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2 Acta Geologica Polonica

2 2023

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Stratigraphic Emmental: High-resolution allostratigraphy reveals multiple intra-formational unconformities in shallow ramp mudstone: Upper Cretaceous, Western Canada Foreland Basin

Hooper Elizabeth A., Plint A. Guy

Acta Geologica Polonica

2023

Vol. 73, no. 4

773--799

Marine mudstone of Coniacian age (c. 89.51–86.49 Ma) was deposited on a storm-dominated ramp spanning the foredeep of the Cretaceous Western Canada Foreland Basin. Marine flooding surfaces define 18 allomembers that thin over 300 km, from c. 140 m in the proximal foredeep to c. 20 m close to the forebulge crest. The broadly conformable succession of allomembers is partitioned into five ‘tectono-stratigraphic units’ by low-angle unconformities that bevel off c. 10 to 20 m of strata over ‘arches’ that have a length scale of c. 50–100 km and are bounded by relatively linear zones of flexure. Depositional history involved two alternate modes: ‘Background’ deposition of subtly-tapered allomembers took place on a planar sea floor, subject to regional flexural subsidence, with sea-level modulated by Milankovitch-scale (c. 125 kyr) eustatic cycles. ‘Flexural’ events deformed the strata into troughs and arches across narrow zones of flexure. Arch crests were bevelled off, probably by submarine wave erosion. Eroded sediment did not accumulate in troughs but was advected beyond the study area by storm-driven processes. Cycles of deposition, warping and erosion were repeated five times on an average timescale of 600 kyr. Arches and troughs do not coincide with Precambrian basement structures, and their origin remains enigmatic. Changes in in-plane stress may have effected the localized vertical motion.

On some Upper Cretaceous ammonites from western Uzbekistan

Ward David J., King Chris, Morris Noel J., Kennedy William James

Acta Geologica Polonica

2023

Vol. 73, no. 4

613–-634

Upper Cretaceous ammonites are described from six horizons in the mammal-bearing terrigenous-clastic sequences of western Uzbekistan, as follows: Upper Cenomanian, with Placenticeras sp. juv. cf. cumminsi Cragin, 1893 and Metoicoceras geslinianum (d’Orbigny, 1850); Lower Turonian with Tragodesmoceras cf. mauryae Kennedy and Wright, 1981, Placenticeras kharesmense (Lahusen, 1884), Watinoceras coloradoense (Henderson, 1908), Watinoceras amudariense (Arkhangelsky, 1916b), Metasigaloceras rusticum (J. Sowerby, 1823), Morrowites wingi (Morrow, 1935), Sciponoceras cf. bohemicum bohemicum (Fritsch, 1872), and Yezoites amudariensis (Arkhangelsky, 1916b); Middle Turonian with Collignoniceras woollgari woollgari (Mantell, 1822); Upper Turonian with Lewesiceras mantelli (Wright and Wright, 1951) and Placenticeras crassum Ilyin, 2020; a single Placenticeras semiornatum (d’Orbigny, 1850) from around the Coniacian/Santonian boundary, and Santonian Placenticeras polyopsis (Dujardin, 1837).