Theodore J. Fremd

Oxygen isotope evidence for progressive uplift of the Cascade Range, Oregon

Oxygen isotope compositions of fossil equid teeth in the Cascade rainshadow reveal a V5x decrease in mean N 18 O since 27 Ma. Isotopic changes are inconsistent with expected effects from global climate change because: (a) the expected isotopic shift to tooth N 18 O values due to global climate change (V1x) is much smaller than the observed shift, (b) predicted and observed isotopic trends are opposite for Oligocene vs. Miocene samples, and (c) average compositions and ranges in compositions remained unchanged for samples from before and after major global cooling in the mid-Miocene. Accounting for a decrease in relative humidity of at least 15%, we infer a topographically driven secular shift in the N 18 O value of rainwater of 6^8x since the late Oligocene, which is approximately equivalent to the modern-day difference in N 18 O values of precipitation and surface waters across the central Cascades. Rise of the central Cascades apparently occurred monotonically over the last 27 Ma, with a hiatus between V15.4 and 7.2 Ma, possibly related to eruption of the Columbia River Basalts. Progressive volcanic accumulation over tens of millions of years best explains the data, rather than a short-lived uplift event. Paleoseasonality, as inferred from isotope zoning and intertooth variability, decreased dramatically from 7^9x at 15.4^7 Ma to V3x at 3 Ma, then increased to 6^ 8x today. The cause of the decrease in seasonality at 3 Ma may reflect either brief warming during the mid-Pliocene within the context of global tectonic reorganization, or consumption by equids of water from an isotopically buffered Lake Idaho. < 2002 Elsevier Science B.V. All rights reserved.

Eocene and Oligocene Paleosols of Central Oregon

Scenic red color-banded claystones of the Clarno and Painted Hills areas of central Oregon are successions of fossil soils that preserve a record of Eocene-Oligocene paleoclimatic change. Conglomerates of the middle Eocene Clarno Formation near Clarno contain largely weakly developed paleosols compatible with an environment of volcanic lahars around a large stratovolcano. Deeply weathered paleosols (Ultisols) around a volcanic dome and overlying these conglomerates indicate a climate that was subtropical (mean annual temperature or MAT 23-25° C) and humid (mean annual precipitation or MAP of 900-2,000 mm). Comparable paleoclimates are indicated by fossil floras from the conglomerates, which show diversity and adaptive features similar to modern vegetation of Volcan San Martin, Mexico. An erosional disconformity in the Clarno area separates these older beds from less deeply weathered red paleosols (Alfisols) in the middle Eocene upper Clarno Formation. The change in paleosols may represent a decline in both temperature (MAT 19-23° C) and rainfall (MAP 900-1,350 mm), with dry seasons. Strongly developed lateritic paleosols (Oxisols and Ultisols) in the uppermost Clarno and lowermost John Day Formations in the Painted Hills record return to more humid conditions during the late Eocene. These paleosols are similar to soils of southern Mexico and Central America in climates that are subtropical (MAT 23-25° C) and humid (MAP 900-2,000 mm). Kaolinitic and iron-rich, red paleosols (Ultisols) of the lower Big Basin Member of the John Day Formation near Clarno and the Painted Hills are erosionally truncated and abruptly overlain by smectitic and tuffaceous paleosols (Inceptisols and Alfisols) of the middle Big Basin Member. This truncation surface can be correlated with the local Eocene-Oligocene boundary. Paleosols of the middle Big Basin Member are most like those of the Central Transmexican Volcanic Belt and indicate an early Oligocene paleoclimate appreciably cooler (MAT 16-18° C) and drier (MAP 600-1,200 mm) than during the late Eocene. Root traces and clay accumulations in the paleosols indicate forest vegetation, also evident from fossil leaves of the lake-margin Bridge Creek flora. The mid-Oligocene upper Big Basin Member of the John Day Formation includes distinctive brown as well as red paleosols (Alfisols). Its paleosols indicate a paleoclimate drier (MAP 500-700 mm) than before, and more grasses in the forest understory. Another erosional truncation marks the base of the late Oligocene (early Arikareean), olive-brown lower Turtle Cove Member of the John Day Formation. Calcareous paleosols with near-granular soil structure (Inceptisols and Aridisols) indicate an even drier (MAP 400-600 mm) climate, more open grassy woodland vegetation than previously, and local wooded grassland of seasonally wet bottomlands. The Clarno-John Day sequence preserves a long-term paleoclimatic record that complements the geological record of global change from deep sea cores and fossil plants. Similarly, it reveals stepwise climatic cooling and drying, with a particularly dramatic climatic deterioration at the Eocene-Oligocene boundary.

