Comparing the depositional architecture of basin floor fans and slope fans in the Pab Sandstone, Maastrichtian, Pakistan

Abstract

Basin-floor fans and slope fans present major differences in their internal architecture related to changes in: (1) margin morphology, (2) relative sea-level change, and (3) sediment supply. These variations are illustrated in the outcrops of the Pab Sandstone in Pakistan. The Pab Sandstone third-order sequence was deposited on the Indo-Pakistani margin during the Upper Maastrichtian. Uplift of the margin induced erosion on the shelf, incision of submarine canyons on the slope and the development of a sand-rich, highefficiency basin-floor fan extending over hundreds of kilometres on the basin floor. During transgression, sediment accumulated in backstepping shoreface deposits on the shelf, and a minor mud-rich slope fan was deposited in the basin. Finally, a sand-rich braided delta prograded across the shelf, feeding a sand-rich slope fan where it reached the shelf margin. This slope fan was of more limited lateral extent. The Lower Pab basin-floor fan shows the effects of flow funnelling and confinement due to a canyon incised into the slope. It consists mainly of channel complexes deposited by superconcentrated density flows to lowdensity turbidity currents. In contrast, the Upper Pab slope fan shows little confinement and low transport efficiency. It consists of tabular lobes, aggrading mid-fan channels and conglomeratic channels in the upper fan. The low transport efficiency of the gravity flows probably explains the low degree of organization of the slope fan.

Connectivity estimation between turbiditic channels and overbank deposits from the modelling of an outcrop analogue (Pab Formation, Maastrichtian, Pakistan)

“…A chronostratigraphical and sequence stratigraphical interpretation of the margin has already been published by Eschard et al (2003Eschard et al ( , 2004. The geometry of the margin was also restored at a regional scale (Eschard et al 2003;Fig.…”

“…1a), with little structural deformation. The Maastrichtian palaeomargin morphology was preserved in these outcrops, and the downstream evolution of a turbiditic system can then be observed almost continuously from the turbiditic canyon area to the lobes (Eschard et al 2004) over more than 200 km. Along the monocline, side gullies also provided three-dimensional (3D) views of the channel complex internal architecture (Fig.…”

“…The Lower Pab Sandstone consists of three superposed channel -lobe complexes (LP-1, LP-2 and LP-3) separated by hemipelagites (Eschard et al 2004) (Fig. 2).…”

“…An interpretation of the depositional model of the Pab turbiditic fans was first proposed at a regional scale (Eschard et al 2004;Umar et al 2011). The facies architecture of the Pab turbitic channel complexes were then studied (Eschard et al 2003;Euzen et al 2007a).…”

The effects of diagenesis on the reservoir characters in sandstones of the Late Cretaceous Pab Formation, Kirthar Fold Belt, southern Pakistan

“…The paleocurrent was consistently from east to west in this part. Two successive deep marine turbidite systems, sand-rich basin floor lobes and sand-rich slope fans, were identified in the Southern Depositional System (Eschard et al, 2003(Eschard et al, , 2004Umar, 2007). The lower part of the formation is regarded as sand-rich basin floor turbidite lobes in a distal setting (north) and their corresponding bypass surface in the proximal setting.…”