5.3: Sedimentary Structures

Sedimentary structures are visible textures or arrangements of sediments within a rock. Geologists use these structures to interpret the processes that made the rock and the environment in which it formed. They use uniformitarianism to usually compare sedimentary structures formed in modern environments to lithified counterparts in ancient rocks. Below is a summary discussion of common sedimentary structures that are useful for interpretations in the rock record.

Bedding Planes

Figure \(\PageIndex{1}\): Horizontal strata in southern Utah.

The most basic sedimentary structure is bedding planes, the planes that separate the layers or strata in sedimentary and some volcanic rocks. Visible in exposed outcroppings, each bedding plane indicates a change in sediment deposition conditions. This change may be subtle. For example, if a section of underlying sediment firms up, this may be enough to create a form or a layer that is dissimilar from the overlying sediment. Each layer is called a bed.

Figure \(\PageIndex{2}\): Students from the University of Wooster examine beds of Ordovician limestone in central Tennessee.

As would be expected, bed thickness can indicate sediment deposition quantity and timing. Technically, a bed is a bedding plane thicker than 1 cm (0.4 in) and the smallest mappable unit.

Graded Bedding

Figure \(\PageIndex{3}\): Image of the classic Bouma sequence. A=coarse- to fine-grained sandstone, possibly with an erosive base. B=laminated medium- to fine-grained sandstone. C=rippled fine-grained sandstone. D=laminated siltstone grading to mudstone.

Graded bedding refers to a sequence of increasingly coarse- or fine-grained sediment layers. Graded bedding often develops when sediment deposition occurs in an environment of decreasing energy. A Bouma sequence is graded bedding observed in a clastic rock called turbidite [24]. Bouma sequence beds are formed by offshore sediment gravity flows, which are underwater flows of sediment. These subsea density flows begin when sediment is stirred up by an energetic process and becomes a dense slurry of mixed grains. The sediment flow courses downward through submarine channels and canyons due to gravity acting on the density difference between the denser slurry and less dense surrounding seawater. As the flow reaches deeper ocean basins it slows down, loses energy, and deposits sediment in a Bouma sequence of coarse grains first, followed by increasingly finer grains (see figure).

Flow and Bedforms

Figure \(\PageIndex{4}\): Bedforms from under increasing flow velocities.

In fluid systems, such as moving water or wind, sand is the most easily transported and deposited sediment grain. Smaller particles like silt and clay are less movable by fluid systems because the tiny grains are chemically attracted to each other and stick to the underlying sediment. Under higher flow rates, the fine silt and clay sediment tend to stay in place and the larger sand grains get picked up and moved.

Ripples

Figure \(\PageIndex{6}\): Modern current ripple in sand from the Netherlands. The flow creates a steep side down current. In this image, the flow is from right to left.

Ripples are known by several names: ripple marks, ripple cross-beds, or ripple cross laminations. The ridges or undulations in the bed are created as sediment grains pile up on top of the plane bed. With the exception of dunes, the scale of these beds is typically measured in centimeters. Occasionally, large flows like glacial lake outbursts can produce ripples as tall as 20 m (66 ft).

Figure \(\PageIndex{7}\): A bidirectional flow creates this symmetrical wave ripple. From rocks in Nomgon, Mongolia. Note the crests of the ripples have been eroded away by subsequent flows in places.

First scientifically described by Hertha Ayrton [26], ripple shapes are determined by flow type and can be straight-crested, sinuous, or complex. Asymmetrical ripples form in a unidirectional flow. Symmetrical ripples are the result of an oscillating back-and-forth flow typical of intertidal swash zones. Climbing ripples are created from high sedimentation rates and appear as overlapping layers of ripple shapes (see figure).

Figure \(\PageIndex{8}\): Climbing ripple deposit from India.

Dunes

Figure \(\PageIndex{9}\): Lithified cross-bedded dunes from the high country of Zion National Park, Utah. The complexity of bedding planes results from the three-dimensional network of ancient dune flows.

Dunes are very large and prominent versions of ripples and typical examples of large cross-bedding [27]. Cross bedding happens when ripples or dunes pile atop one another, interrupting, and/or cutting into the underlying layers. Desert sand dunes are probably the first image conjured up by this category of bedform.

