Michael J. Soreghan

Estimating the age of formation of lakes: An example from Lake Tanganyika, East African Rift system

Age estimates for ancient lakes are important for determining their histories and their rates of biotic and tectonic evolution. In the absence of dated core material from the lake`s sedimentary basement, several techniques have been used to generate such age estimates. The most common of these, herein called the reflection seismic-radiocarbon method (RSRM), combines estimates of short-term sediment-accumulation rates derived from radiocarbon-dated cores and depth-to-basement estimates derived from reflection-seismic data at or near the same locality to estimate an age to basement. Age estimates form the RSRM suggest that the structural basins of central Lake Tanganyika began to form between 9 and 12 Ma. Estimates for the northern and southern basins are younger (7 to 8 Ma and 2 to 4 Ma, respectively). The diachroneity of estimates for different segments of the lake is equivocal, and may be due to erosional loss of record in the northern and southern structural basins or to progressive opening of the rift. The RSRM age estimates for Lake Tanganyika are considerably younger than most prior estimates and clarify the extensional history of the western branch of the East African Rift system. 31 refs., 3 figs., 1 tab.

Anomalous cold in the Pangaean tropics

The late Paleozoic archives the greatest glaciation of the Phanerozoic. Whereas high-latitude Gondwanan strata preserve widespread evidence for continental ice, the Permo-Carboniferous tropics have long been considered analogous to today9s: warm and shielded from the high-latitude cold. Here, we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial Pangaea. An exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral Rocky Mountains. Furthermore, the Permo-Carboniferous archives the only known occurrence of widespread tropical loess in Earth9s history; the volume, chemistry, and provenance of this loess(ite) is most consistent with glacial derivation. Together with emerging indicators for cold elsewhere in low-latitude Pangaea, these results suggest that tropical climate was not buffered from the high latitudes and may record glacial-interglacial climate shifts of very large magnitude. Coupled climate–ice sheet model simulations demonstrate that low atmospheric CO 2 and solar luminosity alone cannot account for such cold, and that other factors must be considered in attempting to explain this “best-known” analogue to our present Earth.

Paleowinds inferred from detrital-zircon geochronology of upper Paleozoic loessite, western equatorial Pangea

U-Pb geochronology of detrital zircons from upper Paleozoic loessite (western United States) provides data bearing on atmospheric circulation within western equatorial Pangea. Zircon age spectra of four loessites from three localities representing middle Pennsylvanian (Desmoinesian) and Early Permian (Wolfcampian) time vary significantly, reflecting changing provenances attributable to temporal and spatial shifts in winds. Zircons from two Desmoinesian samples (from Arizona and Utah) show a dominant mode between 1800 and 1600 Ma, reflecting the Yavapai-Mazatzal terranes that cored the Ancestral Rockies uplifts and suggesting northeasterly winds. Both samples also contain a secondary cluster of Grenvillian grains (1300–1000 Ma), reflecting a south-southeasterly source. Ages for Wolfcampian samples (from New Mexico and Utah) differ from one another; the New Mexico loessite contains a large mode at 1700 Ma, missing in the Utah sample, reflecting their locations on opposing sides of the Ancestral Rocky Mountains, within a westerly wind regime. Inferred easterly winds for middle Pennsylvanian time match model predictions, and the presence of both northerly and southerly directions might reflect time-averaged fluctuation of the Inter-Tropical Convergence Zone. In contrast, monsoonal circulation and attendant westerly winds appear to have been well established by earliest Permian time.

