Plan‐form evolution of ancient meandering rivers reconstructed from longitudinal outcrop sections

Abstract

The mode of channel-bend transformation (i.e. expansion, translation, rotation or a combination thereof) has a direct bearing on the dimensions, shape, bedding architecture and connectivity of point-bar sandstone bodies within a fluvial meander belt, but is generally difficult to recognize in vertical outcrops. This study demonstrates how the bend transformation mode and relative rate of channel-floor aggradation can be deciphered from longitudinal outcrop sections aligned parallel to the meander-belt axis, as a crucial methodological aid to the reconstruction of ancient fluvial systems and the development of outcrop analogue models for fluvial petroleum reservoirs. The study focuses on singlestorey and multi-storey fluvial meander-belt sandstone bodies in the Palaeogene piggyback Boyabat Basin of north-central Turkey. The sandstone bodies are several hundred metres wide, 5 to 40 m thick and encased in muddy floodplain deposits. The individual channel-belt storeys are 5 to 9 m thick and their transverse sections show lateral-accretion bed packages representing point bars. Point bars in longitudinal sections are recognizable as broad mounds whose parts with downstream-inclined, subhorizontal and upstream-inclined bedding represent, respectively, the bar downstream, central and upstream parts. The inter-bar channel thalweg is recognizable as the transition zone between adjacent point-bar bedsets with opposing dip directions into or out of the outcrop section. The diverging or converging adjacent thalweg trajectories, or a trajectory migrating in up-valley direction, indicate point-bar broadening and hence channel-bend expansion. A concurrent down-valley migration of adjacent trajectories indicates channel-bend translation. Bend rotation is recognizable from the replacement of a depositional riffle by an erosional pool zone or vice versa along the thalweg trajectory. The steepness of the thalweg trajectory reflects the relative rate of channel-floor aggradation. This study discusses further how the late-stage foreland tectonics, with its alternating pulses of uplift and subsidence and a progressive narrowing of the basin, has forced aggradation of fluvial channels and caused vertical stacking of meander belts.

A 3D forward stratigraphic model of fluvial meander-bend evolution for prediction of point-bar lithofacies architecture

“…Upstream-bar deposits of the expansional point bar, by comparison, are for the most part well preserved, especially for higher sinuosity meanderbend deposits that occur further away from the axis of the channel belt. This is consistent with field observations reporting that translating point bars exhibit systematic erosion and sediment bypass (Ghinassi and Ielpi, 2015;Ghinassi et al, 2016;Ghinassi et al, 2014;Ielpi and Ghinassi, 2014;Smith et al, 2011;Smith et al, 2009). …”

“…Ghinassi et al 2014); (2) the algorithm to construct internal architecture and geometry of vertical cross sections of point-bar elements; and (3) the algorithm to simulate lithofacies distributions within bar-form elements. This study focuses solely on modelling single point-bar elements in the subsurface.…”

“…Mud drapes in both examples are limited to the top of inclined bar surfaces ( Fig. 11 and 12), akin to field observations in the Scalby Formation (Ghinassi et al, 2016;Ghinassi et al, 2014;Ielpi and Ghinassi, 2014). (dark blue to dark red) scheme is used to differentiate meander positions at different times.…”

“…Several fundamental types of transformations are recognised: expansion, translation, rotation, and combinations thereof (Brice, 1974;Daniel, 1971;Ghinassi et al, 2014;Jackson, 1976;Makaske and Weerts, 2005). However, relationships between the migratory behaviour of a river, the geometry of accumulated sedimentary bodies (e.g., point bars, counter-point bars)…”

Geomorphology Processes of Channel Planform Migration on Meandering Rivers

“…Despite large numbers of literatures from both modern and ancient depositional systems of fluvials, analyzing the dynamic geomorphology evolution of meandering rivers remains a challenge (Willis and Tang, 2010). Attempts have been done to reconstruct the characteristics of palepchannel by understanding the planform migration structure of channel and evolution of dynamic geomorphic process (Ielpi and Ghinassi, 2014;Ghinassi et al, 2014;Wu et al, 2016). Although researches about the morphology and migration of meandering channels are attempted (Schuurman et al, 2016;Rousseau et al, 2016), it still remains poorly undefined how to account for the processes of migration structures.…”