Miocene Tectonics and Climate Forcing of Biodiversity, Western United States

Ungulate and carnivore diversity patterns since 30 Ma in different regions of the western United States suggest abrupt increased diversification at 17–17.5 Ma, followed by decreases ca. 11 Ma, and stasis thereafter. Although global climate change presumably affects evolution, we hypothesize that widespread extensional tectonism in the western U.S. also helped drive diversity increases ca. 17.5 Ma through a topographically induced increase in floral and habitat diversity. The decreases in diversities ca. 11 Ma, as well as the rapid increase in C4 ecosystems (RICE) worldwide at 7–8 Ma may have responded to climate teleconnections and increased seasonality linked to global cooling and growth of orogenic plateaus, particularly the Tibetan Plateau between 13 and 8 Ma. Thus, biodiversity complexly responds both to climate and to tectonics.

Revised Chronostratigraphy and Biostratigraphy of the John Day Formation (Turtle Cove and Kimberly Members), Oregon, with Implications for Updated Calibration of the Arikareean North American Land Mammal Age

Although the Arikareean North American land mammal age was first typified in the Great Plains, the succession there contains significant unconformities, a generally poor magnetic record, relatively sparse radioisotopic calibration, and a major faunal hiatus. In the John Day Valley of central Oregon, however, is a thick, remarkably complete sequence of Oligocene through early Miocene strata (the John Day Formation) potentially amenable to addressing these shortcomings and long known to harbor one of the richest records of mid‐Tertiary mammals in North America. Since Prothero and Rensberger’s first magnetostratigraphic study of the John Day Formation in 1985, new advances in geochronology, together with a more comprehensive suite of paleomagnetic sections keyed to new radioisotopic and biostratigraphic data, have greatly enhanced chronostratigraphic precision. In our attempt to refine John Day chronostratigraphy, we sampled nearly 300 sites for magnetostratigraphy over a 500‐m‐thick interval and used several radioisotopically dated volcanic tuffs for our correlation with the geomagnetic polarity timescale. Many of the rocks analyzed showed unusual magnetic behavior, possibly due to the known zeolitization in this region, thereby precluding an abundance of class 1 polarity determinations. Nevertheless, preliminary results indicate that the Turtle Cove Member stratigraphically upward through the lower Kimberly Member extends from late chron C12n through C7n.1r, or from about 30.6 to 24.1 Ma. Intensive radioisotopic and magnetostratigraphic characterization of these strata provides a framework by which the associated biostratigraphy is assessed for biochronological significance relative to fossiliferous successions of the Great Plains, in turn resulting in reassessment of Arikareean subbiochron (Ar1–Ar4) boundaries. We present a revision of those boundaries that differs from their traditional timing as a hypothesis for testing in other locations.

Glacial-interglacial–scale paleoclimatic change without large ice sheets in the Oligocene of central Oregon

Abundant late Oligocene paleosols in eastern Oregon compose a paleoclimatic archive rivaling the resolution of deep-sea cores, recording 105 Milankovitch-scale cycles over the 5.1 m.y. duration of the middle John Day Formation. Paleoclimatic cycles are apparent from the fossil record of snails, mammals, trace fossils, soil structure, depth to calcic horizon of paleosols, and carbon and oxygen isotopic composition of pedogenic carbonate. Interpreted Oligocene alternation between semiarid sagebrush steppe and subhumid wooded grassland has the same amplitude as that inferred during accumulation of the Quaternary Palouse Loess in Washington and Oregon. This similar amplitude is surprising because large ice caps like those of the Quaternary did not extend across North America or Europe during the Oligocene. Thus ice-albedo amplification of Milankovitch-scale insolation variation cannot explain the similar magnitude of Oligocene paleoclimatic fluctuation. Weak orbital signals were more likely amplified by greenhouse gases such as CO 2 and CH 4 due to changing carbon budgets in the sea and on land.

Stratigraphy, chronology, biogeography, and taxonomy of early Miocene small chalicotheres in North America

Abstract In the late 19th century, rare fossil remains of a small chalicothere were recovered from early Miocene rocks of the John Day Formation of north-central Oregon. Totaling eight isolated teeth and four foot bones, and gathered by various collectors from several localities, these specimens were not at first recognized as chalicotheres, and were originally assigned to a dental species, Lophiodon? oregonensis, and a postcranial species, Moropus distans. All were assigned to the chalicothere genus Moropus by Holland and Peterson in 1914. The lack of precise geographic and stratigraphic data did not permit any definitive assessment of age, origin, or evolutionary stage. We report here newly discovered teeth and foot bones of M. oregonensis found from 1994–1998 at precisely located levels in the John Day Formation. These new discoveries indicate that M. oregonensis occurs in upper John Day units but is absent from the lower part of the formation. One tooth occurs in proximity to a tuff dated at 22.6 ± 0.13 Ma, hence establishes a minimum age for the appearance of the species in North America. New dental and foot elements allow us to synonymize Moropus distans with M. oregonensis, now the type species of the genus Moropus. Additionally, a small chalicothere, probably M. oregonensis, occurs at four Arikareean Gulf Coast sites in north Florida and east Texas. It is suggested that these animals, along with the Oregon chalicotheres, represent relatively primitive populations inhabiting mesic coastal environments of the Pacific margin and southern United States in the early Miocene.