6.7: Sequence Stratigraphy and Walther's Law

Understanding Earth's History Through Sequence Stratigraphy

In the stratigraphic record of a basin, there can be packages of beds that repeat in a cyclic fashion: these related and predictable packages of rock are referred to as sequences. Sometimes, sequences are bounded by clear unconformities and mark major changes in tectonic regimes. In other situations, they are a remnant of changes in sea level. An entire sub-field of stratigraphy, sequence stratigraphy, is dedicated to the study of such cyclic phenomena.

These cycles can occur on a variety of scales. Sometimes, they represent tens of millions of years and sometimes only days. Their causes are open to interpretation, depending upon the scale of time involved, and the evidence contained in the record. One thing that is very clear is the important story these cyclic beds tell about the nature of repetitive phenomenon in nature. Climates change, continents move, and sea levels rise and fall. Such cycles have been occurring during the entirety of Earth’s history. However, stratigraphy can give us important insights into how one cycle differs from another.

Sequence Hierarchy

Stratigraphic sequences are superimposed on one another. This is because environmental phenomena that create cycles of various orders are also happening concurrently. One of the jobs of a sequence stratigrapher is to separate these storylines. Nested within the time span of a first-order megasequence, which has a period of several hundred million years, a number of second-order supersequences will occur. Within a supersequence, a number of smaller sequences occur, and so on. Each of these different packages of strata are bounded by unconformities, created by erosional flooding surfaces, that can be traced over distances. Flooding surfaces are sharp contacts that separate underlying shallower-water facies from overlying deeper water facies. Sequences of any order can be related across time and studied for what they have to teach about the Earth’s past, a region’s past, and a particular location’s past.

Figure \(\PageIndex{1}\): Cratonic Sequences, or Sloss Sequences, as they exist in the Colorado Plateau and Grand Canyon region in the American southwest.

The causes of sequences of various hierarchical levels is subject to debate. First-order megasequences are known to reflect global phenomena, such as the formation and breakup of supercontinents like Rodinia and Pangaea. “The Great Unconformity”, viewable at the Grand Canyon, is a good example of a bounding surface that marks the lower boundary of the megasequence that would eventually lead to the formation of Pangaea, starting with the flooded cratons that make up today’s continents.

Figure \(\PageIndex{2}\): Sedimentary “Sloss” Sequences of North America. White and yellow areas represent major marine transgressions onto the craton while black areas represent major unconformities (sequence boundaries). The Sauk is bounded on the bottom by the “Great Unconformity”, the boundary between the Sauk and Tippecanoe is referred to as the “Knox Unconformity”, and the boundary between the Tippecanoe and Kaskaskia is the “Wallbridge Unconformity”. All six of Sloss’ Sequences are 2nd order supercycles. (Sloss, 1964)

Second-order supersequences are thought to be more regional in scope and again driven by tectonics, but this time within an ocean basin. During the Paleozoic Era, a global megasequence developed, within which four second-order supersequences would progress (the Sauk, Tippecanoe, Kaskaskia, and Absaroka). The Sauk Sequence represents the earliest of these, bounded by the Great Unconformity and the Knox Unconformity. The Tippecanoe Sequence came next, bounded by the Knox Unconformity on the bottom and the Wallbridge Unconformity on top.

Each of these supersequences carry with them signals for shorter, third-order sequences. Within those sequences, fourth-order and fifth-order parasequences, by which level, the data is entirely within a single stratigraphic formation and could reflect a cyclicity of just days. These shortest parasequences can be driven by a number of factors, but Milankovitch orbital variations are likely drivers.

The ultimate goal of sequence stratigraphy is to understand the Earth’s past as recorded in the stratigraphic history of a basin. We can use stratigraphic tools like lithostratigraphy and biostratigraphy to measure and analyze stacks of rock layers. Then, correlating units across a region, it is possible to reconstruct this history. At this point, it becomes possible to quantify changes in water depth, sedimentation rates, rates of tectonic change, and rates of global (eustatic) sea level change.

