John J. Alford

The anomalous Roxana silt and mid-Wisconsinan events in and near southern Michigan

Abstract Thick deposits of Roxana Silt are recognized only along the Illinois River (downstream from the Woodfordian terminal moraine) and are generally interpreted as being mainly loess, with the bulk accumulating from about 40,000 to 30,000 yr ago in association with an Altonian-age glacier in northeastern Illinois. Yet 11 14C dates indicate that southern Michigan was not ice-covered during that interval; thus, any proximate ice must have, at best, been restricted to Great Lakes basins, an interpretation supported by the absence of late Altonian till at critically located Michigan, and nearby, sites. Late mid-Wisconsinan ice did, however, obstruct eastern drainage of the ancestral Great Lakes. Such glacial blockage, the distribution of many Michigan organic deposits within pre-Woodfordian lacustrine sediments, and radiocarbon dates suggest that, more than once, late Altonian lakes associated with the Lake Michigan basin drained into the Illinois River. Erosion of lake and spillway bluffs along with repeated river fluctuations provided a source for the thick, geographically restricted Roxana Silt. Meanwhile, along other nearby rivers the supply was meager and the loess thin.

Terrace stratigraphy in the Tunica Hills of Louisiana

Abstract The age and number of important fossil-producing late Pleistocene terraces in the Tunica Hills have recently become quite controversial. One hypothesis holds that only a single loess-mantled Farmdalian terrace flanks the streams in this area. The other maintains that there are two terraces. The youngest is considered essentially Holocene in age while the older is Sangamon in age. Radiometric and stratigraphic evidence collected for this study indicates that there are two terraces. The youngest is late Woodfordian to Holocene in age while the other is Farmdalian.

Middle Wisconsinan and recent wet site mummified wood, humus, peat, and pollen, Santa Rosa Island, Florida

Abstract Macrobotanical samples obtained from submerged, previously buried, tree stumps in growth position and peat and humus/humate layers from the surf zone of Santa Rosa Island were identified and dated by 14C analysis. Dates obtained clustered in two time spans, 45 440 yr BP. These subfossil macrobotanicals are well preserved in spite of submergence. Conifers of the family Cupressaceae (cf. Juniperus virginiana) and Pinaceae and a dicotyledon of the family Lauraceae (cf. Persea) are the principal tree species identified. Presence of these species in the current surf zone in conjunction with modern vegetation distribution indicates sea level was lower during the Middle Wisconsin and Late Holocene. Microbial decay patterns in a Pleistocene wood sample suggest basidiomycete infestation by brown-rot fungi prior to burial and successional decay by actinomycete bacteria during its waterlogged state. Extensive destruction of S2 microfibrils of the secondary cell wall that left a residual skeleton of lignin and presence of pyrite framboids in Pleistocene wood inform on its depositional history. Pollen/spore assemblages from Holocene humus/humate suggest a former maritime forest habitat with back-barrier freshwater basins. Macrobotanical identification, radiocarbon dates, and palynological indicators suggest Santa Rosa Island is an unstable barrier island at a critical point of degradation due to marine intrusion coincident with global sea-level rise, storm overwash burial of existing maritime plant communities, and accelerated erosion of former vegetational foci by sand aggradation. The 14C date and location of stump sub-samples SRI 1b and SRI 1f in conjunction with previously reported regional 14C dates indicate that SRI 1 occupied a topographic high during marine oxygen isotope chronozone (OIC) 3 rather than being near sea level. Holocene samples spanning the last two millennia verify the occurrence of maritime forest and high dune habitat taxa in the present surf zone.