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Geology.Gsapubs.Org on 27 January 2009 Downloaded from geology.gsapubs.org on 27 January 2009 Geology Mixed carbonate-siliciclastic sedimentation on a tectonically active margin: Example from the Pliocene of Baja California Sur, Mexico Rebecca J. Dorsey and Susan M. Kidwell Geology 1999;27;935-938 doi:10.1130/0091-7613(1999)027<0935:MCSSOA>2.3.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free email alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/index.ac.dtl to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 1999 Geological Society of America Mixed carbonate-siliciclastic sedimentation on a tectonically active margin: Example from the Pliocene of Baja California Sur, Mexico Rebecca J. Dorsey* Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA Susan M. Kidwell Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA ABSTRACT nonmarine, deltaic, and marine environments Bioclast-rich, coarse-grained deposits in the Pliocene Loreto basin provide a record of mixed (Dorsey et al., 1995; Dorsey and Umhoefer, carbonate and siliciclastic sedimentation at the steep hanging-wall margin of this small, fault- 1999). The stratigraphy of the Loreto basin is controlled basin. Sedimentary facies consist of sand- to gravel-sized carbonate debris mixed with divided into four sequences that record fault- volcaniclastic sand and gravel in a proximal to distal facies tract that includes matrix-rich and controlled basin evolution: (1) relatively slow matrix-poor shelly conglomerate, impure calcirudite and calcarenite, mixed-composition turbidites, deposition in a nonmarine half graben; (2) very and bioturbated calcarenitic sandstone. Carbonate material was produced by mollusks and other rapid westward tilting, subsidence, and deposi- benthic organisms on a narrow, high-energy shelf and mixed with volcaniclastic sand and gravel tion of footwall-derived marine Gilbert-type fan in cross-shelf channels. These mixtures were transported down a steep subaqueous slope by debris deltas and associated nonmarine and marine flows, grain flows, and turbidity currents, forming foresets and bottomsets of marine Gilbert-type facies; (3) continued rapid subsidence and depo- deltas. This style of mixed carbonate-siliciclastic sedimentation has not been documented in detail sition of impure bioclastic carbonates derived elsewhere but should be locally abundant in the stratigraphic record of fault-bounded basins, par- from the eastern hanging-wall dip slope; and ticularly those with cool or nutrient-rich waters that support relatively few binding and frame- (4) abrupt foundering of the basin to outer shelf work-building faunas. Recognition of similar facies in other settings can provide useful insights depths followed by filling with marine mudstone into ancient conditions of carbonate production, oceanography, climate, and tectonics. and limestone (Dorsey et al., 1995; Dorsey and Umhoefer, 1999). Sequence 3 (this study) con- INTRODUCTION be locally abundant in the stratigraphic record and tains abundant bioclastic carbonates and asso- Mixed carbonate-siliciclastic sedimentation is can be useful for reconstructing paleogeography, ciated volcaniclastic sediments that accumulated common in middle- to low-latitude shelf and oceanography, climate, and tectonics of ancient in foresets and bottomsets of marine Gilbert-type platform settings and can be controlled by a vari- sedimentary basins. deltas (Fig. 2) at the eastern margin of the Loreto ety of physical and biological processes (Choi basin. These coarse-grained deltas prograded to and Ginsburg, 1982; Mount, 1984; Flood and BASIN SETTING AND STRATIGRAPHY the west and northwest into the basin in response Orme, 1988; Pilkey et al., 1988). Mixing can The Pliocene Loreto basin is a small, trans- to fault-controlled uplift of the hanging-wall tilt occur when terrigenous sand and mud are deliv- tensional half-graben basin that formed during block, which created a new source of siliciclastic ered to a carbonate shelf during storms, causing development of the modern transform-rift plate sediment in the Sierra Microondas (Fig. 1) and a temporary termination or diminution of carbon- boundary in the Gulf of California (Fig. 1; narrow shelf for carbonate