ALBERT

Description: Accurately dating Holocene volcanic rocks poses many challenges but is critical to assessing magmatic evolution and hazard risks at highly active volcanoes. Here we use Ra/Th and 40 Ar/ 39 Ar geochronology to date very young eruptions at Changbaishan volcano, northeastern China, a recently active stratovolcano responsible for one of the most voluminous eruptions in the past ~2000 yr. For Holocene eruptions, 40 Ar/ 39 Ar ages are consistently older than those of both independently determined ages and maximum Ra/Th ages. Overall, Ra/Th ages are most consistent with historical accounts and indicate inaccurate 40 Ar/ 39 Ar ages that are due to extraneous argon in various forms. Ra/Th geochronology also confirms the highly active nature of Changbaishan and supports the continued presence of trachytic magma residing under the volcano that appeared more than ~1100 yr ago.

Description: Essential features of the previously named and described Miocene Crooked Ridge River in northeastern Arizona (USA) are reexamined using new geologic and geochronologic data. Previously it was proposed that Cenozoic alluvium at Crooked Ridge and southern White Mesa was pre–early Miocene, the product of a large, vigorous late Paleogene river draining the 35–23 Ma San Juan Mountains volcanic field of southwestern Colorado. The paleoriver probably breeched the Kaibab uplift and was considered important in the early evolution of the Colorado River and Grand Canyon. In this paper, we reexamine the character and age of these Cenozoic deposits. The alluvial record originally used to propose the hypothetical paleoriver is best exposed on White Mesa, providing the informal name White Mesa alluvium. The alluvium is 20–50 m thick and is in the bedrock-bound White Mesa paleovalley system, which comprises 5 tributary paleochannels. Gravel composition, detrital zircon data, and paleochannel orientation indicate that sediment originated mainly from local Cretaceous bedrock north, northeast, and south of White Mesa. Sedimentologic and fossil evidence imply alluviation in a low-energy suspended sediment fluvial system with abundant fine-grained overbank deposits, indicating a local channel system rather than a vigorous braided river with distant headwaters. The alluvium contains exotic gravel clasts of Proterozoic basement and rare Oligocene volcanic clasts as well as Oligocene–Miocene detrital sanidine related to multiple caldera eruptions of the San Juan Mountains and elsewhere. These exotic clasts and sanidine likely came from ancient rivers draining the San Juan Mountains. However, in this paper we show that the White Mesa alluvium is early Pleistocene (ca. 2 Ma) rather than pre–early Miocene. Combined 40 Ar/ 39 Ar dating of an interbedded tuff and detrital sanidine ages show that the basal White Mesa alluvium was deposited at 1.993 ± 0.002 Ma, consistent with a detrital sanidine maximum depositional age of 2.02 ± 0.02 Ma. Geomorphic relations show that the White Mesa alluvium is older than inset gravels that are interbedded with 1.2–0.8 Ma Bishop–Glass Mountain tuff. The new ca. 2 Ma age for the White Mesa alluvium refutes the hypothesis of a large regional Miocene(?) Crooked Ridge paleoriver that predated carving of the Grand Canyon. Instead, White Mesa paleodrainage was the northernmost extension of the ancestral Little Colorado River drainage basin. This finding is important for understanding Colorado River evolution because it provides a datum for quantifying rapid post–2 Ma regional denudation of the Grand Canyon region.

