sedimentary environments. This phenomenon is prominent in the northern Gulf of California, where coastal carbonate “beachrock” is undergoing erosion by a diverse suite of invertebrates. Near Puerto Peñasco, Sonora, the varied intertidal settings display distinctive associations of macroboring infauna, and accompanying biokarst.

Examination of shell repair in the Recent gastropod species Cerithiumstercusmuscarum from several microhabitats within Bahia la Choya, Sonora, Mexico reveals significant intraspecific variation in shell repair frequencies. Samples include approximately 800 individual gastropods from four distinct localities: two rocky and two sandy intertidal habitats. Biometric measurements and the number of scars on the last whorl were used to statistically investigate the relationships between size, habitat and shell repair.

Shell repair frequencies vary significantly between the microhabitats examined, and this variation is not related to differences in size-frequency distributions between samples. There is no preference for either larger or smaller snails to be attacked in any sample.

Trends in gastropod shell repair frequency provide a method of examining predator-prey relationships through time. However, potential problems, including microhabitat-related intraspecific variation in repair frequency, exist in interpreting repair trends. The results of this study demonstrate that there is significant variation in shell repair frequency through space at any one point in time. Intraspecific variation could, therefore, blur any overall trend in shell repair frequency through time.

Size-frequency distributions and size-survivorship curves were constructed for the following seven species of living gastropods from Bahia la Choya: Cerithideamazatlanica, Cerithideaalbonodosa, Cerithiumstercusmuscarum, Nassariusiodes, Nassariusbrunneostoma, Theodoxusluteofasciatus and Tegularugosa. Size-frequency distributions and size-survivorship curves of empty shells of Cerithideamazatlanica and Cerithiumstercusmuscarum were constructed to evaluate the effects of taphonomic processes on survivorship curves.

All size-survivorship curves are convex-up in shape: small size classes show low mortality and mortality increases with increasing size. Despite post-mortem processes, survivorship curves for empty shells mimic the shape of curves based on live individuals.

I measured the settling velocities and observed the settling behavior of sixteen molluscan species from Bahia la Choya. Bivalve shells stabilize in a concave-up fall position, and exhibit slower settling velocities than gastropod shells of similar weight and volume. Within individual species settling rates increase as the ratio of shell weight to maximum cross-sectional area increases. Five distinct fall-patterns occur: straight-fall, gliding, rotation, rocking, and oscillation.

Taphonomic analysis of fossil remains can provide information on the post-mortem history of the fossils, and may also be useful in interpreting associated sediments. A preliminary survey of the interior surfaces of Recent Chione shells shows that biologic, physical, and chemical agents attacking carbonate substrates in the intertidal environment of Bahia la Choya, Sonora, Mexico, leave behind diagnostic traces of their action.

Shells collected from across the tidal flats and from beach ridge deposits were examined under the SEM and binocular microscope. Tidal flat specimens were dominated by the effects of microboring endoliths. Even recently dead, articulated specimens showed some infestation. Excavation of surfaces by microborers goes through predictable stages, and may therefore be useful for relative “dating” of shells and for estimating burial/sedimentation rates. Beach ridge specimens show a heterogenous array of textures, some inherited from earlier taphonomic episodes, others produced in situ. Textures overprint one another and document transport between habitats.

Similar surface textures should be identifiable in the fossil record and could provide important paleoecologic and sedimentologic information.

Fossil concentrations such as shell beds are preferred collection sites for paleontological and paleoecological data. Although ample attention has been paid to the roles and implications of physical processes in shell bed formation, the role of biogenic sediment reworking in generating shell beds has received little study.

Subsurface shell beds in the Recent intertidal environment at Bahia la Choya form via three distinct processes: storm deposition, tidal channel migration, and biogenic reworking. Shell beds on the outer tidal flats are produced primarily by storm sedimentation. Lateral migration of the main tidal channel produces shell beds along the channel margins. Shell beds on the inner flats distant from the main tidal channel are generated by vertical “conveyorbelt” recycling of fine sediment by deposit feeders. Biogenic reworking concentrates shells at depth into subsurface shell beds, producing a distinct form of biogenic bedding, termed biogenic stratification.

These three modes of shell bed formation leave recognizable taphonomic and sedimentologic signatures. Biogenic stratification in particular has important implications for sedimentation rates, sediment reworking, and faunal condensation in ancient strata.

An actively prograding shelly sand spit forms the eastern margin of Bahia la Choya, northern Gulf of California. Shell-rich deposits exposed on the eastern side of the spit record previous phases of spit growth. Analysis and comparison of faunal composition, biofabric and shell preservation from the beach, estero mouth bar, and the fossil spit deposits suggest 3 major phases of spit evolution: (1) southward progradation of the spit tip/estero mouth bar (lower shell bed), (2) deposition of washover lobes during a major storm (upper shell bed), and (3) eolian accretion (spit crest). A 1300 ± 50 y.b.p. radiocarbon date on a bivalve shell from the storm shell bed provides a maximum age for the storm event.

Taphocoenoses can be powerful indicators of ancient environments, even in depositional settings in which rates of sedimentation are low and tidal currents are strong.

The rich subtropical mollusc fauna of the northern Gulf of California exhibits a distinct zonation across an extensive tidal flat at Bahia la Choya (Sonora, Mexico). The distribution of live faunas is largely controlled by substrate, energy level, availability of food, and period of submergence. Despite strong tidal currents and the effects of time-averaging, the distribution of taphocoenoses closely reflects the distribution of the live communities. Indeed, the zonation of individual species is often reflected in the distribution of their shelly remains.

The outer flat, middle flat, inner flat, tidal channel, and salt marsh subenvironments within the tidal flat complex can be defined by the taxonomic composition, trophic/life habit groups and taphonomic attributes of their taphocoenoses. Gastropods dominate on hard and firm substrates (rocky flats, channel and marsh) while bivalves prevail on sandy substrates. Herbivores are most common on rocky and shelly substrates; detritus feeders occur on the firm substrates of the inner flat and marsh; suspension-feeders predominate in sandy and rocky bottoms. Shell abrasion, encrustation and boring decrease landward.

We tested an integrated approach to paleoenvironmental analysis by examining a Pleistocene shell bed from Bahia la Choya. Taxonomic, ecologic and taphonomic criteria suggest deposition by a storm event in a very shallow subtidal to lowest intertidal environment.