The Providence Island Formation of Early-early Middle Ordovician age occurs in the Champlain Valley and adjacent areas in eastern New York, western Vermont, and southern Quebec. The unit forms part of a carbonate shelf sequence which occupied the eastern margin of the North American continent from Newfoundland to Alabama, and its lithology is representative of the dolostone lithofacies that characterizes the uppermost Beekmantown Group in this region.
This is the first study documenting depositional environments, diagenesis, and stratigraphic correlations of the Providence Island Formation. This formation consists, in decreasing abundance, of dolostones, limestones, shales, and dedolostones. The dominantly fine grain size of the rocks as well as the presence of sedimentary and diagenetic features such as homogeneous and mottled structures (biogenic), stromatolites, mudcracks, herringbone cross-bedding, fenestral cavities, evaporites or their pseudomorphs, diapiric structures, and solution-collapse breccias indicate that these sediments record tidal flat paleoenvironments. These are low tidal flat, high tidal flat and, to a lesser extent, subtidal and supratidal settings similar to those existing in modern arid, restricted marine tidal flats of the Persian Gulf Trucial Coast. The lithofacies and their inferred setting include:
(1) Homogeneous dolostone: subtidal to lower intertidal or, occasionally, supratidal (sabkha).
(2) Homogeneous limestone: lower intertidal.
(3),(4) Mottled dolostone/limestone: lower intertidal.
(5),(6) Laminated dolostone/limestone: upper intertidal.
(7) Skeletal limestone: mostly upper intertidal.
(8) Dedolostone: mostly upper intertidal to supratidal.
(9) Shale: subtidal-lower intertidal to supratidal.
Most laminated dolostones and limestones represent stromatolites. On the basis of composition and texture, the alternating laminae are grouped into two types:
- Dolomitic facies: (I) FM-CPA: (F-fine, M-matrix – C-coarse, P-pyritic, A-allochemical-terrigenous) and (II) F-CA.
- Calcareous facies: (I) Mc-SPA: (Mc-micrite - S-sparite, P-pyrite, A-allochemical-terrigenous) and (II) Mc-SA.
Some laminae are transitional types between these end members. Storms appear to have been the most important factor controlling the type of alternating laminae.
The dolostone portion of the formation displays features that are characteristic of selective, sabkha diagenesis. These include the fine-grained size of the dolomite crystals, the preservation of primary structures (burrows, mudcracks, etc.), and the presence of gypsum and anhydrite crystals as well as their nodular pseudomorphs.
Textural evidence suggests a diagenetic sequence of (1) synsedimentary cementation (or cohesiveness (?)), (2) precipitation of evaporites, dolomitization, and pyritization, (3) precipitation of calcite, (4) dissolution of evaporites and void infill by chert or carbonates, (5) dedolomitization, and (6) compaction and stylolitization. The most important mechanism for the precipitation of evaporites and subsequent dolomitization appears to have been "flooding - reflux" in a highly evaporative environment. Here, downward percolating brines with a high Mg/Ca ratio induced the dolomitization of the sediment in contact with this fluid. The conditions favoring the alternation of dolostones and limestones are unknown.
The upper part of the formation may be divided into four members: the lowest one is about 2 - 3 m thick and is made up of limestones and is followed by a dolostone - dominated sequence (15 m, or more, thick). The overlying calcareous member is similar to, but 0.5- 1 m thicker than, the lower one. It is overlain by a second dolomitic member which could be considered as two units: the lower one (2.5 m) is characterized by laminated facies whereas the upper one (4.5 m or more) consists mainly of homogeneous and mottled facies.
The formation seems to be more than 100 m thick in the central portion of the outcrop belt and reaches approximately 160 m in the area of Shoreham, Vermont. Extensive erosion after the time of deposition of the formation appears to be the cause of lateral differences in thickness, particularly to the south where the formation pinches out and disappears. The post-Beekmantown unconformity probably reflects such an erosive event.
The tidal flats are thought to have developed on a tectonically stable area. Also, it seems that the flats were adjacent to both a very shallow lagoon which possibly was separated from the open (Iapetus) ocean by a physical barrier to the east and the shoreline to the west.
Roma Hernandez, M., 1987. The Providence Island Formation in the
northern Appalachian Region - a Lower-lower Middle Ordovician
analogue to recent arid-semiarid tidal-flat carbonates of the
Persian Gulf Trucial Coast.
Unpublished MSc. thesis, State University of New York at Albany. 209 pp., +xv
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1987 H47
thesis (scanned text) - 20.3MB pdf file
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