Sedimentary structures and stuff

In an earlier post, I mentioned that Jon and I often search for sedimentary structures while measuring sections to get an idea of what the environment was like back when the rock-forming sediments were first deposited. This was a really vague and uninteresting statement, so I'd like to flesh it out and give you all a better look at some of the geology we're working with in Oz. Here's a little photo gallery of some of the most common geologic features we look at, and what they might tell us.

Karst - We wouldn't be able to accomplish much science if we couldn't tell limestone (carbonate) apart from other types of rock. Since we cannot rely on the color of a rock as an indicator, we look for karst, which are a dead giveaway for carbonates. Karst are razor-edged 'ripples' that can be sharp enough to cut you at times. They are a "recent" feature, formed after our ocean sediments became rock and were uplifted into mountains, and after the carbonate sediments were exposed to rainfall. The slightly acidic rain is able to dissolve calcium carbonate, thus leaving behind the razor-sharp ridges that are a bane of weary geologists' butts.

Ribbonite (also known as micrite) - A particularly common facies within the Wonoka Formation is ribbonite, a thinly bedded, wavy laminated fine-grained limestone interbedded with siltstone . Because carbonate is comparatively less resistant to weathering, it often forms as a recessive layer sandwiched between two protruding layers of silt. The presence of ribbonite suggests that the depositional environment was of low energy, the fine sediment grains too small to support the formation of ripples.

Grainstone - Another common facies seen is grainstone, 0.1-meter or thicker deposits of relatively pure carbonate with a larger grain size and often ripples. In contrast to ribbonite, grainstone requires a high-energy environment for deposition.

Ripples - Underwater ripples form in the exact same way that desert sand dunes are created by wind, except on a much finer scale. As a current transports grains along the ocean floor, a small pre-existing mound can act as a catalyst for ripple formation. The sand grains travel up the longer, more shallow stoss side of the ripple, and are then deposited along the steeper lee side.  Thus, a ripple showing a clear asymmetry between its two slopes can give us a general idea of paleocurrent flow direction (however with only two dimensions this can be misleading). Ripples with symmetrical ridges (i.e. no clear stoss and lee) suggest that the ripples formed by waves, not currents, in an intertidal depositional environment.

Climbing ripples - In an environment of fast sedimentation, climbing ripples are often observed. Once one ripple has formed, a new ripple may form on top of the stoss side of the old ripple, and 'climb' up this side as it grows.

Flute casts - Flutes form on sea floor slopes, where slope failure in shallower water sends pulses of sediment-charged water (called 'turbidity flows') to deeper environments. Flute casts are the three-dimensional negative reliefs of these water eddies that have carved depressions into sediment. Paleocurrent is in the direction opposite of that indicated by the pointing 'U'-shape formed by the mounds (in the corresponding photograph, paleoflow would be down).

Flame structures - These mushroom cloud-like structures form as a result of soft-sediment deformation, when the sediments are in the process of lithification but haven't quite turned into rock. It often signifies a rapid rate of deposition; if you pile up sediments quickly, the added load will stress and deform the unlithified layers, squeezing them out like toothpaste.

Stromatolites and microbialites - Mats of cyanobacteria living in the shallow, photic zone of the ocean are often preserved within rock through early lithification. Stromatolites often form rounded mounds that create relief on the paleoseafloor whereas microbialites remain flat-lying.

Intraclastic breccia - Breccias are rocks that contain fragmented clasts of other rocks or minerals, usually within a comparatively finer matrix. Due to storms or slumping, solidified layers of carbonate can break and be transported elsewhere. Intraclastic carbonate breccias contain such pieces that have not traveled very far, and often the matrix also consists of carbonate. Sometimes, the elongate carbonate clasts will deposit parallel to one another to form an imbrication fan.

Debris flow breccia - In contrast to intraclastic breccias, the clasts within debris flow breccias have traveled a greater distance. The layers are often thicker and usually contain a variety of siltstone, carbonate, and sandstone clasts. The matrix is also generally sandier with less pure carbonate. Debris flow breccias suggest an environment with much higher energy than intraclastic breccias, perhaps slope failure on a larger scale, although both are likely controlled by similar processes.

Vugs - These small centimeter-scale pock marks called vugs form when salts like halite, gypsum, and anhydrite have weathered away, leaving spaces in the spots they used to occupy in the rock. Vugs indicate that the sediments lithified in a shallow water environment exposed to the atmosphere, where the waters could evaporate and allow for salt crystal formation.

Color - Although color isn't usually a reliable measure for anything due to weathering and other secondary alterations, it still has its value. For instance, the Bunyeroo siltstone that lies stratigraphically below the Wonoka can have a very distinctive purple color. Looking for the purple silt is an excellent guide to finding the base of the Wonoka. A sudden change in the color of carbonate within a rock can indicate a change in provenance (i.e. the source of silt which gives carbonates color, which can suddenly change from one location to another). And dolomite (magnesium carbonate) is often easily distinguished from limestone by its yellow-orange weathered surface (though luckily we can still sample dolomite because the carbon in the rock was left undisturbed during the mineral replacement of calcium by magnesium).