ZOBODAT - www.zobodat.at Zoologisch-Botanische Datenbank/Zoological-Botanical Database Digitale Literatur/Digital Literature Zeitschrift/Journal: Zitteliana - Abhandlungen der Bayerischen Staatssammlung für Paläontologie und Histor. Geologie Jahr/Year: 1990 Band/Volume: 18 Autor(en)/Author(s): Fürsich Franz Theodor, Flessa Karl W., Aberhan Martin, Feige Andreas, Schödlbauer Susanne Artikel/Article: Sedimentary habitats and molluscan faunas of Bahia la Choya (Gulf of California, Sonora, Mexico) 5-51 © Biodiversity Heritage Library, http://www.biodiversitylibrary.org/; www.zobodat.at 5 Zitteliana 18 5-51 Seiten 10 Tafeln München, 30. 4. 1991 ISSN 0373-9627 Sedimentary habitats and molluscan faunas of Bahia la Choya (Gulf of California, Sonora, Mexico) By FRANZ T. FÜRSICH, KARL \V. FLESSA, MARTIN ABERHAN, ANDREAS FEIGE and SUSANNE SCH ÖD LBAU ER*) With 13 figures in the text and 10 plates ABSTRACT The tidal flat complex of Bahia la Choya can be subdivided molluscs, among them 127 species of bivalves, 88 of gastro­ into fine- to medium-sandy outer flat areas characterised by pods, and three of scaphopods. Highest shell densities occur sand waves, a fine-sandy rippled mid flat, and a fine-sandy to in outermost intertidal areas, low densities are characteristic silty inner flat with a mesita-like topography. Additional ha­ of inner flat firmgrounds. In death assemblages bivalves do­ bitats are a major tidal channel, a supratidal salt marsh, and a minate numerically except in inner flat areas and in the chan­ shelly spit separating the marsh from the tidal flat. Lithified nel; they are also less dominant in hard substrate areas where Pleistocene rocks crop out at various localities, especially in they constitute 40 — 70% of the fauna. Highest diversity va­ the northern part of the bay. Biogenic sedimentary structures lues of the thanatocoenoses are found in outer to mid flat are ubiquitous on the tidal flat and include feeding excava­ areas, whilst lowest values are encountered in the inner flat of tions of rays, crawling traces of gastropods and hermit crabs, the southern part of the bay. These general distribution pat­ shafts of polychaetes, and burrows of crabs and callianassid terns suggest that the distribution of species within death as­ shrimps. A zonation of biogenic sedimentary structures al­ semblages across the tidal flat is largely governed by ecologic lows differentiation of the outer/mid flat and the inner flat/ factors and less by taphonomic processes such as post mortem salt marsh. The shelly benthic macroinvertebrate fauna be­ transport. longs to the Panamic faunal province and is dominated by KURZFASSUNG Das Watt von Bahia la Choya läßt sich in drei Abschnitte Krabben, Calltanassa und Upogebia. Die Zonierung der Spu­ unterteilen: Ein fein- bis mittelkörnigcs äußeres Sandwatt mit ren erlaubt eine Unterscheidung des äußeren und mittleren Großrippein, ein feinkörniges flaches Mittelwatt mit Klein- Watts vom Innenwatt und der Salzmarsch. Das hartteiltra­ rippeln und ein feinsandig bis siltiges Innenwatt mit einer gende Makroinvertebraten-Benthos gehört der Panama-Fau­ „Tafelberg“-Morphologie. Landwärts trennt eine Schillbarre nenprovinz an und besteht vorwiegend aus Mollusken. die Salzmarsch von der Gezeitenfläche. Sowohl Salzmarsch 127 Muschelarten, 88 Gastropodenarten und 3 Scaphopo- wie Watt werden von einer größeren Gezeitenrinne durchzo­ denarten wurden identifiziert. Die größte Schalendichte fin­ gen. Lithifizierte Sedimente des Pleistozäns finden sich in der det sich im äußersten Intertidalbereich; die Festgründe des Umrandung der Bucht und vor allem im nördlichen Teil der Innenwatts besitzen hingegen nur eine geringe Schalendichte. Gezeitenfläche. Lebensspuren sind im gesamten Wattbereich In den Thanatozönosen dominieren die Muscheln zahlenmä­ sehr häufig. Es handelt sich um schüsselförmige Depressio­ ßig bei weitem; lediglich im Innenwatt und in der Gezeiten­ nen, die von Rochen erzeugt wurden, um Kriechspuren von rinne sind Gastropoden häufiger. Auf Hartsubstrat stellen Gastropoden und Einsiedlerkrebsen, um agglutinierte Muscheln 40—70% der Fauna. Die höchsten Diversitäts- Wohnröhren von Polychaeten und um Wohnbauten von werte der Thanatozönosen treten im Außen- und Mittelwatt auf; die geringsten Werte weist das Innenwatt im südlichen Bereich der Bucht auf. Diese allgemeinen Verteilungsmuster F. T. Fürsich, M. Aberhan, A. Feige, Institut für Paläontologie deuten an, daß die Verteilung von Arten in den Thanatozöno­ der Universität, Pleicherwall 1, W-8700 Würzburg, Germany; sen des Watts vorwiegend von ökologischen Faktoren be­ K. W. FlESSA, Department of Geosciences, University of Arizona, Tucson, AZ. 85721, U.S.A.; S. Sc.hOdi Bauer, Franz-Wolter­ stimmt wird und nicht so sehr von taphonomischen Prozes­ Straße 6, W-8000 München 81, Germany. sen wie postmortaler Transport. © Biodiversity Heritage Library, http://www.biodiversitylibrary.org/; www.zobodat.at 6 INTRODUCTION Bahia la Choya, a small protected bay in the northern part Recent habitats of Bahia la Choya. We consider biotic, tapho­ of the Gulf of