Description: Thick late Miocene nonmarine evaporite (mainly halite and gypsum) and related lacustrine limestone deposits compose the upper basin fill in half grabens within the Lake Mead region of the Basin and Range Province directly west of the Colorado Plateau in southern Nevada and northwestern Arizona. Regional relations and geochronologic data indicate that these deposits are late synextensional to postextensional (ca. 12–5 Ma), with major extension bracketed between ca. 16 and 9 Ma and the abrupt western margin of the Colorado Plateau established by ca. 9 Ma. Significant accommodation space in the half grabens allowed for deposition of late Miocene lacustrine and evaporite sediments. Concurrently, waning extension promoted integration of initially isolated basins, progressive enlargement of drainage nets, and development of broad, low gradient plains and shallow water bodies with extensive clastic, carbonate, and/or evaporite sedimentation. The continued subsidence of basins under restricted conditions also allowed for the preservation of particularly thick, localized evaporite sequences prior to development of the through-going Colorado River. The spatial and temporal patterns of deposition indicate increasing amounts of freshwater input during the late Miocene (ca. 12–6 Ma) immediately preceding arrival of the Colorado River between ca. 5.6 and 4.9 Ma. In axial basins along and proximal to the present course of the Colorado River, evaporite deposition (mainly gypsum) transitioned to lacustrine limestone progressively from east to west, beginning ca. 12–11 Ma in the Grand Wash Trough in the east and shortly after ca. 5.6 Ma in the western Lake Mead region. In several satellite basins to both the north and south of the axial basins, evaporite deposition was more extensive, with thick halite (〉200 m to 2.5 km thick) accumulating in the Hualapai, Overton Arm, and northern Detrital basins. Gravity and magnetic lows suggest that thick halite may also lie within the northern Grand Wash, Mesquite, southern Detrital, and northeastern Las Vegas basins. New tephrochronologic data indicate that the upper part of the halite in the Hualapai basin is ca. 5.6 Ma, with rates of deposition of ~190–450 m/m.y., assuming that deposition ceased approximately coincidental with the arrival of the Colorado River. A 2.5-km-thick halite sequence in the Hualapai basin may have accumulated in ~5–7 m.y. or ca. 12–5 Ma, which coincides with lacustrine limestone deposition near the present course of the Colorado River in the region. The distribution and similar age of the limestone and evaporite deposits in the region suggest a system of late Miocene axial lakes and extensive continental playas and salt pans. The playas and salt pans were probably fed by both groundwater discharge and evaporation from shallow lakes, as evidenced by sedimentary textures. The elevated terrain of the Colorado Plateau was likely a major source of water that fed the lakes and playas. The physical relationships in the Lake Mead region suggest that thick nonmarine evaporites are more likely to be late synextensional and accumulate in basins with relatively large catchments proximal to developing river systems or broad elevated terranes. Other basins adjacent to the lower Colorado River downstream of Lake Mead, such as the Dutch Flat, Blythe-McCoy, and Yuma basins, may also contain thick halite deposits.

Description: New single-crystal sanidine 40 Ar/ 39 Ar dates of 12 middle Eocene to Oligocene rhyolitic volcanic ash beds of the Texas coastal plains range from 30.64 ± 0.03 Ma (lower Catahoula Formation) to 41.841 ± 0.016 Ma (lower Crockett Formation). These dates are from Texas coastal plains strata in the Crockett marine transgression and the Yegua and Jackson depositional wedges. Radiometric dating of sanidine and electron microprobe analysis of volcanic apatite phenocrysts validate previous regional correlation of the Upper Alabama Ferry volcanic ash from Brazos County to Houston County, Texas, and provide support for correlation of this bed with the St. Johns bentonite deposit in Louisiana. Middle Eocene ash beds are distinguished by differences in apatite phenocryst composition. The wide distribution of the thick Alabama Ferry volcanic ashes points to an early age (41.841 ± 0.016 Ma) for the start of major explosive rhyolitic volcanism in western North America. Volcanic ash bed samples have subalkaline rhyolite composition with geochemical characteristics consistent with derivation from volcanic arc rocks. Trace-element data show a temporal change in volcanic ashes: Middle Eocene ashes are depleted in heavy rare earth elements compared to late Eocene and Oligocene units. This change in ash composition coincides with the time of transition from subduction compression to arc-rifting on the western margin of Mexico, and compositional data are consistent with the Sierra Madre Occidental being the source for the volcanic ash. Volcanic units of the eastern portion of the Trans-Pecos of Texas and Mogollon-Datil area of New Mexico overlap temporally, but geochemical characteristics and K/Ca ratios in sanidine indicate they are geochemically unlikely to be sources for the Texas coastal plains middle Eocene to Oligocene volcanic ash beds.