Textural and Compositional Variability Across Littoral Segments of Lake Tanganyika: The Effect of Asymmetric Basin Structure on Sedimentation in Large Rift Lakes

Lake Tanganyika, part of the East African rift system, represents one of the most widely cited modern analogs for interpreting ancient rift lakes. To date, few published detailed sedimentologic studies of the modern sediments allow for comparisons to outcrop- and well-bore-scale observations within ancient strata. Four recurrent structural margin types exist along the alternating half-graben structure of the lake: hinged margins, axial margins, accommodation zone margins, and escarpment margins. The hinged margin consists of a series of structurally controlled benches over which long, continuous tracts of bioclastic lag deposits predominate; clastic sands are limited to moderate-size silty deltas and long, narrow shoreface sands. The axial margin is dominated by a wave-dominated, silt-rich delta system. Accommodation zone margins consist of bioclastic lag deposits atop structural highs, whereas carbonate and clastic mud accumulates farther offshore. Escarpment margins contain small fan-delta deposits alternating along shore with talus deposits; offshore carbonate and clastic mud is present away from active gravity-flow deposition. Total organic carbon (TOC) and pyrolysis data from fine-grained samples subtly reflect the contrasts in margin types, but these values are controlled more directly by water depth. Although facies are similar among all margin types, their spatial distribution, in particular the degree to which facies tracts trend parallel to shore, best discriminates among the different margin types. These data suggest that unique but predictable associations of reservoir, seal, and source facies exist along each of the different margin types.

Environmental controls on shell-rich facies in tropical lacustrine rifts: A view from Lake Tanganyika's littoral

Abstract Lake Tanganyika, the worlds largest tropical rift lake, is unique among its counterparts in East Africa for the remarkable diversity of mollusk-rich sediments in its littoral zone. Molluscan shell beds are, however, a common feature of ancient lacustrine rift deposits and thus a better understanding of their spatial and temporal development is important. Targeted surveys across the littoral region of the Kigoma Basin reveal three surficial shell-rich facies that differ widely in depositional style and geometry. A unifying characteristic of these deposits is the volume of shells of Neothauma tanganyicense, a large, viviparous gastropod endemic to the lake. Reservoir-corrected radiocarbon dating indicates that Neothauma deposits in these surficial sediments are time averaged over at least the last ∼1600 calendar years BP. Preservation of fossil Neothauma shells in the littoral zone depends on both environmental conditions and on post-mortem shell modifications. Interaction between shells and mobile siliciclastic grains, facilitated by wave action and storms, represents a particularly destructive taphonomic process in the study area. Rank scoring of damage to Neothauma suggests that stromatolitic encrustations or early calcite coatings may help mitigate shell destruction caused by hydraulic fragmentation and abrasion. Persistence of Neothauma in littoral beds has important implications for the structuring of specialized communities of shallow-water benthos, as well as for improving analog models for hydrocarbon reservoirs in lacustrine carbonates.

An Exhumed Late Paleozoic Canyon in the Rocky Mountains

Landscapes are thought to be youthful, particularly those of active orogenic belts. Unaweep Canyon in the Colorado Rocky Mountains, a large gorge drained by two opposite‐flowing creeks, is an exception. Its origin has long been enigmatic, but new data indicate that it is an exhumed late Paleozoic landform. Its survival within a region of profound late Paleozoic orogenesis demands a reassessment of tectonic models for the Ancestral Rocky Mountains, and its form and genesis have significant implications for understanding late Paleozoic equatorial climate. This discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator.

Processing Backscattered Electron Digital Images of Thin Section

Image analysis of sedimentary particles using backscatter electron (BSE) microscopy shows great promise in paleoclimatic and paleoenvironmental studies. Prior to the last few years BSE microscopy has been used primarily for compositional (provenance) studies. Our preliminary work on Paleozoic loessite, as well as previous work on recent sediments (Francus 1998; Francus and Karabanov 2000), suggests that BSE microscopy image analysis is an effective tool for deriving textural data for use as a paleoclimate proxy. Our data on the Paleozoic loessite shows that we are able to document changes in grain size of quartz through several loessite-paleosol couplets. In each case, the quartz was coarser in the loessite facies relative to the overlying paleosol, which is similar to grain size trends observed in the Quaternary Chinese Loess Plateau. Image acquisition is a critical step in this methodology, however, special precautions are needed to make sure that 1) the samples are suitably prepared, 2) the acquisition instrument’s settings are controlled and maintained, and 3) the acquisition system provides an output of suitable resolution. Processing is similar to other types of imagery subjected to image analysis, and includes calibration, filtering, and image segmentation and thresholding. An important component of processing is testing how different filters affect grain boundaries, particularly if grain size or grain shape is to be measured. In terms of image measurements, the magnification is an important consideration, and should be consistent; with standard BSE detectors, grains smaller than approximately 2 μm can not be resolved because of the size of the interaction volume of the backscatter electrons. As the case study illustrates, measurements of grain size or grain perimeter in this methodology do not translate into actual grain size information because of stereological considerations, however, relative changes in grain parameters yield useful information. The two biggest drawbacks of the present methodology are that it is difficult to keep acquisition conditions constant, and that data collection is time consuming. As instruments with BSE capabilities improve with more digital controls, acquisition will become much more stable, and as protocols are developed, it will be possible to semi-automate the procedure, allowing for a much faster rate of data collection.