California, is a natural laboratory for the study nomic, sedimentologic, and ichnologic environmental indica­ of taphonomic processes and paleoenvironmental indicators. tors. (2) Application and evaluation of these paleoenviron­ The rich molluscan fauna, the low sedimentation rates and the mental indicators in an analysis of adjacent fossiliferous depo­ broad intertidal zonation make the area ideally suited for ac- sits of Pleistocene age. We demonstrate how an integrated ap­ tualistic studies. Nearby fossiliferous deposits of Pleistocene proach to paleoenvironmental analysis is an especially power­ age provide the opportunity to evaluate the utility of such ac- ful technique in paleoecology. tualistic studies for paleoenvironmental purposes. Thus, our A summary of our work is presented in F ursich & F lessa studies have two purposes: (1) Analysis of the nature and di­ (1987) and a field guide to the region is available (F lessa 1987). stribution of potential paleoenvironmental indicators in the THE REGIONAL SETTING Bahia la Choya is located at approximately 31°21' north la­ at Lukeville, Arizona) and the influx of American tourists, titude, 113°37' west longitude, on the northeastern shore of English-language place names are often used in the region. the Gulf of California (Fig. 1), in the Mexican state of Sonora. Bahia la Choya is “Cholla Bay” and Puerto Peñasco is The Bahia la Choya area is shown on the 1981 Puerto Peñasco “Rocky Point” [the literal translation is “ Rocky Port”] to 1 :50,000 topographic sheet (Map code H12A32) published most Americans. by the Comisión de Estudios del Territoria Nacional, Direc­ The region is within the Sonoran Desert. The climate is ción de Estudios Económicos, Mexico City. Puerto Peñasco, arid, with an annual average rainfall of 74 mm (T homson et al. a town of approximately 40,000 inhabitants (in 1988), is 1969). The rainfall is seasonal: part falls during brief but often 10 km to the southeast of Bahia la Choya. violent summer thunderstorms, the remainder falls during the We use Mexican place names throughout. Because of the winter as the remnants of Pacific Ocean storms move across area’s proximity to the United States (100 km from the border the area. Tropical storms and hurricanes occasionally affect Fig. 1: Oblique aerial photograph of Bahia la Choya. M: salt marsh; IF: inner flat; MF: mid flat; OF: outer flat; ST: subtidal; C: channel. © Biodiversity Heritage Library, http://www.biodiversitylibrary.org/; www.zobodat.at 7 the region in the late fall. Evaporation greatly exceeds precipi­ The tidal range in the northern Gulf of California is the tation and vegetation is limited to sparse cactus and desert third highest in North America. Spring tides in the Puerto Pe­ shrubs and grasses. ñasco area often reach 8 to 9 m of vertical displacement. Tides are of the mixed, semidiurnal type: two unequal high and two All streams in the immediate region are ephemeral. The unequal low tides occur each day. The very low slope of the “esteros” of the area are negative esturaries, where evapora­ intertidal area and the very great tidal range result in an exten­ tion significantly exceeds runoff. The only perennial stream sive area of exposure during spring low tides. Approximately entering the northern Gulf of California is the Colorado Ri­ 10 km2 of tidal flats are exposed during exceptionally low ti­ ver, 120 km to the northwest. The influx of both fresh water des. The large tidal range also produces strong currents in the and sediment from the Colorado River is limited. Dams and channels that drain the marsh and the sand flats. We have ob­ irrigation projects have reduced river flow in the delta region served current velocities of 1 m/sec in tidal channels. to such low levels that evaporation eliminates the remaining fresh water during most of the year. The fauna of the Gulf of California is included within the Daily average air temperatures range from 11.6°C in Ja­ Panamic Province, a largely tropical biogeographic province nuary to 30°C in August (T homson 1987). Summer extremes that ranges southward to Panama. The fauna of the northern of 42° C and winter extremes of 0° C are not uncommon in this Gulf has more in common with the truly tropical faunas to the harsh climate. south than it does with the faunas on the Pacific coast of Baja Surface water temperatures range from 10°— 14°C in the California. Brusca (1980) provides a guide to the common in­ winter to 30°—32° in the summer. Salinity in the Gulf of Cali­ tertidal invertebrates of the Gulf. K een (1971) is the authori­ fornia ranges from 35°/c; in offshore surface waters to 39 %o tative reference to the Gulf’s molluscs, and B eckvar et al. in shallow coastal waters. Higher salinities occur in the upper (1987) present a key to the most common molluscs of the Ba­ reaches of esteros and in evaporite basins along the margins of hia la Choya area. Bahia Adair. GEOLOGIC SETTING The Gulf of California is a rift basin which began opening crystalline granite, composed primarily of quartz and micro- approximately four million years ago (L arsfn et al.