Sediment Needs its Space: Accommodation, Sea Level, and Sediment Supply

Basins are not static features. Basins are influenced by the physical processes around them as they record the history of the surrounding land and of their own past. They also come in a variety of shapes and sizes. Some basins, like the Atlantic or Pacific Ocean basins today, are massive. As sediments erode, move downhill via wind and streams, and accumulate, they fill in the “accommodation space” that is available in that basin. Like a bowl of cereal, a basin only holds so much sediment, and fills in over time. Sedimentary systems strive toward an equilibrium between between sediment infill and available accommodation space, but various Earth events will upset this balance.

In order to calculate the accommodation space within a given current or past basin, the following equation is used:

T + E = S + W

Where:

T = Rate of tectonic subsidence, or sinking, in the basin

E = Rate of eustatic, or global, sea level change, relative to a reference ellipsoid

S = Rate of sedimentation/accumulation in the basin

W = Rate of change in water depth within basin

This equation represents a simple balance. If the left side of the equation is positive, then there is increasing accommodation space in the basin and, if negative, decreasing space. On the right, if the number is positive then there is increasing sedimentation and negative if there is not. Thus, if the numbers on both sides of the equation are both positive, then both space and sedimentation (S) are increasing. If the left is negative and the right side positive, then we can predict that the basin will be infilling over time until no more sedimentation is possible. While it seems simple enough, this equation is very difficult to actually use in practice and better serves as a model for thinking about and considering stratigraphic packages of sedimentary rocks (Holland, 2018).

Figure \(\PageIndex{3}\): Cross-section of the Earth showing important sequence stratigraphic definitions (from Holland, 2018).

T: Tectonic Changes

Tectonic changes include variables like the uplift of land, isostatic rebound or the sinking of land, subsidence. Plate tectonic changes are the primary driver of all of the changes in these variables. Recall that oceanic lithosphere is primarily made up of basalt and that continental lithosphere is primarily made up of granite. These different rock types have different bulk densities, basalt being much more dense. This causes oceanic lithosphere to sit lower on the asthenosphere than continental lithosphere. At subduction zones, new mountain ranges are thrust up as lithosphere buckles and thickens. This concentrates mass and leads to the subsidence of the land as foreland basins are created through downward flexure. These kinds of processes tend to deepen basins.

Other situations, such as ice melt on land, tend to shallow basins. During ice ages - times when the continents bear the burden of massive ice sheets - the extra mass of ice weighs down the plates. When the ice melts, local basins will rise as continents isostatically adjust their elevation. The rate of subsidence or uplift (T) is very important to know when calculating the accommodation space of a basin. For the purposes of accommodation space, the rate of subsidence and the rate of eustatic sea level change are the biggest and longest term factors. If eustatic sea level drops at the same time that subsidence increases, no new sedimentation space is created. If eustatic sea level drops at a slower rate than subsidence, new space is made, and available for deposition.

Figure \(\PageIndex{4}\): Glacial Isostatic Adjustment as equivalent \(\ce{H2O}\) thickeness variation rate. In other words, gravitational anomalies (changes) as a result of tectonic rebound from the last glacial advance of the Pleistocene. Note the high levels of adjustment over areas of northern North America, Europe, and also Antarctica. Gravitation, as measured by NASA’s GRACE mission, is balanced against the gradual recovery of ocean basins as they slowly spring upward, causing relative sea level around these noted areas to drop. Gravitational anomalies are due to mass loss, which can be mantle material moving to accommodate areas of the crust that are rebounding. Rates of mass change have been estimated to be as high as 104 gigatons under Greenland (Wu et al., 2010). Image: NASA

E: Absolute Sea Level Changes

Absolute sea level (ASL), also known as eustatic sea level, is the distance between the center of the Earth and the global average sea level. Eustatic sea level fluctuates as the volume of water in the ocean changes, due to changes in glaciation, ocean temperatures, and glacial ice volume. Today, when we think of sea level rise due to global warming, this is the kind of change to which we refer. If so much ice melts in Antarctica or Greenland, sea level globally will rise to some amount (10cm to multiple meters, perhaps) over the next century. Due to global warming, the ocean waters are undergoing thermal expansion, causing the rises we see along our shorelines today.

Figure \(\PageIndex{5}\): Eustatic sea level rise resulting from the melting of glacial ice over the past 24,000 years (Post-Glacial Sea Level by Robert A. Rhode, CC BY-SA 3.0).

