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Stratigraphy, Tectonics, and Basin Evolution in the Anza-Borrego Desert Region

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Anza - C05 Stratigraphy 10/28/05 3:21 PM Page 89 Stratigraphy, Tectonics, and 5Basin Evolution in the Anza-Borrego Desert Region The most beautiful thing we can experience is the mysterious; It is the source of all true art and all science. Albert Einstein Rebecca Dorsey 89 Anza - C05 Stratigraphy 10/28/05 3:22 PM Page 90 Stratigraphy, Tectonics, and Basin Evolution in the Anza-Borrego Desert Region Southern Borrego Badlands Looking West. (Photograph by Rebecca Dorsey) Introduction The fossil record of past life is commonly preserved in ancient sedi- ments and sedimentary rocks. Sediments accumulate in subsiding basins that contain different kinds of depositional environments such as rivers, lakes, deltas, and marine seaways. These environments are friendly to life, and often support assemblages of plants and animals. Through integrative studies of stratigraphy, sedimentology, and paleontology, we can reconstruct ancient life communities and the environments in which they lived. Plate tectonic forces determine where sedimentary basins form, how long and how fast sediments accumulate, and how they may later be faulted, uplifted, and eroded at the surface. Climate also affects basins and sediments; precipitation, wind, and temperature variation affect surface processes such as erosion and soil formation. In the Salton Trough region of southern California, styles, rates, and environments of basin formation have evolved through time in response to complex changes in driving tectonic forces, fault interactions, and climate change. Because of the rich history of geologic research in Anza- Borrego Desert State Park and adjacent areas, it is impossible to summarize all of the knowledge on this subject in a few pages. This chapter presents a brief overview of existing knowledge about the regional stratigraphy, tectonic evolu- tion, and major sedimentary basins preserved in the Park, which have supported a great diversity of plants and animals during the past ~10 million years. Anza-Borrego Desert State Park is located within a complex zone of strike-slip faulting and oblique crustal extension and compression that defines the tectonically active boundary between the North American plate and the 90 Fossil Treasures of the Anza-Borrego Desert Anza - C05 Stratigraphy 10/28/05 3:22 PM Page 91 Pacific plate in southern California (Figures 5.1, 5.2). The southern San 34˚N Andreas fault system, which SGP includes the San Andreas, San SA FZ S ? . Jacinto, and Elsinore faults, is a J R F Z . l broad zone of past and ongoing S Fig. 2 o al C to seismic activity that separates areas n B S belonging to the Pacific plate (Baja Z EF California and southern California) WSDF 33˚N T ro from areas located on North u . I g ila R America (mainland Mexico and the SD OAZ F h G Y U.S.). Long-term northwesterly M S L Z T S U. movement of the Pacific plate rela- F C S. P A. F Mex tive to North America has resulted LS ico in progressive right-lateral fault dis- C ∞ D D placements and related crustal 32˚N deformation during the past ~25 to E 30 million years, producing a com- A BF plicated network of faults, rotating crustal blocks, mountain ranges, and Gu WB sedimentary basins. The two plates lf o f are also diverging slightly in some 31˚N SF Ca areas, which has caused the Salton li S fo S r Trough and Gulf of California to P n km M ia F open up by oblique rifting and 0 50 100 extension during the past 10 to 15 117˚W 116˚W 115˚W 114˚W million years. These aspects of rela- tive plate motion and the develop- ment of geologic structures on a regional scale are known from landmark stud- Figure 5.1 Faults and Topography of the ies by Atwater (1970), Lonsdale (1989), Stock and Hodges (1989), Powell et al. Northern Gulf of California and (1993), DeMets (1995), Dickinson (1996), Atwater and Stock (1998), Axen and Salton Trough Region. Fletcher (1998), Oskin and Stock (2003), and others. Decorated thicker lines are detachment faults, tick marks on As described below, we now know that regional subsidence related to upper plate; plain lines are high- angle normal and strike-slip crustal extension and transtension (a combination of strike-slip movement and faults. Explanation: ABF, Agua oblique extension of a fault) produced a number of fault-bounded basins that Blanca fault; BSZ, Brawley filled with sediments from Miocene to Pleistocene time (Figures 5.2, 5.3, 5.4). Spreading zone; CDD, Cañada In the recent geologic past (last 1 to 2 million