20.4: Cap carbonates

One surprising feature of Snowball Earth glaciogenic deposits is that many of these distinctive sedimentary rocks with their characteristic signatures of glaciation are comformably overlain by deposits of limestone and/or dolostone. Because the carbonate strata always appear on top of the glaciogenic strata, they are dubbed “cap carbonates.”

Figure \(\PageIndex{1}\): Noonday dolomite cap carbonate comformably overlies glaciogenic Kingston Peak diamictite in Sperry Wash, near southern Death Valley. (Callan Bentley photo)

The most astounding thing that cap carbonates tell us is that temperature conditions abruptly switched from the chill of glacial conditions to balmy tropical warmth. Shallow water limestones form best today in places like the Bahamas, or offshore of Florida. The cap carbonates are conformable with respect to the glaciogenic sediments on which they are deposited. They are also much more extensive, covering vastly more area than the glaciogenic sediments alone, suggesting they were deposited during a major transgressive event. Several lines of evidence point to the rate of cap carbonate deposition being very rapid. The comformable relationship between pre-Snowball carbonates (warm) to during-Snowball glaciogenic sediments (cold) to post-Snowball cap carbonates (warm again) implies extreme temperature oscillations during the Neoproterozoic. These strata stretch our conception of the limits of climate change on Earth.

Overall, the cap carbonates show a transgression (as glacial ice melted and was added as water to the ocean) and a massive chemical precipitation of carbon-containing rock (which has major implications for climate controls). These extensive deposits indicate the ocean was in a state of carbonate oversaturation despite what must have been extraordinary levels of \(\ce{CO2}\)-induced ocean acidification in the wake of millions of years of volcanic outgassing with no carbon-removal processes operating on Earth’s surface (see discussion below for details).

The cap carbonate transgressive sequences for Sturtian and Marinoan glaciations are distinctive and consistent in deposits of that age around the world.

Unusual characteristics

Figure \(\PageIndex{2}\): Columnar “tubestone” stromatolites from Maieberg Formation, Namibia. (Photo by Jay Kaufman, reproduced with permission.)

Many cap carbonates show some primary sedimentary structures that suggest the post-Snowball hothouse Earth was a crazy place.

Not only was the temperature quite high, but apparently the cap carbonates were deposited very rapidly. This makes sense if we consider that the world’s surface consisted of a lot of pulverized rock interacting in shallow water with an atmosphere supercharged with carbon dioxide. As the \(\ce{CO2}\) dissolved into the seawater to combine with calcium ions (and some magnesium ions too), huge quantities of carbonate were precipitated and blanketed the seafloor. As the glaciers melted, a lot of fresh water was added to the oceans, and sea level rose globally, flooding the recently-deglaciated land surface.

Overall, the cap carbonates include two major subunits: an initial dolostone, representing shallow conditions, and a later limestone, representing a deeper depositional setting. Within this transgressive sequence are several distinctive primary sedimentary structures:

1. tubestone stromatolites

2. giant wave ripples

3. aragonite crystal fans

4. barite seafloor cements

Figure \(\PageIndex{3}\): Columnar “tubestone” stromatolites from Sete Lagoas, Brazil. (Photo by Jay Kaufman, reproduced with permission.)

A distinctive primary sedimentary structure in the cap carbonates is the tubestone stromatolite. Stromatolites are fossilized microbial mats as we see in other carbonate rocks throughout geologic time. However, the cap carbonate stromatolites are often very thin, and grow in tight proximity to other another.

One interpretation is that these stromatolites have this distinctive form because they were growing in relatively shallow water (within the depths to which sunlight penetrates) and that the water was precipitating fine-grained carbonate rapidly, so that a form of natural selection takes over, and only those stromatolites that grow upward are those that don’t get buried survive to photosynthesize another day. The result is a dense collection of finger-like projections, with space in between, sometimes with short-lived microbial lamination bridging the gap over the fresh deposits of chemically-precipitated fine-grained carbonate. These finger-like stromatolites are sometimes organized into broad dome-like structures or mounds.

Above the tubestone stromatolites are accumulations of structures that resemble ripple marks, a signature bedform occurs-the giant wave ripple. These odd, large (1-6 m wavelength), symmetrical ripples grew upward without much sideways movement of the wave crest. They were initially thought to represent violent hurricane waves in the post-Snowball greenhouse climate, but this doesn’t explain why they grew straight upward. A preferred explanation is that they represent the record of a deeper wave base combined with the rapid cementation of the seafloor during cap carbonate deposition.

Figure \(\PageIndex{4}\): Multiple layers of aragonite crystal fans in cap carbonate at Sete Lagoas, Brazil. (Photo by Jay Kaufman, reproduced with permission.)

Another interesting feature that suggests very rapid deposition of the carbonate is layers full of aragonite crystal fans. (Aragonite is a polymorph of calcite.) These large blade-like crystals of \(\ce{CaCO3}\) occur in layers within the overall cap carbonate sedimentary sequence. The long axis of the crystals is perpendicular to the ancient seafloor. Astonishingly, these crystals seem to imply such rapid crystallization of carbonate from the seawater that huge crystals several centimeters long grew upward into the supersaturated water column, and were buried as fast as they could form! It’s very difficult to imagine conditions stable enough for 10 cm tall crystals poking up all over the seafloor, like a ‘bed of nails,’ and persisting for any length of time in violently oscillating ocean waves.

In Marinoan cap carbonates (but not Sturtian), barite seafloor cements appear exactly at the transition between the older, shallow-water dolostone deposits and the younger, deeper-water limestone deposits. In structure, these are similar to the aragonite crystal fans described above, but the mineral is different: it is a barium sulfate called barite, \(\ce{BaSO4}\). The inference is that this distinctive layer formed when sulfate from surface meltwaters and barium (derived from feldspar weathering?) in deeper ocean waters mixed for the first time. If this is right, then the barite seafloor cements mark passage of the chemocline between these two reservoirs as transgression proceeded.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

At what rate were cap carbonates laid down?

a. Rapidly

b. Very gradually

Answer

a. Rapidly

Exercise \(\PageIndex{2}\)

Which of the following primary sedimentary structures are distinctive to cap carbonates?

a. Giant wave ripples, graded beds, and desiccation cracks (mud cracks).

b. Giant wave ripples, tubestone stromatolites, and seafloor crystal fans of aragonite.

c. Ball and pillow structure, tubestone stromatolites, and spinifex texture.

d. Aeolian cross-beds, coral reefs, and seafloor crystal fans of aragonite.

Answer

b. Giant wave ripples, tubestone stromatolites, and seafloor crystal fans of aragonite.

Exercise \(\PageIndex{3}\)

Cap carbonate sequences show an overall _________________ sequence.

a. transgressive

b. conservative

c. regressive

Answer

a. transgressive