Soft‐sediment deformation structures in late Albian to Cenomanian deposits, São Luís Basin, northern Brazil: evidence for palaeoseismicity

Abstract

Soft‐sediment deformation structures from the Alcântara Formation (late Albian to Cenomanian), São Luís Basin, northern Brazil, consist of (1) contorted structures, which include convolute folds, ball‐and‐pillow structures, concave‐up paths with consolidation lamination, recumbently folded cross‐stratification and irregular convolute stratification that grades into massive beds; (2) intruded structures, which include pillars, dykes, cusps and subsidence lobes; and (3) brittle structures, represented by fractures and faults displaying planes with a delicate, ragged morphology and sharp peaks. These structures result from a complex combination of processes, mostly including reverse density gradients, fluidization and liquefaction. Reverse density gradients, promoted by differential liquefaction associated with different degrees of sediment compaction, led to the genesis of convolute folds. More intense deformation promoted the development of ball‐and‐pillow structures, subsidence lobes and sand rolls, which are attributed to denser, and thus more compacted (less liquefied), portions that sank down into less dense, more liquefied sediments. Irregular convolute stratification that grades into massive beds would have formed at periods of maximum deformation. The subsidence of beds was accompanied by lateral current drag and fluid escape from water‐saturated sands. In addition, the fractures and faults record brittle deformation penecontemporaneous with sediment deposition. All these mechanisms were triggered by a seismic agent, as suggested by a combination of criteria, including (1) the position of the study area at the edge of a major strike‐slip fault zone that was reactivated several times from the Albian to the Holocene; (2) a relative increase in the degree of deformation in sites located closer to the fault zone; (3) continuity of the deformed beds over large distances (several kilometres); (4) restriction of soft‐sediment deformation structures to single stratigraphic intervals bounded by entirely undeformed strata; (5) recurrence through time; and (6) similarities to many other earthquake‐induced deformational structures.

The Traces of Earthquake (Seismites): Examples from Lake Van Deposits (Turkey)

“…Some structures are remaining connected to the overlying bed or the others are completely isolated from the bed. These structures in Lake Van sediments are very similar to presented in previous works (Hempton et al, 1983;Allen, 1986;Rossetti, 1999). Ball and pillow structures occur with liquefaction of unconsolidated sediments.…”

“…Deformation structures exist in different levels of these shallow water deposits with the other sedimentary structures as cross-beds and wave ripples. Soft sediment deformation structures are classified differently according to morphologic properties or occurrence processes of the structure (Rossetti, 1999;Dramis & Blumetti, 2005;Neuwerth et al, 2006;Taşgın & Türkmen, 2009). In this study, soft sediment deformation structures, observed in lacustrine deposits of Lake Van, were classified as contorted structures (simple-complex convolute bedding and ball-pillow structures), load structures (flame structures) and water escape structures (dish and pillar structures).…”

“…The most known occurence of soft sediment deformation structures are overpressure of sediments (Lowe & LoPiccolo, 1974;Lowe, 1975), storm waves (Molina et al, 1998;Alfaro et al, 2002) and seismic shakings (Seilacher, 1969;Lowe, 1975;Sims, 1975;Rossetti, 1999;Vanneste et al, 1999;Jones & Omoto, 2000;Rodriguez-Pascua et al, 2000;Bowman et al, 2004). Deformation structures in lacustrine deposits of Lake Van were evaluated in the light of these trigger mechanism.…”

Distal turbidite fan/lobe succession of the Late Oligocene Zuberec Fm. – architecture and hierarchy (Central Western Carpathians, Orava–Podhale basin)

“…The generating mechanism of convolute structures is linked to fluidization processes, which create gravitational instabilities [73][74][75]. The triggering mechanism of these structures is often related to processes of sediment gravity flows, overloading of sandstone beds [e.g.…”

W jaki sposób badać sejsmity? Przegląd metod badawczych

“…Dokładny opis litofacjalny, a następnie interpretacja genetyczna warunków środowiska jest niezbędna do odczytania prawidłowych implikacji paleogeograficznych. Powszechnie wiadomo, że struktury deformacyjne spowodowane rozchodzeniem fali sejsmicznej zapisują się przede wszystkim w osadach uwodnionych i udział wody jest niezbędny do ich powstania (Lowe 1976;Allen 1982;Rossetti 1999;Owen 1996;Van VlietLanoë i in. 2004).…”