Revised large mammal biostratigraphy and biochronology of the Barstow Formation (Middle Miocene), California

This paper presents a new means of interpreting the distribution of taxa among taxonomic lists. Traditionally, “similarity” indices have been used to compare lists, and “association” measures have been used to compare taxonomic distributions. It is argued that when sampling regimes are poorly understood, all similarity and association indices are unjustifiable. However, observations that taxa have overlapping (conjunct) or nonoverlapping (disjunct) distributions are universally meaningful. Because greater sampling can only increase the number of known conjunctions and because long lists serve as conjunctional Rosetta Stones, conjunction data sets can be far more reliable than the lists that generate them. One way to account for patterns of conjunction and disjunction is to create theoretical spaces composed of two distributional boundaries or edges for each taxon in each dimension. A parsimonious arrangement of edges implies distributions that always overlap when taxa are conjunct and infrequently overlap when taxa are disjunct. A procedure involving correspondence analysis is shown to minimize the number of implied disjunctions. The method is used to analyze a set of 271 Miocene large-mammal genus lists from the Great Plains region. Biostratigraphic and radiometric data demonstrate that the “best” arrangement of distributional edges is temporal and corresponds to an age-range chart. The edge sequence is calibrated to the radiometric time scale and used to compute a detailed genus-level diversity curve and to redefine the boundaries between Miocene Land-Mammal Ages and Subages in the Great Plains. Each of the Miocene Ages can be divided into two or three Subages. Diversity levels are apparently low throughout the Early Miocene and rise sharply during the Middle Miocene. Significant drops in diversity occurred at about 17.4, 11.6, 8.7, and 6.5 Ma. The base of the Barstovian corresponds to the base of the Middle Miocene, and the Clarendonian straddles the Middle-Late Miocene boundary.

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THE INFLUENCE OF DEPOSITIONAL ENVIRONMENT AND BASIN HISTORY ON THE TAPHONOMY OF MAMMALIAN ASSEMBLAGES FROM THE BARSTOW FORMATION (MIDDLE MIOCENE), CALIFORNIA

Palaios ◽

10.2110/palo.2019.067 ◽

2020 ◽

Vol 35 (4) ◽

pp. 175-190

Author(s):

KATHARINE M. LOUGHNEY ◽

CATHERINE BADGLEY

Keyword(s):

Middle Miocene ◽

Element Composition ◽

Depositional Environments ◽

Skeletal Element ◽

Ephemeral Wetland ◽

Facies Associations ◽

History Of ◽

Fossil Assemblages ◽

Depositional Setting ◽

Barstow Formation

ABSTRACT The Barstow Formation in the Mojave region of California was deposited in an extensional-basin setting of the Basin and Range province and preserves diverse middle Miocene mammalian assemblages. Six facies associations represent the dominant depositional environments in the basin, which changed through time from alluvial-fan and playa-dominated settings to floodplains and spring-fed wetlands. The majority of fossil localities and specimens occur in later-forming facies associations. We analyzed the taphonomic characteristics of fossil assemblages to test whether basin-scale facies associations or locality-scale facies exert more control on the preservational features of mammalian assemblages through the formation. We documented the facies settings of 47 vertebrate localities in the field in order to interpret depositional setting and the mode of accumulation for fossil assemblages. We evaluated skeletal material in museum collections for taphonomic indicators, including weathering stage, original bone-damage patterns, hydraulic equivalence, and skeletal-element composition. We evaluated four alternative modes of accumulation, including attritional accumulation on the land surface, accumulation by fluvial processes, carnivore or scavenger accumulations, and mass-death events. The majority of localities represent attritional accumulations at sites of long-term mortality in channel-margin, abandoned-channel, poorly drained floodplain, and ephemeral-wetland settings. Skeletal-element composition and taphonomic characteristics varied among facies, indicating an important role for depositional setting and landscape position on fossil-assemblage preservation. We find that locality-scale facies have a greater influence on the taphonomic characteristics of fossil assemblages; the taphonomy of each facies association is influenced by the facies that compose it. The facies composition and distribution within facies associations change through the formation, with a greater variety of depositional settings forming later in the history of the basin. Heterogeneous landscapes present more settings for fossil accumulation, contributing to the increase in fossil occurrence through the depositional history of the formation.

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Petrographic and trace element analysis of uranium-rich tufa calcite, middle Miocene Barstow Formation, California, USA

Sedimentology ◽

10.1111/j.1365-3091.2004.00631.x ◽

2004 ◽

Vol 51 (3) ◽

pp. 433-453 ◽

Cited By ~ 19

Author(s):

Jennifer M. Cole ◽

E. Troy Rasbury ◽

Isabel P. Montañez ◽

Vicki A. Pedone ◽

Antonio Lanzirotti ◽

...

Keyword(s):

Trace Element ◽

Middle Miocene ◽

Trace Element Analysis ◽

Element Analysis ◽

Barstow Formation

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Vegetation and habitat change in southern California through the Middle Miocene Climatic Optimum: Paleoenvironmental records from the Barstow Formation, Mojave Desert, USA

Geological Society of America Bulletin ◽

10.1130/b35061.1 ◽

2019 ◽

Vol 132 (1-2) ◽

pp. 113-129 ◽

Cited By ~ 2

Author(s):

Katharine M. Loughney ◽

Michael T. Hren ◽

Selena Y. Smith ◽

Janice L. Pappas

Keyword(s):

Southern California ◽

Middle Miocene ◽

Climatic Variability ◽

Tectonic Activity ◽

High Pco2 ◽

Vegetation Composition ◽

Isotopic Analyses ◽

Precipitation Regimes ◽

Climatic Optimum ◽

Barstow Formation

Abstract The Barstow Formation (ca. 19–13 Ma) of southern California, USA, is a terrestrial sequence known for its diverse Miocene mammalian faunas. The formation spans the Middle Miocene Climatic Optimum (MMCO; 17–14 Ma) and offers the opportunity to study environmental change during the last major interval of global warming of the Cenozoic. We combined isotopic analyses of carbon and hydrogen from sedimentary n-alkanes and bulk soil organic matter with analysis of phytoliths (plant silica) and diatoms to reconstruct vegetation composition, habitat structure, and moisture dynamics through the formation. The δ13C and δD of long-carbon-chain n-alkanes derived from terrestrial plants and preserved in sediments of the Barstow Formation record drying prior to the MMCO and through the Middle Miocene Climatic Transition (MMCT; beginning ca. 14 Ma), punctuated by increased moisture in fluvial environments during the peak of the MMCO. Variable isotopic results relate to changes in moisture, driven partly by facies, vegetation composition, tectonic activity, and climatic variability coinciding with intervals of high pCO2. Phytolith assemblages are dominated by forest indicators in riparian habitats during the MMCO, and grass morphotypes constitute significant components of phytolith assemblages after the MMCO, indicating a shift to drier, more open-canopy habitats. The establishment of dry, wooded grasslands in the Barstow Basin coincides with the beginning of the MMCT and cooling sea-surface temperatures in the Pacific Ocean. Our results indicate that moist, closed-canopy habitats formed in southern California during climatic warming, followed by savannah during climatic cooling and the shift to seasonal precipitation regimes.

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