Figure \(\PageIndex{10}\): The modern sand dune in Morocco.

Dunes are the most common sedimentary structure found within channelized flows of air or water. The biggest difference between river dunes and air-formed (desert) dunes is the depth of the fluid system. Since the atmosphere’s depth is immense when compared to a river channel, desert dunes are much taller than those found in rivers. Some famous air-formed dune landscapes include the Sahara Desert, Death Valley, and the Gobi Desert [28].

As airflow moves sediment along, the grains accumulate on the dune’s windward surface (facing the wind). The angle of the windward side is typically shallower than the leeward (downwind) side, which has grains falling down over it. This difference in slopes can be seen in a bed cross-section and indicates the direction of the flow in the past. There are typically two styles of dune beds: the more common trough cross-beds with curved windward surfaces, and rarer planar cross-beds with flat windward surfaces.

In tidal locations with strong in-and-out flows, dunes can develop in opposite directions. This produces a feature called herringbone cross-bedding.

Figure \(\PageIndex{11}\): Herringbone cross-bedding from the Mazomanie Formation, upper Cambrian of Minnesota.

Bioturbation

Figure \(\PageIndex{14}\): Bioturbated dolomitic siltstone from Kentucky.

Bioturbation is the result of organisms burrowing through soft sediment, which disrupts the bedding layers. These tunnels are back filled and eventually preserved when the sediment becomes rock. Bioturbation happens most commonly in shallow, marine environments, and can be used to indicate water depth [30].

Mudcracks

Figure \(\PageIndex{15}\): Lithified mud cracks from Maryland.

Mudcracks occur in clay-rich sediment that is submerged underwater and later dries out. Water fills voids in the clay’s crystalline structure, causing the sediment grains to swell. When this waterlogged sediment begins to dry out, the clay grains shrink. The sediment layer forms deep polygonal cracks with tapered openings toward the surface [31], which can be seen in profile. The cracks fill with new sediment and become visible veins running through the lithified rock. These dried-out clay beds are a major source of mud chips, small fragments of mud or shale, which commonly become inclusions in sandstone and conglomerate. What makes this sedimentary structure so important to geologists is that they only form in certain depositional environments—such as tidal flats that form underwater and are later exposed to air.

Raindrop Impressions

Figure \(\PageIndex{19}\): Mississippian raindrop impressions over wave ripples from Nova Scotia.

Like their name implies, raindrop impressions are small pits or bumps found in soft sediment. While they are generally believed to be created by rainfall, they may be caused by other agents such as escaping gas bubbles [35].

Imbrication

Figure \(\PageIndex{20}\): Cobbles in this conglomerate are positioned in a way that they are stacked on each other, which occurred as flow went from left to right.

Imbrication is a stack of large and usually flat clasts—cobbles, gravels, mud chips, etc—that are aligned in the direction of fluid flow [36]. The clasts may be stacked in rows, with their edges dipping down and flat surfaces aligned to face the flow (see figure). Or their flat surfaces may be parallel to the layer and long axes aligned with the flow. Imbrications are useful for analyzing paleocurrents, or currents found in the geologic past.

References

• 24. Bouma, A. H., Kuenen, P. H. & Shepard, F. P. Sedimentology of some flysch deposits: a graphic approach to facies interpretation. 168, (Elsevier Amsterdam, 1962).
• 26. Ayrton, H. The origin and growth of ripple-mark. Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character 84, 285–310 (1910).
• 27. Ashley, G. M. Classification of large-scale subaqueous bedforms: a new look at an old problem-SEPM bedforms and bedding structures. J. Sediment. Res. 60, (1990).
• 28. Dingus, L. & Loope, D. Death in the Dunes. Nat. Hist. 109, 50–55 (2000).
• 31. Plummer, P. S. & Gostin, V. A. Shrinkage cracks: desiccation or synaeresis? J. Sediment. Res. 51, (1981).
• 35. Metz, R. Why not raindrop impressions? J. Sediment. Res. 51, (1981).
• 36. Karátson, D., Sztanó, O. & Telbisz, T. Preferred clast orientation in volcaniclastic mass-flow deposits: application of a new photo-statistical method. J. Sediment. Res. 72, 823–835 (2002).