skip to main content

Title: Paleoenvironments, taphonomy, and stable isotopic content of the terrestrial, fossil-vertebrate–bearing sequence of the El Disecado Member, El Gallo Formation, Upper Cretaceous, Baja California, México
Abstract The Late Campanian (Late Cretaceous), upper part of the El Disecado Member, El Gallo Formation, Baja California, México, preserves a rich fossil assemblage of microvertebrates and macrovertebrates, silicified logs, macroscopic plant remains, and pollen that was likely deposited as the distal part of a subaerial fan. The unit was episodic and high energy, with its salient features deriving from active river channels and sheet, debris-flow deposits. Landscape stability is indicated by the presence of compound paleosol horizons, containing Fe2O3 mottling in B horizons, cutans, and calcium carbonate concretions. All of these features indicate wet/dry cyclicity in subsurface horizons, likely attributable to such cyclicity in the climate. Drainage was largely to the north and to a lesser extent, the west; however, some current flow to the south and east is preserved which, in conjunction with the proximal location of marginal marine deposits, suggest the influence of tides in this setting. The fossil vertebrates preserved in this part of the El Disecado Member are almost exclusively allochthonous, preserved as disarticulated isolated clasts in hydraulic equivalence in the braided fluvial system. A relatively diverse microvertebrate assemblage is preserved, the largest components of which are first, dinosaurs, and second, turtles. Non-tetrapod fossils are relatively uncommon, perhaps reflecting an absence of permanent standing water in this depositional setting. Here we report a high-precision U-Pb date of 74.706 + 0.028 Ma (2σ internal uncertainty), obtained from zircons in an airfall tuff. The tuff is located low within the sequence studied; therefore, most of the sedimentology and fossils reported here are slightly younger. This date, which improves upon previously published 40Ar/39Ar geochronology, ultimately allows for comparison of these El Gallo faunas and environments with coeval ones globally. Primary stable isotopic nodules associated with roots in the paleosols of the terrestrial portion of the El Disecado Member are compared with ratios from similar sources from coeval northern and eastern localities in North America. Distinctive latitudinal gradients are observed in both δ13C and δ18O, reflecting the unique southern and western, coastal geographic position of this locality. These differences are best explained by differences in the floras that populated the northern and eastern localities, relative to the southern and western floras reported here.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Page Range / eLocation ID:
991 to 1011
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Drillholes represent one of the clearest lines of evidence for predation of benthic invertebrates in the fossil record and are frequently used as a primary proxy for predation intensity in the fossil record. Drillholes are abundant in the late Cretaceous and Cenozoic, but their occurrence is patchy in older deposits of the Mesozoic. The inconsistent record of drillholes in pre-Cretaceous deposits of Mesozoic age are problematic for interpretations of predation-prey dynamics and adaptive radiations, and the role of taphonomy or diagenesis have not been resolved. Here we present drilling percentages for assemblages of well-preserved shelly benthic invertebrates (mainly comprised of bivalves and rare gastropods) from the upper Norian (Upper Triassic) in northern Italy in order to compare these values with reported drilling percentages from the Carnian San Cassiano Formation, a rare Triassic sedimentary unit that has yielded many drilled fossils. The Norian fossil deposits reported here are comparable to those of the San Cassiano in terms of depositional environment, preservation, and region, and can be reasonably compared to the drilling percentage of fossils from the San Cassiano. The sampled deposits are collected from marly limestone horizons in the Argillite di Riva di Solto in the Southern Italian Alps, deposited in the Lombardian Basin, and which are interbedded with shale units containing well-preserved fish and arthropod fossils, enabling a correlation between paleoecological structure of the shelly benthos and the demersal-pelagic predator diversity. Over four hundred bivalve fossils yielded a drilling percentage of 0.24% (1/406), which is typical for fossil assemblages of this age, but the single occurrence of a drillhole in this study is in marked contrast to the many drilled specimens reported from the San Cassiano Formation deposit in Italy. The drilled specimen (with complete drillhole) was an infaunal bivalve and no incomplete drillholes were observed in other specimens. Thus, drilling percentages for the Triassic are consistently low, but present, suggesting that drilling predation was an ecologically minimal influence to benthic communities and unlikely to have driven the significant ecological changes observed in benthic communities during the Late Triassic. Although drilling predation occurred during the Late Triassic, we present an updated database of specialized durophagous predators (including fishes, sharks, and reptiles) that are likely to have been more ecologically influential on benthic communities during the Norian Stage, fishes in particular. 
    more » « less
  2. Abstract

