Sedimentology and sequence architecture of incised-valley fills and interfluvial deposits: upper Mannville interval (Lower Cretaceous), Little Bow-Turin area, southern Alberta

摘要

Deposition of the upper Mannville interval (Lower Cretaceous) in the Little Bow-Turin area in southern Alberta occurred on the cratonic side of the Cordilleran foreland basin and evolved in response to changes in base level and sediment supply. Incised-valley fills and interfluvial paleosols in two stratigraphic units of the upper Mannville interval, the Glauconitic Sandstone and the overlying division A of an upper lithic member, provide evidence for multiple cycles of fluvial incision and subaerial exposure followed by deposition in fluvial, estuarine and coastal plain environments. These cycles, which are interpreted to have formed predominantly in response to fluctuations of base level, resulted in stacked, high-frequency, unconformity-bounded sequences. Incised valleys in the Glauconitic Sandstone are filled predominantly with inner to middle estuarine deposits, whereas those in division A are filled predominantly with fluvial deposits. The infills of division A valleys thus formed in a more proximal landward position than those of Glauconitic valleys. Geographic position on the coastal plain therefore had an important influence on the depositional style of the high-frequency sequences. A change in the ratio of sediment supply to creation of accommodation space (supply/space ratio) may also have played a significant role in -producing the distinctive depositional differences between Glauconitic and division A sequences. The predominance of estuarine sediments in Glauconitic valleys suggests that facies shifted rapidly landward during valley filling due to a low supply/space ratio. The predominance of fluvial deposits and lesser amounts of estuarine deposits in division A valleys suggests that facies may have shifted more slowly landward during valley filling due to a higher supply/space ratio.%La sédimentation de l'intervalle du Mannville supérieur (Crétacé inférieur) de la région Little Bow-Turin du sud de l'Alberta s'est produite du côté cratonique du bassin sédimentaire de l'avant-pays de la cordillère et s'est développ╚ en réponse aux changements du seuil et de la réserve de sédiments. Les remblais des vallées encaissées et les paléosols interfluviaux dans deux unités stratigraphiques de l'intervalle Mannville supérieur, le grès glauconieux et la division A sus-jacente d'un membre supérieur de dépôts à nombreux fragments de roches plus anciennes, offrent une preuve des cycles multiples d'encaissements fluviatiles et d'affleurements subaériens suivis par la sédimentation d'environnements fluviatiles, estuariens et de plaine du littoral. Ces cycles, qui sont interprétés comme ayant été formés de façon prédominante en réponse aux fluctuations du niveau du seuil, ont produit des séquences empilées, de hautes fréquences limitées par une discordance.