S: Sedimentation Rates

Sedimentation rates (S) vary a great deal. As already seen, they are most closely linked with the rate of change in water level (W) in the equation above. How do sedimentation rates change? There are a wide variety of factors. As mountains rise, sedimentation rates will also rise, as the land moves upward into the atmosphere and is exposed to ever more chemical and physical weathering. Likewise, the eventual erosion of such mountains will lead to drop offs in sedimentation into the basin. Sedimentation is also affected by the climate and latitude. Humid environments will have more runoff. This will lead to higher sedimentation rates. The type of geologic setting also matters. Siliciclastic environments source their sediment from outside the basin and so in these situations sediment supply is a much more significant factor in basin analysis. In carbonate settings, the sediment is produced within the basin by organisms precipitating \(\ce{CaCO3}\) directly from seawater.

When it comes to sediment supply, we can also describe how relative sea level behaves with respect to sedimentation rates. When sediment supplies are increasing more rapidly than water levels, it is said that the coastal land is prograding. When sediment supplies are decreasing, it is said that the coastal land is retrograding. Thus, the rate of sedimentation (S) is critical to grasp when trying to discern whether sedimentation or water level is controlling changes in accommodation space. If progradation of a coastline occurs and sea level regresses, then accommodation space decreases. If coastlines are in retrograde and sea levels are transgress, then accommodation space in the basin will increase.

W: Relative Sea Level Changes

Seas are dynamic. It is a bit misleading to refer to “sea level”, in the sense that there is no absolute level at which the water rests, when measured against the floor of a basin. Measured relative sea level is an average depth, relative to this basin floor in a particular region. Relative sea level (RSL) is the water depth (W) of the basin. It is affected by many factors that can cause it to rise or fall. Some of these factors include 1) increases in water mass (from melting glacial ice), 2) thermal expansion of water with a rise in temperature, 3) changes in ocean currents or seawater salinity, and 4) isostatic rebound adjustments of continents.

When we look at NOAA tidal gage data today, we can see that the eastern shore of Virginia is experiencing rising seas, while the Gulf coast of Alaska is experiencing a net drop in sea level. What explains the difference in relative sea level trends?

Figure \(\PageIndex{6}\): Relative sea level (RSL) trends for two sites on opposite sides of the North American craton. Kiptopeke, Virginia, is experiencing a relative rise in sea level of 3.97 mm/yr where Kodiak, Alaska (southern coast) is experiencing a drop in relative sea level of -9.92 mm/yr. While global (eustatic) sea levels are rising, local records will vary. Forecast trends for both sites, according to NOAA, indicate positive sea level trends into the next century. Note: The Kodiak, Alaska data has a large gap around the time of the 1964 Good Friday earthquake. Prior to the quake, the sea level trend was flat. After the quake, the sea level trend is negative. This negative trend is temporary, however, and may be a tectonic remnant of this event.

Virginia's relative sea level reflects the eustatic trend in increasing global sea level due to thermal expansion of the oceans, but the tidal gauge data for Alaska nicely illustrates the effect tectonics can have on relative sea level. Prior to the great 1964 “Good Friday” quake, sea level was stable. After the quake, not only did sea level start out much higher than the original datum prior to 1964, but it has been dropping ever since. This indicates that the land surface is undergoing uplift due to tectonic changes since the earthquake. Long term trends (not shown) according to NOAA do see sea level eventually rising here also, but overcoming tectonic effects takes time.

Transgressions, Regressions, and Walther’s Law

As mentioned above, when sediments are deposited into a basin, they are affected by and also affect the basin. Where sediments ultimately come to rest depends on the energy level of the system and how it is able to handle sediment grain size. Generally, the energy level of water moving sediment decreases as gradient shallows, from mountains to the sea, gradually leaving behind successively smaller particles. Beaches, locations many of us are very familiar with, are full of sand because the energy levels at the seashore are well-suited to that grain size. In deeper water, grain sizes diminish even more, until all you find are marine muds well offshore. These successive sedimentological changes, along with the flora and fauna adapted to living in them and the unique structures created, are how we determine local facies. The facies are the characteristics of a rock, formed under its unique depositional environment. Along a seashore, sedimentary facies will display a gradual decrease in grain size from areas well above the tidal zone (supratidal) to areas well below wave base (deep marine).

