LOCATION AND COMPOSITION OF SOURCE AREAS FOR THE MIDDLE AND LATE ORDOVICIAN BLOUNTIAN (TACONIC) FORELAND BASIN, SOUTHEASTERN CORNER OF LAURENTIA

Irregular lateral and vertical variations in grain size and composition of detritus in the Middle and Upper Ordovician Blountian foreland strata suggest a complex distribution of uplifts. We compare composition and stratigraphic trends of foreland strata for determination of rock types in source areas and location of uplifts.

Conglomerate beds overlie the post-Knox unconformity both in distal and proximal foreland areas, and the proximal foreland succession includes conglomerate beds. In distal areas, conglomerates consist exclusively of chert clasts and are interbedded with and/or overlain by peritidal carbonates (Lenoir, Chickamauga). In proximal areas, conglomerate beds overlying the unconformity and interbedded with deep-water shales (Athens) consist dominantly of limestone and dolomite clasts; conglomerates near the top of a prograding siliciclastic succession (Chota) include more sandstone and siltstone clasts.

Sandstones are divided into three petrofacies. Matrix-rich arkoses to subarkoses are the dominant component of the underfilled (lower) Blountian clastic wedge (Athens, lower and middle Greensport). Plagioclase, orthoclase, and microcline are identified; however, albitization obscures feldspar identification. The overfilled (upper) Blountian clastic wedge consists of subarkose to quartzarenite petrofacies in the distal foreland (upper Greensport, Colvin Mountain, Sequatchie) and sublitharenite petrofacies in the proximal foreland. In general, sedimentary lithic grains (shale and siltstone) dominate over metamorphic lithic grains (quartzite and schist); feldspar-quartz aggregates are subordinate. Micritic carbonate fragments are common in the northeasternmost proximal section. Open-marine skeletal fragments are present within sandstones that record structural instability of the carbonate platform.

The match of conglomerate clasts with Knox-Lenoir strata supports the interpretation of basement fault reactivation affecting the foreland plate. A northwestward break-forward propagation of the orogenic belt, consisting of Laurentian basement and sedimentary cover, is recorded by the upsection increase of compositional maturity in distal foreland sandstones, and a switch from subarkoses to sublitharenites in proximal foreland sandstones.