years) many of these basins have David detachment; CPF, Cerro been uplifted and eroded to reveal the diverse stratigraphic record of their tec- Prieto fault; E, Ensenada; IF, tonic, climatic, and paleontologic evolution. In the Salton Sea, fault-controlled Imperial fault; SAFZ, San Andreas fault zone; SD, San subsidence has continued to the present day, accumulating a thick section of Diego; SGP,San Gorgonio Pass; young sediments that are buried in the modern basin beneath the surface. SF, San Felipe; SJFZ, San Jacinto fault zone; SSPMF, Sierra San Pedro Martir fault;T,Tijuana; and WB,Wagner basin. (Shaded- relief map base courtesy of H. Magistrale) Stratigraphy, Tectonics, and Basin Evolution in the Anza-Borrego Desert Region 91 Anza - C05 Stratigraphy 10/28/05 3:22 PM Page 92 Prior Work Our understanding of geological events summarized below is based on decades of research by many scientists. This chapter does not present new data, it is simply an attempt to synthesize a vast body of existing knowledge and make it accessible to a broad audience. Some of the more influential studies of regional stratigraphy, basin evolution, and related structures in the western Salton Trough appear in papers and theses by: Axen and Fletcher (1998), Bartholomew (1968, 1970), Brown et al. (1991), Dean (1988, 1996), Dibblee (1954, 1984, 1996a, 1996b), Dronyk FAULTS ANTICLINE Plutonic and (1977), Feragen (1986), Frost et al. STRIKE-SLIPANTICLINES S SA Met. Rocks an F ta PREHISTORICDETACHMT (1996a, 1996b), Girty and Armitage R THRUST Plio-Quat. os ? SHORELINE 33° a Sediments (1989), Ingle (1974), Johnson et al. M 30' t 6 s Alluvium 9 (1983), Kerr (1982, 1984), Kerr and S CF A Kidwell (1991), Lough (1993, LT O N 1998), Merriam and Brady (1965), B Muffler and Doe (1968), Opdyke et C R C F A S W al. (1977), Quinn and Cronin (1984), E water A L 33° BB E surface Y -234' Remeika (1995), Remeika and 15' SFH Beske-Diehl (1996), Schultejann SF F Quat. (1984), Sharp (1982), Stinson Rhyolite F a J r a xt (1990), Stinson and Gastil (1996), c E Z in O E Ct VF Vallecito Mts Co Shore- F N Tarbet and Holman (1944), Wells F F R E Z line E (1987), Winker (1987), Winker and WP 33° Kidwell (1986, 1996), Woodard Fish SH Mesquite VCFCB Creek F Mts Basin (1963, 1974). This list is only a par- SM TBM F tial sampling of many theses, papers, and abstracts that have contributed to our understanding of this fascinat- 0 10 20 30 km EF IF 32° ing region. Interested readers are 45' 116°30' 116°15' 116° 115°45' 115°30' encouraged to explore the original literature upon which the following Figure 5.2 summary is based. Geologic Map of the Salton Trough Region. Explanation: BB, Borrego Badlands; CF, Clark fault; CCF, Coyote Creek fault; EF, Elsinore fault; ERF, Elmore Ranch fault; EVF, Earthquake Valley fault; VCFCB,Vallecito Creek-Fish Creek basin; IF, Imperial fault; SFF, San Felipe fault; SFH, San Felipe Hills; SHF, Superstition Hills fault; SMF, Superstition Mtn fault;TBM, Tierra Blanca Mts; and WP, Whale Peak. (Map compilation courtesy of L. Seeber) 92 Fossil Treasures of the Anza-Borrego Desert Anza - C05 Stratigraphy 10/28/05 3:22 PM Page 93 Compiled from Winker and Kidwell (1996) terrace gravels, alluvium, etc. PLEIS Qt, Qal ~0.9 Ma (p-mag) Hu. Hueso locally derived streams 2.3±0.4 Ma Ta. Tapiado Fm. (lacustrine) (ash date) Canebrake / Olla / Diablo Palm Spring Group kno w n fro m vertebrate paleo m a t e l y e C g i O l l a F m . ages also approx l o F m Diablo Fm. al (Colorado delta-plain); a n e b r ~ 4.5 k m o f d i p p i n g s t r a t a i a b Olla Fm. (local streams); D. and Canebrake Cgl. Ck (alluvial fans) E N E IC ~4.2 Ma (p-mag) PL O Jackson F ork Mbr. Camels Head Mbr. kno w n fro m micro-paleo ages also approxim ately Im perial Group (muddy tidal flats) Deguynos F m. Yuha Mbr. (delta front with oyster beds) coarsening-up, delta progradation Mud Hills Mbr. (pro-delta marine mudstone) sandy turbidites 5.3 Ma a n i (biostr.) upper megabreccia a t r sandy turbidites ~5.5-6.3 Ma La Fish Creek Gypsum (biostrS p.) l i t M t . lower megabreccia G r o u p ≤1 km Elephant Trees Cgl. 22-14 Ma (K-Ar)E N E IC MO ? Red Rock Fm. Alverson P z , M z Volcanics igneous and metamorphic rocks Figure 5.3 Generalized Stratigraphic Column for the Vallecito Creek-Fish Creek Stratigraphic Section. Paleomagnetic and ash dates are from Opdyke et al. (1977) and Johnson et al. (1983). Biostratigraphic age controls are from studies of Stump (1972), Downs and White (1968), Ingle (1974), Pappajohn (1980), Dean (1988), and McDougall (personal communication, cited in Winker and Kidwell, 1996).
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