David B. Loope

A 10,000 year record of dune activity；dust storms；and severe drought in the central Great Plains

Dune fi elds and loess deposits of the Great Plains of North America contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought. We present a 10,000 yr reconstruction of dune activity and dust production in the central Great Plains region, based on 95 optically stimulated luminescence ages. The integration of data from both eolian sand and loess is an important new aspect of this record. Clusters of ages defi ne episodes of extensive eolian activity, which we interpret as a response to frequent severe drought, at 1.0‐0.7 ka and 2.3‐4.5 ka (with peaks centered on 2.5 and 3.8 ka); sustained eolian activity occurred from 9.6 to 6.5 ka. Parts of this record may be consistent with hypotheses linking Holocene drought to sea surface temperature anomalies in the Pacifi c or Atlantic oceans, or to the El Nino-Southern Oscillation phenomenon, but the record as a whole is diffi cult to reconcile with any of these hypotheses.

Dune-dammed paleovalleys of the Nebraska Sand Hills: Intrinsic versus climatic controls on the accumulation of lake and marsh sediments

Although running water is the dominant geomorphic agent on Earth, eolian processes can gain ascendancy in regions where the climate is arid, vegetation is sparse, and abundant sand is available for transport. With climate change, the boundaries between fluvial-dominated and eolian-dominated areas may shift. Although there have been few reports in the North American literature of river systems blocked by dune sand, our work in the Nebraska Sand Hills provides evidence of multiple episodes of such blockage events. During prolonged arid intervals in latest Pleistocene and middle Holocene time, eolian dune sand blocked two large valley systems in western Nebraska. These blockages raised the water table of the High Plains aquifer as much as 25 m over an area of 7000 km2 and created over one thousand lakes. Wetlands far removed from the discharge points of the buried paleovalley system are strongly alkaline (exceeding 250 000 mg/L total dissolved solids [TDS]). Relatively fresh (280 mg/L TDS), flow-through lakes are present at the distal end of the system where the gradient of the water table is steep and the cross section of the buried valley is large. Anomalously thick marsh and lake sediments accumulated in deep paleovalleys upstream of dune dams near the southern margin of the Sand Hills. Our cores and radiocarbon dates from Blue and Crescent Lakes reveal their histories to be quite distinct from adjacent Swan Lake; these differences are best explained by multiple blockage events. Our work explains why lakes are most abundant in the driest part of the Sand Hills. It also provides another line of evidence for major dune activity in the Sand Hills region during Holocene time and shows that factors other than regional climate, specifically location, height, and hydraulic conductivity of dune dams, can control the rise and fall of the ground-water table and the chemistry of lakes.

Life and death in a Late Cretaceous dune field, Nemegt basin, Mongolia

For more than 70 years, red sandstones of the Gobi Desert have yielded abundant articulated skeletons of Late Cretaceous dinosaurs, lizards, and mammals. At Ukhaa Tolgod, structureless sandstones are the only fossiliferous facies, and we present new evidence for deposition on dune-sand–sourced alluvial fans that were built at the margins of stabilized eolian bedforms during mesic climatic episodes. In laterally and vertically adjacent large-scale eolian cross-strata in which skeletons are absent, we have found abundant tracks of dinosaurs that walked on sparsely vegetated dunes that were active under xeric conditions. Our study of calcareous concretions in vaguely bedded eolian sandstones suggests that bedding was nearly destroyed by burrowing invertebrates and trampling dinosaurs. The accumulation of illuvial clays and pedogenic calcite in these sediments reduced infiltration of rainwater and, with attendant climatic change and heavy rainfall events, led to fan development.

Episodic deposition and preservation of eolian sands: A late Paleozoic example from southeastern Utah

The Upper Pennsylvanian and Lower Permian stratigraphic sequence in Canyonlands National Park contains about 40 flat-topped eolian sand bodies. Rhizoliths, burrows, and traces of former evaporites abound at the tops of these tabular genetic units. Upper surfaces of units were created by eolian deflation to the level of the groundwater table. Analysis of cross-stratification reveals that sands accumulated via migration and climb of relatively small dunes. Brief episodes of sand accumulation were triggered by regression and were terminated when upwind sand supplies were depleted. Upper surfaces of genetic units are diastems and represent much more geologic time than the rocks themselves.

