ABSTRACT Previous work shows that the Burnt Bluff Group was deposited as a series of shallow- to moderate-water-depth facies in a tropical marine setting in the Michigan Basin during the Llandoverian. New interest in the unit for both hydrocarbon resources (subsurface) and aggregate resources (outcrop) is driving research in this poorly understood unit. New cores, as well as investigation of the outcrop belt in the Upper Peninsula of Michigan, allow further elaboration of the depositional model. The traditional interpretation of the Burnt Bluff Group consists, in stratigraphic order, of the open-marine deposits of the Lime Island Formation, the restricted lagoonal–tidal flat Byron Formation, and the open-marine deposits of the Hendricks Formation. The contacts with the underlying Cataract Group and overlying Manistique Group are gradational. Identification and description of facies in both cores and outcrop sections provide new constraints on the stratigraphic nomenclature of the Burnt Bluff Group. New outcrop and core data require the revision of the depositional model for portions of the group because typical Byron-like facies are found interbedded with Hendricks-like facies in both the Byron and Hendricks Formations. Limited age data from published research combined with the new facies model for the Burnt Bluff Group suggest that the unit was deposited as a time-transgressive package on a carbonate ramp during the Llandoverian.

Abstract Determining how autogenic and allogenic processes and responses in deltas scale up from meter-scale laboratory experiments to actual field examples remains a challenge. This study was devised to bridge that scale gap using field data from small, hundreds of meter-scale natural deltas. Ground-penetrating radar and core data were collected from four different river-dominated delta morphotypes developing at the margins of freshwater coastal lagoons in southern Brazil. Since the sediment supplying these deltas is sourced from a nearby dune field and is similar between the deltas, it is hypothesized that major morphological differences in the four deltas are primarily the result of differences in sediment discharge rates (sediment-water ratio). As observed in published tank experiments, channel cross-section and distributary channel patterns in the deltas vary as a function of sediment discharge, from shallow sheet-like flow at high discharge to well-established, stable distributary channels (i.e., birdsfoot pattern). A contributing factor may be the development of vegetation on the slower growing deltas influencing sediment cohesion, a key control in laboratory-scale deltas. As in many tank experiments, these lagoon deltas are steep and sandy, with the Froude number modulated to just below Froude critical flow (i.e., they are Froude-scaled). Ground-penetrating radar sections were processed, interpreted, and integrated with cores, allowing the definition of radar units. Analysis of the radar units demonstrates the presence of both allogenic and autogenic signals. Allogenic control is identified in the stacking of clinoforms and is perceptible in both sides of a single delta (delta 4), as well as in two other deltas (deltas 1 and 2). An autogenic signal varies according to delta planform shape and was identified by the stacking of lobe elements, both in dip and strike. Base-level change (lake level) and autogenic avulsion cycles occur on similar timescales, and therefore it is a significant challenge to separate these different processes in the stratigraphy. The potential uses of these types of data include understanding the link between delta dynamics, channel patterns, and stratigraphy to develop improved genetic models of steep sandy deltas common in the stratigraphic record.

Duxbury Beach, Massachusetts, is a retreating, transgressive barrier that is effectively managed to meet a range of competing land uses. While the barrier is heralded as a natural coastal setting, the entire landform is methodically engineered on an ongoing basis to best accomplish the goals established for the beach within a context of natural processes. Historical and geological data indicate that the natural barrier form includes numerous ephemeral tidal inlets (some of which have migrated) and overwash channels, and low discontinuous dunes. At present, the managed barrier has a continuous vegetated foredune and broad backdune. Management techniques have evolved over the past several decades based on growing experience and understanding of the coastal processes and of wildlife habitats. Although the foredune crest is reconstructed each spring, the entire beach is gradually being allowed to retreat to remain in equilibrium with rising sea level. The lagoonal shore is being widened through beach nourishment and through proposed creation of back-barrier salt marshes using silty dredge spoil. Uses of the barrier include town and public recreational beaches, off-road vehicle access, a right-of-way to isolated communities, flood protection of landward areas, and shorebird nesting habitat.