Paleohydrology and 3D Facies Architecture of Ancient Point Bars, Ferron Sandstone, Notom Delta, South-Central Utah, U.S.A

Abstract

Most outcrops of fluvial deposits consist of a series of cliff exposures, either natural or manmade (e.g., roadcuts). Predictions, and especially observation of plan-form geometry, such as might be made with numerical experiments or from studies of modern rivers, are challenging to test in most ancient outcrops. This study examines ancient exhumed channel belts from the Cretaceous Notom Delta of the Ferron Sandstone Member in south-central Utah. Extensive plan-view exposures with local vertical cliff exposures allowed documentation of channel plan-form, channel-belt dimension, bar migration patterns (translation versus expansion), and cross-sectional facies architecture. Channel-fill thickness and bedding structure, documented from the cliff exposures, were used in paleohydraulic reconstructions. Approximately 270 paleocurrent directions were integrated with grain-size measurements to reconstruct the 3D facies architecture. Paleocurrent measurements are consistent within specific facies architectural units (such as unit bars) and show systematic variation at the channel-belt scale that can be used to infer channel and bar migration patterns. In this example, the migration pattern of the single-thread channel bend was interpreted to change from expansion to translation with a corresponding bend sinuosity that increased from 1.01 to 1.44. Inclined large-scale foresets are interpreted to be indicative of unit bars. Empirical equations result in estimated average channel depths from 1.7 m to 3.6 m with corresponding widths of 23 m to 89 m respectively. These empirical estimates match the dimensions measured in the field. For example, channel widths of around 50 m were measured from abandoned channel fills. Bar thickness, measured from vertical outcrops, ranges from 5.4 m to 6.3 m, which yields a narrower estimate of channel width ranging from 47 to 59 m. Integration of sediment size, bedforms type, and channel depth were used to estimate the discharge of the river, which is on the order of , 400 m 3 /s. This suggests that the Ferron deltas were characterized by small, steep-gradient ''dirty'' rivers, which is consistent with the hyperpycnal nature of linked downstream deltaic systems.

Integrated sedimentology and ichnology of Late Jurassic fluvial point‐bars – facies architecture and colonization styles (Lourinhã Formation, Lusitanian Basin, western Portugal)

“…An independent check on this figure was provided by relating cross‐set data to dune height and hence formative flow depth (Bridge & Tye, ; Leclair & Bridge, ; Wu et al ., ). This was feasible because the standard deviation to mean thickness ratio of the cross‐sets was approximately equal to 0·88(±0·3) (Bridge & Tye, ).…”

“…Palaeocurrent data were collected to determine local palaeotransport directions, whilst dune cross‐set thicknesses were collected for the purpose of calculating dune heights and hence formative flow depths (cf. Bridge & Tye, ; Leclair & Bridge, ; Wu et al ., ). Dip azimuth data for internal depositional surfaces were also collected.…”

Recognition criteria, characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation, Argentina)

“…At dune-scale, the bidirectionality could be tidal or an apparent feature due to 2D exposure of trough cross-bedding or of the fluvial bar. For example, detailed work from the Ferron Sandstone in Utah, USA (Wu et al, 2015), indicated a high spread of palaeocurrents that was explained by the stacking pattern of point-bar deposits, and probably purely fluvial conditions. Am4: heterolithic deposits Heterolithic deposits (Fig.…”

Formation of point bars through rising and falling flood stages: Evidence from bar morphology, sediment transport and bed shear stress

“…Point bars with ‘ridge and swale’ topography are traditionally thought to be common features in meandering rivers (Gibling & Rust, ), whereas amalgamated compound point bars are increasingly recognized as important architectural components in low sinuosity, meandering or braided rivers (Lunt & Bridge, ; Moody & Meade, ; Wu et al ., ). Despite having a distinct facies architecture, formative sinuous channels associated with point bars show migration patterns ranging from lateral expansion to downstream translation in a number of modern (Hickin, ) and ancient examples (Wu et al ., ) and in several flume and theoretical studies (Peakall et al ., ; Duan & Julien, ). Moreover, recent flume studies (van de Lageweg et al ., ) and theoretical models (Eke et al ., ) show that the deposition of point bars in meandering rivers could be a passive response of bank retreat (also known as ‘bank pull’).…”

“…Flow processes associated with different types of point bars have rarely been documented in ancient fluvial deposits, because along‐stream changes of bar characteristics are difficult to document in two‐dimensional (2D) outcrops and their plan‐view forms must be extrapolated. Despite an increased focus on plan‐view macroform geometry in ancient outcrops (Brooks, ; Foix et al ., ; Ielpi & Ghinassi, ; Bhattacharyya et al ., ; Wu et al ., ), the signatures of flow processes preserved in the rock record have rarely been investigated.…”