DISCUSIÓN DE LOS RESULTADOS

shallow subaqueous water escape (c). Both types vary in size, in regularity of shape and in depth of penetration into the underlying sediment (up to several tens of cm). The filled V-shaped cracks in cross section may become ptygmatically folded due to compaction.

PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3

3.82 Thin horizon showing load-induced convolute lamina- tion in volcaniclastic siltstone– mudstone turbidite sandwiched between thick-bedded sandstone turbidites. Width of view 15cm. Pliocene, Boso Peninsula, near Tokyo, Japan.

3.83 Highly disorganized and contorted siltstone–mudstone units within turbidite slope suc- cession; interpreted as very rapid dumping of load from turbidity current as it spills over channel levee. Note that this is not bio- turbation. Hammer 25cm. Triassic–Jurassic, Los Molles, west central Chile.

3.84 Loading and possible scouring of siltstone/sandstone into mudstone, within inter - bedded siltstone/sandstone turbidites and dark grey mud- stone turbidites and hemi - pelagites. Hammer 30cm. Eocene, near Annot, SE France.

Deformed bedding and shale clasts

PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3

3.85Load (L) and flame (F) structures at base of normally graded volcaniclastic turbidite. Pale mud floating clasts within turbidite probably derived from detachment of flame structures. Miocene, Miura Basin, south central Japan.

3.86 Loads, flames and pseudonodules – P (as marked) within volcaniclastic turbidite succession. Width of view 25cm. Photo by Bob Foster.

Muzwezwe River, Zimbabwe.

3.87 Loads (L), flames (F), and pseudonodules (P) along base of graded sandstone turbidite (Bouma divisions ABC as marked). Width of view 30cm. Paleogene, S California, USA. F L L L F F P P C B A F P L

PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3 3.88 Shale-clast horizon (arrow) within structureless sandstone, representing broken- up mudstone interval between sandy turbidites.

Width of view 40cm. Eocene, Pera Cava, SE France.

3.89 Shale-clast-rich turbidite sandstone within deep-water turbidite succession; clasts possibly derived from local channel-bank collapse. The many different types of shale clast, their origins and occur- rence are illustrated in Figs 3.16, 3.17. Hammer 25cm. Cretaceous, Carmelo, central California, USA.

PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3 3.90 Water-escape burst-through structures (B) deforming parallel and cross- lamination into convolute lamination; deep-water turbidite succession. Width of view 30cm. Oligocene, Reitano Flysch, NE Sicily, Italy.

3.91 Large tepee structure in peritidal limestones, formed on the lee side of a barrier island. Tepee structures range from small-scale buckled polygonal desiccation cracks (typical desic- cation structures on carbonate tidal flats) to the larger and more complex features pictured here. Photo by Paul Potter. Width of view 2.5m. Permian, Carlsbad Cavern National Park, W Texas, USA.

3.92 Water-escape dish structures, together with burst- through/short pipe structures, in deep-water massive (turbidite) sandstone succession. Width of view 40cm. Eocene, Cantua Basin, central California, USA. B B B Water-escape and dessication structures

3.93 Water-escape dish structures (detail) in deep-water massive (turbidite) sandstone succession. Width of view 20cm. Eocene, Cantua Basin, central California, USA.

3.94 Water-escape pipe and sheet structures in deep-water massive (turbidite) sandstones. Width of view 30cm. Oligo–Miocene, Numidian Flysch, N Sicily, Italy.

3.95 Convolute lamination and burst-through structure (B) at top of sandstone turbidite bed. Lens cap 6cm.

Paleogene, southern California, USA. PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3 B

3.96 Vertical pipe/chimney structure indicative of large- scale water escape, in fan-delta sandstone succession. Hammer 45cm. Pliocene, near Carboneras, SE Spain.

3.97 Vent of small sand volcano (left of lens cap) through cal- carenite turbidite, now carbon- ate cemented and capped with parallel-laminated sandstone (upper division of turbidite). Lens cap 6cm.

Cretaceous, Ifach, SE Spain.

PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3

3.98 Bedding plane view of syneresis cracks (trilete and irregular star-shaped) on surface of mudstone bed. These form by subaqueous dewatering of sediments.

Jurassic, near Whitby, NE England.

3.99 Present day mudcracks (desiccation cracks) on dried out surface of raised mudflat. Recent, East Anglia, E England.

3.100Open irregular polygonal mudcracks (desiccation cracks) formed on ancient supratidal carbonate mudflats. Photo by Paul Potter. Hammer 25cm. Ordovician, Burksville, Cumberland, Kentucky, USA.

PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3

Biogenic sedimentary structures

THERE ARE MANYstructures formed in sedi- ments by the action of plants and animals. These include irregular disruption of the sedi- ments (bioturbation), discrete organized markings (trace fossils or ichnofossils), and biogenic growth structures (e.g. stromato- lites). Some modern organic markings (such as borings and surface trails on rocks), as well as inorganic structures (such as syneresis cracks, water-escape structures, concretions) can be confused with trace fossils in certain instances.

Bioturbation

(Fig. 3.20; Plates3.101– 3.120)

Bioturbation refers to the irregular disruption of sediment by plants and animals, rather than organized and recognizable burrows or other traces. It is commonly described in terms of intensity (or percentage bioturbated), ranging from no or sparse bioturbation to

intense or complete bioturbation. The se di- ment appearance changes from slightly mottled, often with distinct trace fossils, to thoroughly churned. Bioturbation always accompanies trace fossil activity.

Observe and measure:

1. The degree or intensity of bioturbation. 2. The different sediment types intermixed. 3. Any distinct trace fossils (see below).

Trace fossils

(Fig. 3.21, 3.22, 3.23; Plates3.101– 3.122) The study of trace fossils (ichnofossils) as part of sediment facies can reveal much comple- mentary information to that gained from observing primary (dynamic) structures on the one hand and true body fossils on the other. Although they are treated in some respects as any other type of fossil, having ichnogenus and ichnospecies names, they are formed very much by the interaction of organisms with the sediment and only very rarely reveal the true identity of their archi- tects (ie the organism that formed them).

Trace fossils can be classified in terms of their mode of preservation, within a mud- stone or sandstone bed or at a boundary between the two, but this does little more than convey information regarding their mor- phological expression (toponomy). A more revealing classification, preferred here, is in terms of the behaviour (ethology) of the organism that formed them. This, after all, provides greater insight into the depositional environment. There are now 13 different groups recognized in this classification, neces- sarily with some behavioural overlap between groups. PRINCIP AL CHARACTERISTIC S OF SEDIMENT ARY ROCKS 3 0 0 No bioturbation

1 1–4 Sparse bioturbation, bedding distinct, few discrete traces and/ or escape structures

2 5–30 Low bioturbation, bedding distinct, low trace density, escape structures may be common 3 31–60 Moderate bioturbation, bedding

boundaries sharp, traces discrete, overlap rare

4 61–90 High bioturbation, bedding boundaries indistinct, high trace density with overlap common 5 91–99 Intense bioturbation, bedding completely disturbed (but just visible), limited reworking, later burrows discrete

6 100 Complete bioturbation, sediment reworking due to repeated overprinting

Grade Percent Classification bioturbated

3.20 Bioturbation index (after Tucker 1996).