Trace fossils in estuarine deposits of different ages have been compared to evaluate colonization history of brackish-water ecosystems and to calibrate trace-fossil, brackish-water models with respect to geologic time. This comparative analysis reveals that, although the colonization of marginal-marine, brackish-water environments was a long-term process that spanned most of the Phanerozoic, this process of invasion of fully marine organisms into restricted, marginal-marine habitats did not occur at a constant rate. Five major colonization phases can be distinguished. The first phase (Ediacaran–Ordovician) represents a prelude to the major invasion that occurred during the rest of the Paleozoic. While Ediacaran–Cambrian ichnofaunas seem to be restricted to the outermost zones of marginal-marine depositional systems, Ordovician assemblages show some degree of landward expansion within brackish-water ecosystems. Intensity of bioturbation and ichnodiversity levels were relatively low during this pha...

https://bioone.org/journals/palaios/volume-20/issue-4/palo.2004.p04-32/Colonization-of-Brackish-Water-Systems-through-Time--Evidence-from/10.2110/palo.2004.p04-32.pdf

Colonization of Brackish-Water Systems through Time: Evidence from the Trace-Fossil Record

Publisher Summary This chapter reviews the historical context and describes the different contourite facies recognized. The contourite facies models are outlined and the interpretation of contourite sequences is discussed. Some hybrid contourite facies are considered and criteria for the distinction between contourites and associated facies in deep-water systems are focused on. The problems and controversy surrounding the recognition of fossil contourites in the ancient records arealso described. The range of recognized contourite facies, including the facies groups, are tabulated. . Muddy contourites are homogeneous and appear as thick featureless units. Silty contourites have a larger silt-sized component and potential for revealing some internal structure. Gravel-rich contourites and gravel-bearing contourites are common in drifts at high latitudes as a result of input from ice-rafted material. The facies models for both muddy and sandy contourites are originally based on data from many examples of contourites. Composite contourite facies model showing grain-size variation through the standard mud-silt-sand contourite sequence, linked to variation in contour-current velocity is diagrammatically represented.

Chapter 13 Contourite Facies and the Facies Model

Deep-marine tidal bottom currents and their reworked sands in modern and ancient submarine canyons

An occurrence of inclined heterolithic stratification (IHS) is described from a tidal point bar in a 40-m-deep distributary of the macrotidal (tidal range 3.6-7.8 m), Han River delta, Korea. The channel bank demonstrates a convex-upward profile with intermittent presence of wave-formed scarps and terraces near the low-water level. The vertical succession of IHS is approximately 25 m thick and dips into the channel with angles reaching 14°. The IHS overlies 15 m of trough cross-bedded sand deposited in the channel thalweg and lower point bar. Even though the channel as a whole is ebb dominated, the preserved cross bedding is predominantly flood directed because the mutually evasive nature of the ebb and flood currents causes the point-bar surface to be flood dominated. This pattern may be a common feature of tidal point bars. The IHS itself consists of interstratified fine sand, sandy silt, and silt with an upward-fining textural trend. Tidal rhythmites are well developed in the middle and upper intertidal zone, and may also be present in the subtidal zone, but are poorly developed near the low-water level because of wave action. Seasonal discharge variations of the Han River are not obvious in the deposits, because the large size, distal location, and energetic tidal environment of the studied channel reduces the impact of river-stage fluctuations. Despite the moderate salinity levels, bioturbation is rare, except in the upper intertidal zone, because of the rapid sedimentation and energetic conditions.

Sedimentology of Modern, Inclined Heterolithic Stratification (IHS) in the Macrotidal Han River Delta, Korea

There has been a lively debate since the 1980s on distinguishing between paleo-tsunami deposits and paleo-cyclone deposits using sedimentological criteria. Tsunami waves not only cause erosion and deposition during inundation of coastlines in subaerial environments, but also trigger backwash flows in submarine environments. These incoming waves and outgoing flows emplace sediment in a wide range of environments, which include coastal lake, beach, marsh, lagoon, bay, open shelf, slope and basin. Holocene deposits of tsunami-related processes from these environments exhibit a multitude of physical, biological and geochemical features. These features include basal erosional surfaces, anomalously coarse sand layers, imbricated boulders, chaotic bedding, rip-up mud clasts, normal grading, inverse grading, landward-fining trend, horizontal planar laminae, cross-stratification, hummocky cross-stratification, massive sand rich in marine fossils, sand with high K, Mg and Na elemental concentrations and sand injections. These sedimentological features imply extreme variability in processes that include erosion, bed load (traction), lower flow regime currents, upper-flow regime currents, oscillatory flows, combined flows, bidirectional currents, mass emplacement, freezing en masse, settling from suspension and sand injection. The notion that a ‘tsunami’ event represents a single (unique) depositional process is a myth. Although many sedimentary features are considered to be reliable criteria for recognizing potential paleo-tsunami deposits, similar features are also common in cyclone-induced deposits. At present, paleo-tsunami deposits cannot be distinguished from paleo-cyclone deposits using sedimentological features alone, without historical information. The future success of distinguishing paleo-tsunami deposits depends on the development of criteria based on systematic synthesis of copious modern examples worldwide and on the precise application of basic principles of process sedimentology.

Process-sedimentological challenges in distinguishing paleo-tsunami deposits

The article by Shanmugam et al. (2000) is a welcome addition to the scant sedimentological literature on the Oriente Basin and its continuation in Colombia, the Putumayo Basin (Higgs, 1997a). Shanmugam et al. (2000) reinterpreted the Hollin and Napo T-U sands as tide-dominated estuarine deposits, rejecting a previous deltaic model. Neither model, however, accords with the stated lack of evidence for subaerial emergence. An alternative, tidal shelf interpretation is promoted here, and petroleum reservoir implications discussed. Also, although Shanmugam et al. (2000) did not report any sequence boundaries, their data suggest that angular (tectonic) sequence boundaries occur at the base of the A and B limestones, signifying possible incised valleys cutting into the U and T sands, again important for oil exploration.

### Lack of Emergence Indicators

The 516 ft (157 m) of cores from seven wells examined by Shanmugam et al. (2000) show a simple facies association of shales, heterolithics, and cross-bedded sands. "[N]o evidence for subaerial exposure" exists (Shanmugam et al., 2000, p. 674); for example, "rooting is lacking" (Shanmugam et al., 2000, p. 660) and no coal beds, paleosoils, or desiccation cracks were reported. Sparse interpreted marsh deposits (<1% of one 58 ft core) (Shanmugam et al., 2000, table 2) are based on nondiagnostic interpretation criteria, that is, sandstone with "intervals of concentrated carbonaceous fragments … may be interpreted as a marsh environment" (Shanmugam et al., 2000, p. 660). Alternatively, such plant-flake concentrations could occur on a shelf because of tidal-current segregation of particles. An interpreted "fluvial channel" is based solely on "cross-stratification and basal lags" (Shanmugam et al., 2000, p. 656), both of which can also apply to submarine tidal bars (e.g., Stride et al., 1982, figure 5.22; Dalrymple, 1992, figure 29C).

The same facies association and absence or scarcity of emergence indicators characterizes the Hollin and Napo T-U …

J. Toro Álava

Papers

Tide-Dominated Estuarine Facies in the Hollin and Napo ("T" and "U") Formations (Cretaceous), Sacha Field, Oriente Basin, Ecuador