Modern coral reefs and carbonate platforms provide the key ingredients of process and product to interpret ancient carbonate deposits. Here we can observe directly the relationship among all those processes – biological, physical, chemical – that contribute to the construction of solid rock.
Contributors
The inaugural collection of images for modern coral reefs has been generously donated by Charlie Kerans, the Department Chair and Robert K. Goldhammer Chair in Carbonate Geology in the Department of Geological Sciences, Jackson School of Geosciences, The Univeristy of Texas at Austin. Charlie is a carbonate specialist. I first met Charlie at Carleton Univeristy in Ottawa, where we shared an office whilst both of us were undertaking Doctoral research. Both of us worked on Precambrian rocks; Charlie on carbonates. He hasn’t stopped looking at carbonates.
The collections here are from Palancar Caves, and Columbia reef, Cozumel, off the Caribbean coast of Yucatan Peninsula. They are popular diving spots, for good reason as the images will attest. The water is clear and there is great diversity of reef and off-reef fauna and flora. It is a fantastic location to look at analogues for ancient reef systems – hard corals, soft fan corals, algae (particularly Halimeda), sponges, bryozoa, fish.
I’d be grateful for any corrections and clarifications to the species identifications.
The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite and professional thing to do). Copyright of images is retained by the owner, as indicated. Contact Charlie for further information (link above).
This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images. There, you will also find links to the other Atlas categories.
Click on an image for an expanded view, then the ‘back page’ arrow to return to the Atlas.
The images: Palancar Cave, Cozumel
Columbia Reef, Cozumel
This reef, part of the reef system along the west coast of Cozumel, is located a little south of the Palancar reefs.
Sediment that is moved along a substrate (e.g. the sea floor, river bed, submarine channel, wind-blown surface) will commonly generate structures that record its passing. Sedimentary structures that preserve directionality (paleoflow) are indispensable for deciphering whence the sediment came and where it went; for interpreting sedimentary facies (local scale) and sedimentary basins (regional scale). Paleocurrents are a measure of these ancient flows.
A single structure, such as a ripple will give a unique measure of paleoflow at a certain point in space and time. An important question for this single piece of data is – how relevant is it to the bigger picture of sediment dispersal? To get a sense of regional flow and sediment transport patterns, we need many measurements so that we can tease the overall pattern of flow from whatever local variations might exist.
We can illustrate this central problem by looking at flow in a fluvial meander belt with depositional settings like the main channel (arrows), point-bars and adjacent flood plain. This snapshot in time shows clearly the huge variation in local flow directions. We also need to account for other ‘snapshots’ in time, because even at a local scale (e.g. one meander bend and point-bar), the directions of flow and sediment transport will vary from flood to low water stage. We can try to circumvent this problem if we measure a large number of flow directions over an equally large area of the river and floodplain. In modern drainage basins this is straight forward but for the rock record, exposure is likely to be discontinuous and even structurally disjointed.
Structures indicating unique flow directions
Subaqueous dunes and ripples: These bedforms are built by 2-dimensional (straight-crested) dunes and ripples. Hence, the boundaries between adjacent crossbed sets tend to be planar (cf. trough crossbeds). Flow direction is approximately at right angles to dune or ripple crests.
Trough crossbed, or 3D subaqueous dunes Spoon-shaped troughs filled by migrating, sinuous dunes produce trough crossbedding. This kind of crossbed is common in confined, channelized flow (e.g. fluvial and tidal channels). The mean flow direction is along the axis of the trough.
Left: Festooned trough crossbeds exposed approximately parallel to bedding. Paleoflow is the direction of the hammer handle Proterozoic Loaf Fm.). Right: Cross-section view of multiple trough crossbeds – only apparent flow directions can be surmised from outcrop (Eocene Buchanan Lake Fm.).
A caution about wave-formed ripples; This bedform does not arise from bedload transport in flowing currents, but from wave orbitals. Wave ripples are not paleocurrent indicators. However, wave ripple crests will be oriented approximately parallel to the strike of the ancient shoreline.