    The James Ross Basin, in the northern Antarctic Peninsula, exposes which is probably the world thickest and most complete Late Cretaceous sedimentary succession of southern high latitudes. Despite its very good exposures and varied and abundant fossil fauna, precise chronological determination of its infill is still lacking. We report results from a magnetostratigraphic study on shelfal sedimentary rocks of the Marambio Group, southeastern James Ross Basin, Antarctica. The succession studied covers a ~1,200 m‐thick stratigraphic interval within the Hamilton Point, Sanctuary Cliffs and Karlsen Cliffs Members of the Snow Hill Island Formation, the Haslum Crag Formation, and the lower López de Bertodano Formation. The basic chronological reference framework is given by ammonite assemblages, which indicate a Late Campanian – Early Maastrichtian age for the studied units. Magnetostratigraphic samples were obtained from five partial sections located on James Ross and Snow Hill islands, the results from which agree partially with this previous biostratigraphical framework. Seven geomagnetic polarity reversals are identified in this work, allowing to identify the Chron C32/C33 boundary in Ammonite Assemblage 8‐1, confirming the Late Campanian age of the Hamilton Point Member. However, the identification of the Chron C32/C31 boundary in Ammonite Assemblage 8‐2 assigns the base of the Sanctuary Cliffs Member to the early Maastrichtian, which differs from the Late Campanian age previously assigned by ammonite biostratigraphy. This magnetostratigraphy spans ~14 Ma of sedimentary succession and together with previous partial magnetostratigraphies on Early‐Mid Campanian and Middle Maastrichtian to Danian columns permits a complete and continuous record of the Late Cretaceous distal deposits of the James Ross Basin. This provides the required chronological resolution to solve the intra‐basin and global correlation problems of the Late Cretaceous in the Southern Hemisphere in general and in the Weddellian province in particular, given by endemism and diachronic extinctions on invertebrate fossils, including ammonites. The new chronostratigraphic scheme allowed us to calculate sediment accumulation rates for almost the entire Late Cretaceous infill of the distal James Ross Basin (the Marambio Group), showing a monotonous accumulation for more than 8 Myr during the upper Campanian and a dramatic increase during the early Maastrichtian, controlled by tectonic and/or eustatic causes.

    more » « less
  3. Haasgat is a primate-rich fossil locality in the northeastern part of the Fossil Hominid Sites of South Africa UNESCO World Heritage Site. Here we report the first hominin identified from Haasgat, a partial maxillary molar (HGT 500), that was recovered from anex situcalcified sediment block sampled from the locality. Thein situfossil bearing deposits of the Haasgat paleokarstic deposits are estimated to date to slightly older than 1.95 Ma based on magnetobiostratigraphy. This places the hominin specimen at a critical time period in South Africa that marks the last occurrence ofAustralopithecusaround 1.98 Ma and the first evidence ofParanthropusandHomoin the region between ∼2.0 and 1.8 Ma. A comprehensive morphological evaluation of the Haasgat hominin molar was conducted against the current South African catalogue of hominin dental remains and imaging analyses using micro-CT, electron and confocal microscopy. The preserved occlusal morphology is most similar toAustralopithecus africanusor earlyHomospecimens but different fromParanthropus. Occlusal linear enamel thickness measured from micro-CT scans provides an average of ∼2.0 mm consistent withAustralopithecusand earlyHomo. Analysis of the enamel microstructure suggests an estimated periodicity of 7–9 days. Hunter–Schreger bands appear long and straight as in someParanthropus, but contrast with this genus in the short shape of the striae of Retzius. Taken together, these data suggests that the maxillary fragment recovered from Haasgat best fits within theAustralopithecus—earlyHomohypodigms to the exclusion of the genusParanthropus. At ∼1.95 Ma this specimen would either represent another example of late occurringAustralopithecusor one of the earliest examples ofHomoin the region. While the identification of this first hominin specimen from Haasgat is not unexpected given the composition of other South African penecontemporaneous site deposits, it represents one of the few hominin localities in the topographically-distinct northern World Heritage Site. When coupled with the substantial differences in the mammalian faunal communities between the northern localities (e.g., Haasgat, Gondolin) and well-sampled Bloubank Valley sites (e.g., Sterkfontein, Swartkrans, Kromdraai), the recovery of the HGT 500 specimen highlights the potential for further research at the Haasgat locality for understanding the distribution and interactions of hominin populations across the landscape, ecosystems and fossil mammalian communities of early Pleistocene South Africa. Such contextual data from sites like Haasgat is critical for understanding the transition in hominin representation at ∼2 Ma sites in the region fromAustralopithecustoParanthropusand earlyHomo.

    more » « less
  4. The Huolinhe Formation in the Huolinhe Basin, eastern Inner Mongolia is one of the most important Lower Cretaceous coal-bearing strata in China, yielding abundant, diverse, and well-preserved plant fossils. Its precise age, however, is poorly known due to lack of associated marine deposits and volcanic beds. Here we present U-Pb zircon ages, and the associated palynological assemblages of an ash layer of the Huolinhe Formation recently discovered at the Zhahanaoer open-cast coal mine in Jarud Banner. Stratigraphic analyses based on boreholes suggest that the ash layer occurs near the bottom of the “lower coal-bearing member” of the Huolinhe Formation. U-Pb zircon geochronology using the SIMS method constrains the depositional age of the ash layer to be 125.6 1.0 Ma (late Barremian–earliest Aptian), and this is consistent with the result from LA-ICP-MS analyses of the same sample. A late Barremian–earliest Aptian age for the ash layer is also supported by the palynological assemblage associated with the layer, in which the pollen of gymnosperms and the spores of ferns and bryophytes are dominant, angiosperm pollen is very rare and represented by only Clavatipollenites. This study contributes important new data for understanding the age of the entire Huolinhe Formation and also provides a more precise maximum age for the key plant fossils preserved in the deposits above the ash layer. 
    more » « less
  5. Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 40Ar/ 39Ar sanidine age data. In contrast to the dominantly southerly or southeasterly paleoflow exhibited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast-flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin. In the eastern and northern parts of the San Juan Basin, paleoflow was generally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ∼15 km south of Dulce, New Mexico. There, an ∼130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 40Ar/ 39Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin. Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin. 
    more » « less