Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin, North-eastern Spain

Abstract

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.

The Oligocene‐Miocene Guadalope‐Matarranya Fan, Spain, as an Analog for Long‐Lived, Ridge‐Bearing Megafans on Mars

“…We interpret the ridges in the Guadalope-Matarranya fan to be exhumed and eroded channel belts, rather than inverted channels, similar to previous work in the area (Cuevas Martínez et al, 2007Gonzàlez-Bonarino et al, 2010;Friend et al, 1979;Mohrig et al, 2000;Williams, 1975). Ridge caprocks exhibit amalgamated strata from dunes and bars (Figures 3 and 6), indicating channel aggradation and channel lateral migration created a channel belt.…”

“…Cuevas Martínez et al (2007Martínez et al ( , 2010 observed pedogenically altered mudstone laminae between some downstream-accreting bars and putative ant nests within the bars, both of which they interpreted to represent intermittent fluvial activity. Gonzàlez-Bonarino et al (2010) also found bioturbation and small-scale sedimentary structures and interpreted that low or no flows existed between the main bar-forming flood events. Milankovitch-cycle-paced cyclicity is also evident in the lacustrine deposits of the Los Monegros Formation at the toe of the Guadalope-Matarranya fan system (Pérez-Rivarés et al, 2004;Valero et al, 2014).…”

“…Channel belts range from 1 to 15 m thick and are up to 90 m wide, with width‐to‐thickness ratios averaging 6 with standard deviation of 2 (Cabrera et al., 1985; Cuevas Martínez et al., 2010). The most common sedimentary structures in the channel belts are bar‐scale cross‐beds, interpreted to represent downstream‐migrating alternate bars, and laterally accreting bank‐attached bars based on whether their accretion directions are more parallel or perpendicular, respectively, to the accretion direction of paleoflow indicators like dune cross‐sets (Cuevas Martínez et al., 2007; Friend et al., 1986; Gonzàlez‐Bonarino et al., 2010; Mohrig et al., 2000). Many channel bodies are amalgamations of distinct generations of fluvial deposits, some of which are separated by thin mudstone drapes.…”

Form, evolution and controls of a Jurassic incised valley‐fill: middle part of the Western Sichuan Depression, China

“…Tectonic uplift and climatic changes can drive base level; however, their influence varies from upstream to downstream. The mid‐ to downstream segments of rivers are primarily controlled by lake or sea‐level (analogous to the base level) changes (Hampson et al ., ; Alqahtani et al ., ), while the upstream segment of rivers may be more controlled by tectonism (Shanley and McCabe, ; Mattheus et al ., ; González‐Bonorino et al ., ; Alqahtani et al ., ).…”

“…in the late lowstand systems tract (LST) and early transgressive systems tract (TST) (Vail et al ., ; Posamentier and Vail, ; Van Wagoner et al ., ; Zaitlin et al ., ; Willis, ; Veiga et al ., ; Catuneanu, ; Catuneanu et al ., ). New models (Rajchl and Ulicny, ; Gouw and Autin, ; González‐Bonorino et al ., ; Hampson et al ., ; Alqahtani et al ., ) focus on the planform geometric variation in incised valleys and analyse their controlling factors from upstream to downstream.…”