Imbrication Flat and platy clasts are commonly oriented by strong currents, such that the ‘plates’ dip upstream. These fabrics are common in gravelly fluvial deposits.
Imbricated platy cobbles and pebbles in a modern stream. Flow is to the right.
Flute casts Flutes originate from erosion of a soft, commonly muddy substrate and are filled with sand – they are part of the overlying bed and are usually seen as casts on the sole of the overlying bed. Flow direction is towards the open, shallow end of the flute.
Large flute casts on a turbidite bed sole (Omarolluk Fm, Belcher Islands). Flow was from top left to bottom right
Structures indicating ambiguous flow directions:
Groove casts Objects dragged across a soft substrate by strong currents (e.g. bottom currents, turbidity currents) will scour linear grooves that become filled by the overlying sedimentary layer. Like flutes, they are usually seen as casts on the soles of beds. In the absence of other indicators, the two possible paleoflow directions are 180o apart.
Groove casts on a bed sole, indicate flow in either direction. other criteria, like flute casts, are need to specify unambiguous flow directions.
Parting lineation These are subtle structures 2 or 3 grains thick, that are visible only on exposed laminated bedding. The word ‘Parting’ refers to rock breakage along planar laminations. Parting lineation is attributed to high flow velocities where the long axes of sand grains become aligned (in Flow Regime terminology this corresponds to Upper Plane Bed conditions). Paleocurrents are measured parallel to the long direction of parting, but like groove casts, are ambiguous.
Paleoflow indicated in this parting lineation was either to the left or right.
Current alignment of elongate fossils, rod-shaped clasts, or bits of wood can generally be treated like groove casts in terms of their paleocurrent value. There are exceptions; for example turreted gastropods may be aligned with their apices pointing downstream. The example shown here shows fairly consistent alignment of Permian Fusulinid foraminifera parallel to the prevailing flow (but the actual flow direction is ambiguous).
The classic text that deals with paleocurrent analysis is – Potter, P.E. and Pettijohn, F.J. (1977) Paleocurrents and Basin Analysis. 2nd Edition, Springer-Verlag, New York, 425 p.
Determining the orientation of a plane given the points for three intersecting wells.
Mapping is the essence of geology. Geology maps provide the wherewithal to decipher the time and space organization of Earth’s solid and fluid spheres. We map the outer veneer by directly observing rocks and fluids, ‘walking out’ rock units, measuring, sampling and imaging as we go. More recent tools include all manner of remotely sensed data and satellite imagery (seismic, Lidar, radar, Landsat). We apply the same tools to map other planetary surfaces, although the walking-out is done by remotely controlled rovers.
Subsurface mapping is the essence of all explorations for aquifers, hydrocarbons, minerals, geothermal energy and geotechnical constructions. Subsurface mapping provides us with a deeper (sic) understanding of how the Earth works. Subsurface mapping is entirely dependent on remote sensing (e.g. seismic, gravity, radar) and borehole probing.
The orientation of geological planes in the subsurface is no less important than in surface mapping, but the database is commonly one-dimensional (e.g. borehole depths and lithologies). For example, a zone of mineralization at depth lies beneath an unconformity; knowing the orientation of the unconformity plane will give us more confidence predicting the trend of mineralization (assuming the unconformity is reasonably flat). To solve the problem, we need depths from three borehole intersections with the plane. The solution is commonly referred to as the 3-point problem. It is based on an understanding of dip and strike.
Geometric calculation of dip and strike in a 3-point problem
A graphical solution is shown in the animation. You need paper, a ruler and a protractor. This method requires the horizontal and vertical (depth or elevation) scales to be the same (no vertical exaggeration). Normally the construction would be done on the plane itself (i.e. 2-dimensional) – here the 3-dimensional view has been added to help you visualize the problem.
The animation was made from still images: use the pause and play buttons as you work through the exercise.
