Bedrock Islands Research Articles

Paint Rock and Southwest Paint Rock Fields, Concho County, Texas: Strawn Analogs of Modern Island Carbonate Facies of Ambergris Cay, Belize: ABSTRACT

Lower Strawn (Desmoinesian Goen Limestone) reservoirs at Paint Rock and Southwest Paint Rock fields are a complex of carbonate and associated facies interpreted as having been deposited in various environments on and around large, emergent islands on shallow carbonate shelves. The origin and geometries of the component lithofacies in these fields, and their reservoir diagenetic histories, are similar to those presently accumulating on Ambergris Cay, a linear island complex on the northern shelf of Belize. Paint Rock field originated as a narrow, elongate Chaetetes reef trend that formed the foundation on which the overlying island facies were deposited. As on Ambergris Cay, these reef limestones developed extensive porosity during postdepositional subaerial exposure due to meteoric leaching. In contrast, Southwest Paint Rock field is cored by older island deposits rather than reef limestones. With ensuing stillstand or subsequent sea level rise, beach grainstones were deposited along the windward and leeward margins of the foundation highs in these fields. Tight lagoonal micrites and coals (peat-swamp facies) comprise the inner island facies, and are locally associated with porous supratidal dolomites. These island complexes are transected locally by tidal channels that are filled with nonporous micrites. Repeated sea level fluctuations during the history ofmore » these fields resulted in a characteristic cyclic stratigraphy of stacked island facies and reservoirs. The reservoirs in the field are developed in the bedrock or older island cores, as well as in the overlying beach facies and supratidal dolomites. These fields are mappable as linear stratigraphic traps with low-relief closure, and are readily identified by subsurface geologic and facies analyses. Similar shelf island-type fields analogous to these strawn and Holocene Belizean examples are found throughout the Midland basin and Eastern shelf.« less

An Emerging Esker in Southern Finland

A partially submerged esker situated among a group of rocky islands on the northern coast of the Gulf of Finland is examined. As the esker emerged from the sea to form separate islands, the glaciofluvial material was deformed by current and wave action and redeposited. The contemporary rate of emergence is 1.8 mm per year. The structural differences in the primary esker have been emphasized by selective marine erosion. Differences in wave energy are reflected in the length of the fetch, which shortens towards the mainland and with land uplift. Shortening of the lateral fetch has prevented further marine erosion once the summit of the esker has been eroded to the level of the bedrock islands. Only in the direction of the open sea does the force of the waves continue unabated. This wave action has resulted in the formation of beach ridges on the southernmost tip of the esker. Elsewhere this same action causes longshore drifting, which has resulted in the development of spits and tombolos on the lee side of the esker islands.

Bay of Fundy Intertidal Zone Sediments

ABSTRACT Sedimentation occurs in 4 environments in the Bay of Fundy intertidal zone. These environments are (1) wave-cut benches, (2) estuarine clay flats, (3) tidal flats in lee of bedrock islands, and (4) salt marshes. Thin sediment veneers deposited on a wave-cut bench comprise 75 percent of the intertidal zone sediments of the Bay of Fundy. Such sediments are characterized by texture and composition which is similar to that of the underlying bedrock from which it was derived. Wave action and braided streams rework the sediments and leave the following combination of primary structures: lenticular cross-stratification; flutes; grooves; flat-topped, scoured, current, oscillation and interference ripple marks; current lineation; megaripples, and scour pits on the lee side of pebbles. Estuarine clay flats are underlain by unstratified silt and clay which is drained by meandering creeks. Tidal marsh sediments consist of a silt and clay bank overlain by a thin zone of decayed plant debris. Tidal flats in the lee of bedrock islands were subdivided into a higher tidal flat characterized by gravel and sand, and a lower tidal flat characterized by gravel, sand, silt and clay. The higher tidal flats are drained by braided streams. Meandering creeks drain the lower tidal flats and develop a lag concentrate of gravel, sand, and clam shells at the channel bottoms by the process of lateral sedimentation. Comparison of Bay of Fundy intertidal zone sediments and Triassic redbeds of the Canadian Maritime Provinces showed that both are characterized by the same color, texture, and mineral composition because the modern sediments were derived from Triassic bedrock. However, the Triassic redbeds are of different origin as their characteristic combinations of primary structures suggests fluvial and lacustrine conditions.