Annual monsoon rains recorded by Jurassic dunes

Pangaea, the largest landmass in the Earths history, was nearly bisected by the Equator during the late Palaeozoic and early Mesozoic eras. Modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme seasonality, but direct evidence for annual rainfall periodicity has been lacking. In the Mesozoic era, about 190 million years ago, thick deposits of wind-blown sand accumulated in dunes of a vast, low-latitude desert at Pangaeas western margin. These deposits are now situated in the southwestern USA. Here we analyse slump masses in the annual depositional cycles within these deposits, which have been described for some outcrops of the Navajo Sandstone. Twenty-four slumps, which were generated by heavy rainfall, appear within one interval representing 36 years of dune migration. We interpret the positions of 20 of these masses to indicate slumping during summer monsoon rains, with the other four having been the result of winter storms. The slumped lee faces of these Jurassic dunes therefore represent a prehistoric record of yearly rain events.

Long‐Lived Pluvial Episodes during Deposition of the Navajo Sandstone

The Navajo Sandstone of the American Southwest was deposited at approximately 190 Ma in a giant, subtropical dune field near the western margin of Pangea. From this unit, we report thick intervals of dune cross‐strata that were churned by insects and trampled by reptiles. Although dunes continued to migrate freely, the distribution of trace fossils shows that plant life in wet interdune areas sustained high levels of animal activity on the dunes for many thousands of years. We interpret this suite of structures as the record of a pluvial episode climatologically similar to the period of “greening” in the Sahara 4000–10,000 yr ago. A high percentage of the rainfall on the Navajo erg recharged the water table and led to the development of highly dilute, local groundwater flow systems that discharged into interdune areas.

Rhizoliths in ancient eolianites

Abstract Rhizoliths are abundant within Permo-Pennsylvanian eolian rocks of southeastern Utah. They are also present at the tops of several eolian sand bodies within the Pennsylvanian Fountain Formation of southeastern Wyoming and at a single stratigraphic level within the Jurassic Navajo Sandstone of southeastern Utah. Ranging in diameter from less than 1 mm to 15 cm, these cylindrical structures are composed of siliciclastic-free micrite and calcite spar. Cylinders ramify throughout the uppermost 2–3 m of thin, flat-topped sand bodies. Large cylinders are compound rhizoliths and display alveolar fabric, a feature resulting from rootlet calcification that is characteristic of modern and ancient calcretes. Development of rhizoliths required flat, geomorphically stable substrates for plant growth, highly permeable surficial sediments, and sources of calcium ions. The restriction of rhizoliths to planar deflation surfaces suggests that plants were unable to colonize or stabilize eolian bedforms. Paleozoic rhizoliths appear to be restricted to coastal areas with onshore winds. Associated evaporites suggest an arid to semi-arid paleoclimate. Rhizoliths formed when carbonate allochems derived from an upwind epeiric sea were dissolved and calcite reprecipitated around plant roots. Quaternary rhizoliths are best developed in similar sedimentologic and geographic settings. The abundance of rhizoliths and carbonate allochems in Quaternary and late Paleozoic low-latitude coastal dunes suggests that they are genetically related to glacio-eustatic sea-level fluctuations.

Lethal Sandslides from Eolian Dunes

Fossil vertebrates entombed within the Upper Cretaceous Djadokhta Formation of southern Mongolia bear testimony to a heretofore unknown geologic phenomenon: mass wasting of eolian dunes during heavy rainstorms. Evaporation of shallow‐penetrating rainwater led to progressive calcite accumulation in a thin layer of sand about 0.5 m below the surface of dune lee slopes. During rare heavy rainstorms, a perched water table developed at the top of calcitic zones. Positive pore water pressure led to translational slides and fast‐moving sediment gravity flows that overwhelmed animals on the lee slopes of large dunes and in interdune areas.

