Classification. (Fig. 8.)
Cu 8. Estuarine Sands.
Cu 7. Tawera Silts.
Cu 6. Upper Castlecliff Shellbed.
Cu 5. Zethalia-Amphidesma Sands.
Cu 4. Stirocolpus Silts.
Cu 3. Tainui Shellbed.
Cu 2. Antisolarium Sands.
Cu 1. Basal Upper Castlecliff Sands.
Cu 1. Basal Upper Castlecliff Sands (R. S. A. 8 and 9).
Exposure: Between 280 and 125 chains north-west from Castlecliff.
Lithology: Conglomerate, current-bedded sands, alternating sands and silts. Basal conglomerate contains andesite and rhyolite pebbles.
Fauna: Rolled mollusca in basal conglomerate; biocoenotic mollusca in uppermost sand (Notovola, Myadora, Amphidesma).
Contacts: Strongly disconformable on bored surface of Cl 11 below; conformably overlain by Cu 2a above.
Thickness: 60–65 feet.
Subdivision: Basal conglomerate (Cu 1a); 3–5 feet of pebbly sands, coarsely current-bedded, with rare rolled shells (Cu 1b); alternating loose sands and bedded muddy sands or silts, in beds 1–3 feet thick, barren (Cu 1c); coarse loose sand with fossils (2 feet, Cu 1d).
Cu 2. Antisolarium Sands (R. S. A. 6 and 7).
Exposure: From 205 to 95 chains north-west of Castlecliff. The sequence Cu 1 to Cu 3 is well shown at the “Pinnacles”, a gulch in the cliff flanked on either side by a pinnacle of fossiliferous siltstone, 162 chains north of Castlecliff.
Lithology: Muddy medium micaceous sandstone; rare mudstone.
Fauna: Abundant mollúsca, scattered throughout the bed and aggregated into a basal shellbed (Cu 2a). Antisolarium egenum (Gld.) is strikingly dominant.
Contacts: An undulating parting separates the fine muddy sand of Cl 2a from the uncoherent coarse loose fossiliferous sand at the top of Cl 1. The subdivisions of Cl 2 are perfectly conformable with each other. At the top the Cu 2 sands become rapidly finer and pass into silts at the base of Cu 3, where fossils become more abundant.
Thickness: 30–40 feet.
Subdivision: The basal 3 ft. shellbed (Cu 2a = R. S.A. 7) packed with fossils (Venericardia locally dominant) passes abruptly into Cu 2b, muddy sands with abundant scattered Antisolarium and other mollusca, which in places grade laterally into bedded silty mudstone with lenses of sand packed with Antisolarium. Cu 2c and Cu 2d form the uppermost five feet of the bed in which Antisolarium is virtually absent and lines of Tawera, Notocallista and Glycymeris are developed parallel to the bedding. Cu 2d is the uppermost 9 inches immediately below the base of Cu 3 (defined by the lowest occurring Notovola tainui and by rapid replacement of sand by silty mud) and is separated from Cu 2c for purely faunal reasons.
Cu 3. Tainui Shellbed (R. S. A. 5).
Exposure: Forming a “buttress” projecting on to the beach at high tide mark, 90 chains north-west of Castlecliff (Plate 30, Fig. 10) and rising to the top of the cliff 185 chains farther north-west.
Lithology: Fine, massive, smooth mudstone, unctuous (but not plastic) when wet.
Fauna: Abundant mollusca, bryozoa and brachiopods. Mollusca concentrated along bedding planes; bryozoa in lens-like colonies (“bioherms”).
Contacts: Conformable, above and below.
Thickness: 18–20 feet.
Remarks: This is one of the best-known Castlecliffian horizons, being the most important part of the blue clays with shells (bed 4) of Park, rightly correlated with the lower beds at Shakespeare Cliff, the Castlecliff “blue clays” and “papa” of most of Finlay's molluscan descriptions, and the “basal papa” of Powell (1931). The mollusca are beautifully preserved. Notovola tainui Fin. and Ptero-chelus zelandicus (Hutton) are conspicuous and apparently restricted fossils.
