Anatomy of straight vs meandering fluvial channels in Triassic red beds of S Spain and W Morocco. Implications as reservoir rocks

César Viseras, Juan Fernández Martínez, Saturnina Henares, Sila Pla Pueyo

Research output: Contribution to conferenceAbstract


Two successions of Triassic red beds developed in a continental rift tectonic setting have been studied. One
of them is located in the tabular cover of the Iberian Meseta, Spain (TIBEM) and the other in the western end of
the High Atlas, Morocco (THATLAS). Among the most characteristic architectural elements of both successions
are sandy sedimentary bodies resulting from the dynamics of meandering and straight channels.
Deposition in meandering channels can lead to the formation of sedimentary bodies of high lateral extent (up
to 400-500 m). However, in the two studied examples we found that under high subsidence or rising base level
conditions, frequent avulsion processes have resulted in the development of ribbon-type bodies with similar
geometry to that of the straight channels (2-4 m thick lenses with 30-60 m of lateral continuity). Both types of
channels can therefore be easily confused in exploration geology.
The bodies generated by the dynamics of meandering channels consist of three elements corresponding to
the sub-environments of point bar, sinuous channel and chute channel. The point bar displays the lateral
accretion units with epsilon cross-bedding and irregularities at the top due to scroll bars and swell depressions
between them. The sand layers of the lateral accretion units (thickness between 10 and 30 cm) show
interbedding of centimetric clay levels deposited at times of inactivity of the bar. The sinuous channel is filled by
rippled sand that alternates with settling of lutite. The chute channels have a similar behaviour to that of braided
or straight channels.
The straight channels show a characteristic expansion wing towards the top. Their filling occurs in several
phases related to flow regime fluctuations, giving way to the development of megarriples and scouring surfaces.
The abandonment phase produces climbing ripples and a large proportion of shaly sediment.
As they may result in reservoirs of similar size and shape, the differentiation between both types of bodies
from subsurface data may have an important implication from the exploration point of view. The bodies
generated by high sinuosity fluvial dynamics present their most porous facies in the lateral accretion units of the
point bars, although the clay beds developed during inactivity times can act as barriers to the movements of
fluids. The sand bodies related to straight channels behave as more homogeneous reservoirs, with much more
porous facies at the lower part and a less porous top transitioning to the seal, represented by the overlying
overbank fines.
Original languageEnglish
Number of pages1
Publication statusPublished - 2011
Event28th IAS Meeting of Sedimentology - Zaragoza, Spain
Duration: 5 Jul 20118 Jul 2011


Conference28th IAS Meeting of Sedimentology


  • fluvial
  • Triassic
  • Morocco
  • Spain
  • red beds
  • Reservoirs

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