The Kat O Formation crops out in scattered areas at the northern tip of Crooked Island, on Robinson Island (Plate 8.5), Ap Lo Chun, Sai Ap Chau, Pak Tun Pai, Ledge Point and North Point. At the designated type section on Crooked Island (Figure 8.4), the formation is estimated to have a minimum thickness of 100 m. At this locality the formation dips gently (5º) to the north and rests unconformably on fine ash tuff of the Early Cretaceous Ngo Mei Chau Formation.

The Kat O Formation comprises dominantly calcareous breccia, conglomerate and coarse-grained sandstone and is distinguished from the Pat Sin Leng and Port Island formations by the presence of abundant calcite cement. The formation fills a small depositional basin bounded by faults in Ap Chau Hoi and Crooked Harbour. The formation is exposed at the coastline of Lau Fau Shan, and occurs in a marine borehole in Deep Bay, where it comprises granite clast-bearing breccia and sandstone.

Owing to the limited exposure available, facies modelling of the Kat O Formation has not been attempted.

Details

Cheung Pai Tau. The formation here comprises calcareous breccia with coarse sandstone. Bedding dips very gently, at 5^o, to the north. A 15 m-wide fault zone, marking the southern margin of the basin, dips at 48^o to the north. There is a block-bearing tuff within the fault zone that includes blocks of limestone, black mudstone, lava and tuff. The limestone is dark grey to light grey and pink, and is commonly cut by small veins of calcite (Williams, 1943). Fossils of Fusulina and Fusulinella of Permian age were recovered from one limestone clast (Lee, in Williams, 1943). A quarry where the limestone was extracted is now disused and flooded.

Ap Chau (including Sai Ap Chau, Ap Lo Chun, Ap Chau Pak Tun Pai and Ap Chau Mei Pak Tun Pai). The Kat O Formation on these islands is mainly reddish brown, thickly bedded breccia with thinly bedded coarse-grained sandstone, cemented with a calcareous and clayey matrix. Some veins are characterized by large crystals of calcite. Lithic clasts contain tuff, rhyolite and vein quartz and are 30 to 600 mm across. The bedding is gently dipping at 5-10^o to the north. The strata are cut by minor normal faults. The calcareous minerals in the rocks are susceptible to marine erosion leading to the formation of sea caves, cliffs, arches and wave-cut platforms (Plate 8.A3). A large sea arch occurs at Kau Tau Shek (845630 846030 Kk-1), north of Ap Chau (Plate 8.A4).

Chek Kok Tau. This headland, on the northern coast of Kat O, exposes a widespread, reddish brown breccia and overlies pale green lapilli tuff of the Ngo Mei Chau Formation. Fine-grained detrital rocks, locally occurring near the base of the formation are seen only on the sea shore (847950 846140 Kk-2). Grey to greyish white breccia and coarse sandstone, 1.5 m thick, lie directly on the tuff, passing upwards into 1 m of grey sandy siltstone, 0.5 to 1 m of greyish white soft mudstone, and 1 to 2 m of greyish white sandstone. This sequence is overlain by the thickly bedded calcareous reddish brown breccia and coarse-grained sandstone that make up the rest of the headland.

Lau Fau Shan and Deep Bay. The formation is exposed at the coastline of Lau Fau Shan (816550 836930 Kk-3), and occurs in a marine borehole in Deep Bay (807150 831330 Kk-4), where it comprises granite clast-bearing breccia and sandstone.