Age of Tunica Hills (Louisiana-Mississippi) Quaternary fossiliferous creek deposits; Problems of radiocarbon dates and intermediate valley terraces in coastal plains

Among numerous stream-valley terrace deposits of the Gulf and Atlantic coastal plains that occupy chronologically and spatially intermediate positions between the youngest coastwise (coast-parallel) Pleistocene surface and the present, active floodplain, those of the Tunica Hills seem to provide by far the best opportunity for radiometric dating. Earlier views on the age of the Little Bayou Sara creek alluvium, represented by a single, 8 to 10-m-thick unit, ranged between the last interglaciation and middle Holocene. Reexamination of these deposits in the Little Bayou Sara and adjacent valleys clearly suggests their Late Pleistocene (apparently Farmdalian Interstade) age. The majority of the 14 available dates from the Little Bayou Sara and Tunica Bayou valleys proved to be too young, due to postdepositinal contamination. Dates ranging between 33,720 and 25,965 yr B.P. came from samples thought to be uncontaminated. Plant and faunal elements with boreal affinities in the unique fossil assemblage appear to be relicts of a preceding, full-glacial period, as regarded by Brown (1938). The absence of colder climate taxa from the Wilcox Bluff flora on Bayou Sara is insufficient evidence for a suggested Sangamon Interglacial age of the flora, and the terrace stratigraphy holds no proof for that view either. Only a single, loess-mantled, constructional, Quaternary, valley-terrace surface is present in the area. A narrow, low, actively developing floodplain terrace along Little Bayou Sara, cut into the Pleistocene alluvial unit, is primarily erosional in origin and has no bearing on the age of that unit. The age of the Tunica Hills terrace unit may provide comparison for dating intermediate valley-terrace deposits in favorable coastal settings elsewhere.     

The geoarchaeology of Iron Age “moated” sites of the Upper Mae Nam Mun Valley,N.E. Thailand. I: Palaeodrainage,site–landscape relationships and the origins of the “moats”

The study focuses on the “moated” Iron Age sites of N.E. Thailand, first identified as significant prehistoric settlement sites in the 1940s from aerial photography. Two more recent photograph sets are used to map the surficial geology and prehistoric site distribution for a study area west of Phimai, N.E. Thailand, with a focus on site–landscape relationships and, in particular, relationships between site location and form and patterns of palaeodrainage. The derived record of the surficial geology reflects several phases of palaeodrainage, characterized by differing locations and types of former river channels. Of note is the differentiation between a recent period in which river channels, including those presently active, are single-string meandering channels, and an older period of broad belts of meandering multistring channels. The prehistoric site distribution correlates closely with the older drainage, and for many, the encircling channels (the “moats”) are closely associated with former river channels. These relationships provide a critical and novel model for site distribution; several implications arise, supported by emerging field evidence, and introducing issues for archaeological debate: (i) there is no need, as has been done in the past, to invoke prehistoric artificial forms of drainage associated with the sites; (ii) the definition of the encircling channels as “moats” is seriously called into question; and (iii) the inferred geomorphological evolution of the floodplain implies past changes in environmental parameters such as run-off, climate or biophysical environments. Since the sites are all located in or beside ancient meander belts, these parameters should now be introduced into archaeological discussions regarding the establishment, history, evolution, and abandonment of the Iron Age sites. Methodologically, this article illustrates the need to be aware of the complexity of aerial photograph interpretation in archaeological survey, showing that careful analysis of aerial photograph information may have a significant impact upon the modeling of prehistoric interpretations. Further stratigraphical studies will be reported subsequently, and will refine the models presented here. © 1999 John Wiley & Sons, Inc.     

Comment on ‘Testing earthquake prediction methods: “The West Pacific short-term forecast of earthquakes with magnitude MwHRV ≥ 5.8”’ by V.G. Kossobokov

In his paper Kossobokov investigates the efficiency of our short-term forecast for two western Pacific regions. Although we agree with the basic results of his evaluation that the forecast statistics is much better than a random guess, we have reservations about his definition of earthquake prediction, some of his tests, and his interpretation of the test results. We distinguish between deterministic earthquake predictions and statistical forecasts. We argue that some techniques used by Kossobokov may not be appropriate for testing our forecasts and discuss other testing methods, based on the likelihood function. We demonstrate that Kossobokov's null hypothesis may be biased, and this bias can influence some of his conclusions. We show that contrary to Kossobokov's statement, our algorithm predicts mainshocks when they are preceded by foreshocks.     

Comment on “Reinterpretation of Cl data: physical processes, hydraulic interconnections and age estimates in groundwater systems” by E. Mazor

Mazor (1992) has reinterpreted several previous ³⁶Cl studies. The studies he revised used extensive physical hydrogeological data to aid in interpretation of the ³⁶Cl measurements. Major ion chemistry and other isotope tracers were considered in order to evaluate the groundwater geochemistry. The studies then used simple geochemical models to account for Cl behavior in the subsurface. The result of these ³⁶Cl dating studies was in reasonable agreement with both numerical models of the aquifer systems and with independent geochemical studies.Mazor (1992) has reinterpreted these studies based on the assumption that spatial variation in chemical and isotope data should be attributed to unspecified “discontinuities” in the flow regime. The conceptual models of aquifer hydrodynamics resulting from this approach differ radically not only from the previous ³⁶Cl interpretations, but also from the findings of virtually all previous hydrogeological and geochemical investigations. Although Mazor's “reinterpretations” are provocative, he does not show how they explain the data of the numerous previous studies better than the conceptual models presented by the authors of those studies, nor has he demonstrated that his new models are consistent with the fundamental physical laws governing groundwater flow. Until this is satisfactorily accomplished I will continue to prefer the original ³⁶Cl interpretations.