Cu 4. Stiracolpus Silts (R. S. A. 4).
Exposure: From 50 to 160 chains north-west from the end of the cliff at Castlecliff.
Lithology: Fine, massive, micaceous siltstone, coarser above.
Fauna: Scattered mollusca, with Stiracolpus dominant.
Fig. 9 View of coastal section between Kai Iwi Beach and Castlecliff looking south-east from a point 140 chains south-east of the mouth of Mowhanau Stream.
Fig. 10. Chione-Potamopyrgus silts (Nu 7). Ototoka. Stunted biocoenotic Chione (larger shells) accompanying the fresh water gasteropod Potamopyrgus (small specks) in a low salinity estuarine habitat.
Fig. 11. Tainui shellbed (Cu 3) at buttress, 90 chains north-west of Castlecliff.
Fig. 12 Zethalia-Amphidesma sands (Cu 5). Detail of a shelly lens, from a fallen block; Castlecliff.
Contacts: Conformable rapid passage from Cu 3. A possible disconformity separates from Cu 5.
Thickness: 30 feet, thinning to 20 feet north-west.
Subdivision: The upper 6 feet of the bed (Cu 4b) contains more abundant fossils and is coarser than the lower part (Cu 4a).
Cu 5. Zethalia-Amphidesma Sands (R. S. A. 3).
Exposure: From 20 to 140 chains north-west of end of cliff at Castlecliff.
Lithology: Loose, coarse, current-bedded, micaceous sand, with lenses of shell-sand, and basal shellbed.
Fauna: Abundant fossils in basal shellbed (Dosinula and Ostrea dominant), and in lenses (Zethalia, Amphidesma, etc., and the echinoderm Arachnoides, Plate 30, Fig. 12).
Contacts: The base appears to truncate the underlying bed, causing it to thin to the north-west; there is no clear erosion surface, so that submarine scouring is possibly indicated at the time of rapidly changing sedimentation. The muddy sand of Cu 6 overlies the underlying loose sands conformably.
Thickness: 36 feet, locally reduced to nothing by a scour channel filled with Cu 8.
Subdivision: The 2 ft. basal shellbed of large pelecypods in coarse matrix is a conspicuous aquifer (Cu 5a). The shellbeds in the overlying sands (Cu 5b) are lenticular and developed at a number of horizons in the sand. They have not been mapped individually.
Cu 6. Upper Castlecliff Shellbed (R. S. A. 2).
Exposure: 30 to 140 chains north-west of the end of the cliff at Castlecliff, cut out for some 20 chains by scouring of Cu 8.
Lithology: Medium to fine muddy sandstone.
Fauna: Abundant mollusca and brachiopods.
Contacts: Conformable above and below.
Thickness: 7 feet.
Remarks: Although the fauna is rich, the bed has been somewhat leached and preservation is variable.
Cu 7. Tawera Silts (R. S. A. 1).
Exposure: For 120 chains north-west of Castlecliff, interrupted by the scour channel of Cu 8.
Lithology: Massive or poorly bedded fine micaceous grey siltstone with rare carbonaceous bands.
Fauna: Scattered mollusea; discontinuous lensing bands of shell dominated by Tawera 15–25 feet above base.
Contacts: Conformable over Cu 6; overlain by Cu 8 with strong disconformity.
Thickness: Up to about 35 feet, but cut out locally by Cu 8.
Subdivision: Cu 7a silts below Tawera band (Cu 7b); Cu 7c barren silts above Tawera band.
Cu 8. Estuarine Sands.
Exposure: For 145 chains north-west of Castleeliff.
Lithology: Bedded alternating sands, silts, and muds with scattered pebbles at base and carbonaceous partings; locally with fine current bedding.
Fauna: Rare shelly partings near base, and rolled estuarine shells at base, which is locally a surface bored by Anchomasa.
Contacts: Strongly disconformable on Cu 8, with a deep channel. about 20 chains wide in the section, scouring down to the base of Cu 5. Top not visible.
Thickness: 35 feet +; thickens to 50 feet +, in scour channel.