Figure \(\PageIndex{7}\): Overview of transgression (bottom three time slices) and regression (top three time slices) as recorded by changes in water depth, shoreline position, and changes in depositional environments through time (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

In these seashore locations, one of the key variables that affects how strata develop through facies are changes in sea level. Sea level change is continuous. Sediment deposition along shorelines is also continuous during these changes in sea level. When sea level rises, we call it a transgression. When it falls, we call it a regression. As water level rises, or transgresses, something interesting happens. These facies migrate shoreward also! Likewise, as water level drops, or regresses, the facies migrate seaward.

Named after German geologist Johannes Walther, Walther’s Law proposes that facies in continuous vertical successions were also deposited in laterally adjacent environments. In the diagram provided above, we have three laterally adjacent environments: a shoreface (likely sandstone), offshore transition (likely mudrock), and shelf (likely carbonate). If a geologist were given Core 1, they'd see these facies stacked on top of one another, from sandstone, to mudrock, to carbonate, and conclude that the water got progressively deeper through time to account for the facies changes. It would be reasonable to interpret this conformable succession as recording a transgression. In Core 2, they would see the opposite trend: a facies succession that records progressive shallowing, reasonably interpreted as a regression.

Please watch this video below for an excellent explanation.

As seen in the video above, Walther’s Law applies to both siliciclastic basins and carbonate basins. In siliciclastic basins, our shoreward to deep marine progression might begin with conglomerate and then continue offshore with sandstone, siltstone, and end with mudstone. If sea level rises, deep marine facies will be pushed shoreward and deposited on top of shallower water units in a fining-upward sequence, typically of a transgressive sequence. Likewise, if sea level regresses, shoreward facies will migrate toward the sea, being deposited on top of deeper water facies, creating a stratigraphic succession of coarsening-upward layers.

In carbonate systems, the shoreward to deep marine sequence runs from grainstone (mostly fossil material) to packstone, wackestone (more mud), and ends with a limey mudstone. The same transgressive and regressive sequencing applies with changes in sea level up and down in these environments.

Packages of stratigraphic sequences can be transgressive or regressive in nature. When a transgressive sequence lies below a regressive sequence, the boundary between them is referred to a as a Maximum Flooding Surface. Likewise, the top of a regressive sequence is erosional and referred to as a Sequence Boundary.

Key Terms

• regression – a geologic process in which the sea level lowers, exposing previously submerged areas and leading to the deposition of progressively shallower-water sediments over deeper-water sediments
• sequence – a relatively conformable package of sedimentary strata bounded above and below by unconformities (erosional or non-depositional surfaces) that record a cycle of sea-level change and sediment deposition
• transgression – a geologic process in which the sea level rises, resulting in progressively deeper-water sediments being deposited over shallower-water sediments
• Walther’s Law – a principle stating that the vertical order of sedimentary facies in a stratigraphic sequence reflects the lateral changes that existed at the time of deposition, facies that are found adjacent to one another in space will succeed one another vertically in the rock record

References:

Holland, Steven (2018). An Online Guide to Sequence Stratigraphy. https://strata.uga.edu/sequence/index.html.

Wilson, J. Tuzo (1966). Did the Atlantic close and then re-open?. Nature, 211, 676-681.

Wright, K. (2013). Seismic Stratigraphy and Geomorphology of Palaeocene Volcanic Rocks, Faroe-Shetland Basin, Unpublished PhD Thesis, Durham University.

Wu, X. P., M. B. Heflin, H. Schotman, B. L. A. Vermeersen, D. A. Dong, R. S. Gross, E. R. Ivins, A. Moore, and S. E. Owen (2010), Simultaneous estimation of global present-day water transport and glacial isostatic adjustment, Nature Geoscience, 3(9), 642-646, doi: http://dx.doi.org/10.1038/NGEO938.

Van Wagoner, J. C., 1995, Overview of sequence stratigraphic foreland basin deposits: terminology, summary of papers, and glossary of sequence stratigraphy, in J. C. Van Wagoner, and G. T. Bertram, eds., Sequence Stratigraphy of Foreland Basin Deposits: Outcrop and Subsurface Examples from the Cretaceous of North America: AAPG Memoir 64