An Exhumed Late Paleozoic Canyon in the Rocky Mountains : A Reply

We thank William Hood for his discussion of Soreghan et al. (2007). We recognize that our interpretation of the events leading to the formation of Unaweep Canyon, particularly our proposed Paleozoic age of the (ancestral) canyon, represents a significant departure from established models. Validation of our hypotheses regarding its age and origin would force revision of several longaccepted models, ranging from the Cenozoic tectonic and geomorphic evolution of this region to the climatic and perhaps tectonic framework of the Permo-Pennsylvanian tropics represented by this system. Accordingly, our work deserves close scrutiny, and Hood’s discussion provides such an opportunity. Hood (2009) begins his discussion by stating that Soreghan et al. (2007) presented the hypothesis that Unaweep Canyon is a Permian glacial valley that was filled by Paleozoic sediment and subsequently exhumed by Cenozoic rivers. To clarify, the focus of Soreghan et al. (2007) is the hypothesized Paleozoic age of the canyon, although we posed the question of a possible glacial origin in the final sentences of the article. A more complete analysis of the evidence for a glacial origin, however, appears in Soreghan et al. (2008), although only abstracts of this aspect (e.g., Soreghan et al. 2004) were published at the time that Hood submitted his discussion. Nevertheless, here we address all of the points raised by Hood; we treat each of his points using the subheadings he provides. Reexamination of the Field Evidence and Paleomagnetic Data

Image Calibration, Filtering, and Processing

In this chapter we discussed image calibration, filtering, and processing techniques, which are used to prepare an image for subsequent data extraction and analysis. Size measurements from a digital image are calibrated by imaging objects with a known size. Pixel intensity is a measure for the composition of the imaged object and can be calibrated by imaging objects with known composition. Methods depend on the type of material and imaging technique. We discuss colour calibration, as colour is one of the most widely used types of data in image analysis. Filtering is performed on an image to remove artefacts that are unrelated to the object of study. The challenge is to find the best filter, one that removes all noise with minimum change to the actual information in the image. Described are techniques to remove the effects caused by uneven illumination during imaging, and methods to filter camera related noise. Image processing involves modification and/or enhancement of the image in such a way that the required numerical data can be extracted more easily. Processing techniques that are outlined include edge detection, segmentation, and processing of binary images.

A Multi-Week Basin-Analysis Lab for Sedimentary Geology

We describe a multi-week basin-analysis lab for sedimentary geology that utilizes construction, analysis, and interpretation of 1) sedimentary facies and structures, 2) paleocurrent data, 3) subsurface data, 4) paleogeographic data, 5) bio- and magnetostratigraphic data, and 6) subsidence curve(s) associated with a hypothetical sedimentary basin. Students collect most of the data in the field during trips that occur prior to and during the lab, and analyze their data by hand and with the aid of selected computer programs. Ultimately, students synthesize an integrated analysis and discuss a series of questions about basin history (for example, paleogeography, paleotectonics) and economic potential (for example, hydrocarbon systems). Students individually collect and analyze most of the data but orally present selected aspects of their synthesis in teams. The exercise allows students to develop their skills in field-data collection, and analysis and interpretation of a variety of sedimentarygeology data typ...