麻豆传媒

The Windsor Group (Visean) carbonates at Gays River lie unconformably upon a NE trending anticlinal high of Cambro-Ordivician metagreywackes belonging to the Goldenville Formation. The distribution of carbonates sediments is affected by minor relief features found on the anticline which take the form of N-NW trending, 90 meter high spurs with associated re-entrants. These spurs are steep-sloped, and stepped as a result of strongly-developed intersecting joint sets (087/47N, 150/70W).

Physical weathering, prior to carbonate deposition, produced a scree deposit of basement material. The scree has a discontinuous distribution with wedge-shaped accumulations (3m) located at the foot of the spurs, in the re-entrants, and on the basement steps. During initial transgression of the Windsorian sea, the scree was partially reworked and a carbonate matrix was added. The resulting deposit is termed the basal conglomerate.

Carbonate rock types present in the decline can be divided into two lithologic groups. The first group represents a flank deposit derived from the second group of rocks. This flank deposit overlies the basal conglomerate and/or the basement in the re-entrants. The flank deposit consist of a wedge or fan of thinly-bedded lithic skeletal grainstones which are interstratified with thin back carbonaceous laminae. The wedge dips basinward (N-NW) at 25 to 30 degrees. Gypsiferous, peloidal wackestone discontinuously overlies the lithic skeletal grainstones. In turn, the wackestone is overlain by more than 50 meters of anhydrite and gypsum.

Group two rock types consist of algal bindstone and bafflestone, lithic skeletal grainstones and packstones, and algal fenestral wackestone-packstone. In addition, a calcrete profile is present. The rocks of group two are located higher up the basement slope than those of group one. The algal bindstone, algal bafflestones, lithic skeletal grainstones and packstones represent a algal dominated, wave-resistant, organic-framework reef (as defined by Heckel, 1974). The algal fenestral wackestone-packstone records a restricted, shallow-water, lagoonal deposit; which is both adjacent to, and overlying the bindstone and bafflestone rock types. A thin zone of low-middle intertidal sediment is present near the top of wackestone-packstone. This zone exhibits an irregular fenestral fabric. The calcrete profile indicated the presence of an unconformity of subaerial exposure within the depositional history of the carbonates.

Lithologic relations within and among group two rocks are very complex. Lithostratigraphic, let alone chronostratigraphic, correlation of the carbonate units using drill core from adjacent holes (60m spacing) is not possible on the scale used while mapping underground (1cm-25cm).

Keywords:
Pages:141
Supervisor: Paul Schenk