production. ate production (e.g., Kreisa, 1981). Storms may Umhoefer et al., 1994; Umhoefer and Dorsey, winnow and transport skeletal carbonate material 1997). The basin formed by westward tilting and SEDIMENTOLOGY OF MIXED FACIES from outer reef or bank settings into inner shelf asymmetric subsidence between ca. 5(?) and Mixed carbonate-siliciclastic deposits in se- and lagoonal areas of fair-weather siliciclastic 2 Ma, and it contains a thick, diverse assemblage quence 3 vary considerably in grain size, sorting, sedimentation, producing stratified bioclastic of sedimentary rocks that accumulated rapidly in sedimentary structures, thickness, and relative deposits and interbedded mixtures (e.g., Kelling and Mullin, 1975). Rarely, clastic carbonate material is derived from older lithified carbonate terranes, in which case production of carbonate sediments is unrelated to basinal dynamics or Figure 1. Geologic map of paleoecology of the shelf (Mount, 1984). Loreto basin and regional In this paper we introduce a previously undoc- tectonic setting (inset). Bio- umented style of mixed carbonate and siliciclastic clastic carbonates include sedimentation by using an example from the tec- mixed carbonate and vol- caniclastic deposits of this tonically active Pliocene Loreto basin in Baja study (box shows location California Sur, Mexico (Fig. 1). In this setting, of this study). Abbrevia- molluscan carbonates were produced on a narrow, tions: BC—Baja Califor- high-energy shelf rimmed by Miocene volcanic nia; BCS—Baja California Sur; Kg—Cretaceous gran- bedrock, mixed with volcaniclastic sand and itoids; L—Loreto basin; gravel in cross-shelf channels, and transported m—pre-Cretaceous meta- down steep, unstable slopes into the flanking morphic rocks; Mv—Mio- basin by a variety of subaqueous sediment-gravity cene volcanic rocks; Qt— flows. These flows produced a diverse array of Quaternary terrace depos- its; Qal—Quaternary allu- mixed bioclastic and volcaniclastic deposits that vium; Tv—Tertiary vol- are described and interpreted in the following. canic rocks; sst—sand- Although not previously documented, this style of stone. mixed carbonate-siliciclastic sedimentation should *E-mail: [email protected]. Geology; October 1999; v. 27; no. 10; p. 935–938; 4 figures; 1 table. 935 overlain by massive pebbly calcirudite and sandy Figure 2. View looking north calcarenite (Fig. 3, B and C; Table 1). Carbonate at large-scale foresets of Gilbert-type fan delta, show- grains are well sorted and in extreme instances ing transport toward west. consist of closely stacked concave-up valves of a Upper foresets (dark colored) single scallop or barnacle species. Inverse and nor- are matrix-rich shelly con- mal grading in basal gravel layers, abrupt but glomerate (facies 2); lower foresets (light-colored) are nonerosive contacts between gravel and calciru- transported calcirudite and dite, and close association with subaqueous debris calcarenite (facies 4) and bi- flows and turbidites suggest deposition from high- partite conglomerate and cal- concentration grain flows and overflowing, lower carenite (facies 3). Thickness density turbulent suspensions (e.g., Lowe, 1982; of exposed foresets is ~50 m. Nemec, 1990; Falk and Dorsey, 1998). Concentra- tion of calcirudite and calcarenite in the upper parts of these deposits is attributed to hydraulic sorting and segregation of lighter carbonate shell percent of carbonate and siliciclastic components hash refers to carbonate grains 2–10 mm in diam- debris from heavier volcaniclastic gravel. (Table 1; Fig. 3). These deposits are exposed in a eter, and calcirudite indicates gravel-sized car- Facies 4 consists of massive to relict-bedded laterally contiguous facies tract that displays an bonate particles (Table 1). calcirudite and calcarenite with relatively minor overall decrease in grain size from proximal Facies 1 is preserved locally as thin topset lithic sand, granules, and small pebbles (Fig. 3C). Gilbert-type delta foresets (Fig. 2) to medial and deposits and consists of thin- to medium-bedded Calcirudite and calcarenite record deposition distal bottomsets. Carbonate grains consist clast-supported conglomerate, shelly sandstone, from bioclastic turbulent suspensions,
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