New Stratigraphic Subdivision, Depositional Environment, and Age Estimate for the Upper Cretaceous Djadokhta Formation, Southern Ulan Nur Basin, Mongolia

Abstract Studies of key and newly discovered sections of the Upper Cretaceous Djadokhta Formation along the southern margin of the Ulan Nur Basin allow a new subdivision based on lithology. The formation and its members were mapped at both Bayn Dzak, an area that includes the Flaming Cliffs, and Tugrugyin Shireh, an area about 50 km to the northwest of Bayn Dzak. Stratigraphic sections at both localities were remeasured. The considerably enlarged formation comprises a lower Bayn Dzak Member, dominated by moderate reddish orange sands with subordinate mudstone units, and an upper Tugrugyin Member, composed of pale orange to light gray sands. Investigations of key sections at Tsonzh and Alag Teer demonstrate the presence of transitional mudstone lenses between these members within the Djadokhta Formation. Two distinct, sandy, sedimentologic facies are recognized in both members. Cross-bedded intervals, occasionally exhibiting wind-ripple cross lamination, document the presence of a Cretaceous dunefield in the Ulan Nur Basin. Structureless intervals are interpreted to represent wet sandy fluvial deposits and debris flows that moved down the dune faces. In the Bayn Dzak Member, lenses of brownish mudstone are interpreted to represent interdune deposition in shallow ponds by fluvial action. Fluvial action is also represented in the Bayn Dzak Member by beds of caliche, which contain conglomerate at the base but fine upward into limestone. The vertebrate fauna from the Djadokhta Formation is summarized. Although the Bayn Dzak fauna lived somewhat earlier than that from Tugrugyin Shireh based on the superposition of the members, it is not clear how much earlier. The fauna from the Djadokhta Formation has previously been assigned ages from Cenomanian to earliest Maastrichtian. New magnetostratigraphic data document a sequence of normal and reversed magnetozones through the Bayn Dzak Member up into the basal Tugrugyin Member. The presence of reversed magnetozones establishes that the sediments containing the faunas were probably deposited after C34n. The quick stratigraphic succession of normal and reversed magnetozones suggests, but does not clearly establish, that the sediments may have been deposited during the rapid sequence of polarity changes in the late part of the Campanian between about 75 to 71 Ma.

Holocene history of lacustrine and marsh sediments in a dune-blocked drainage, Southwestern Nebraska Sand Hills, U.S.A.

As many as 2500 interdune lakes lie within the Nebraska Sand Hills, a 50 000 km stabilized sand sea. The few published data on cores from these lakes indicate they are typically underlain by less than two m of Holocene lacustrine sediments. However, three lakes in the southwestern Sand Hills, Swan, Blue, and Crescent, contain anomalously thick marsh (peat) and lacustrine (gyttja) sediments. Swan Lake basin contains as much as 8 m of peat, which was deposited between about 9000 and 3300 years ago. This peat is conformably overlain by as much as 10.5 m of gyttja. The sediment record in Blue lake, which is 3 km downgradient from Swan lake, dates back to only about 6000 years ago. Less than two m of peat, which was deposited from 6000 to 5000 years ago, is overlain by 12 m of gyttja deposited in the last 4300 years. Crescent Lake basin, one km downgradient from Blue Lake, has a similar sediment history except for a lack of known peat deposits. Recently, a 8-km long segment of a paleovalley was documented running beneath the three lakes and connecting to the head of Blue Creek Valley. Blockage of this paleovalley by dune sand during two arid intervals, one shortly before 10 500 yr BP and one in the mid-Holocene, has resulted in a 25 m rise in the regional water table. This made possible the deposition of organic-rich sediment in all three lakes. Although these lakes, especially Swan, would seem ideal places to look for a nearly complete record of Holocene climatic fluctuations, the paleoclimatic record is confounded by the effect dune dams have on the water table. In Swan Lake, the abrupt conversion from marsh to lacustrine deposition 3300 years ago does not simply record the change to a wetter regional climate; it reflects the complex local hydrologic changes surrounding the emplacement and sealing of dune dams, as well as regional climate.