Once a cross-section has been plotted and lithostratigraphic or time-stratigraphic units have been correlated, an interpretation of the sedimentary history is possible. In Labs 1 to 3 you learned how to describe sedimentary rocks, facies, and stratigraphy. Below you will find a summary of key principles and concepts that help with the geological interpretation of stratigraphic logs and cross-sections.
Lab 1: Relating Clastic Rocks to How and Where They Formed
a. The location from which clastic sediments are derived is termed the sourceland. Sourcelands are typically terrestrial and represent the exposed and eroding continental landmasses.
b. Sediment colour is controlled by many factors but can reveal the extent of oxidation during deposition and provide a clue to location. Brown or red typically indicates an arid terrestrial environment. Grey/green is common in for shallow and/or circulating water, and black typically indicates deep water and/or oxygen depleted water.
c. Grain size of clastic sediments relates to the energy of the transporting mechanism. High energy environments transport large grain sizes and small grain sizes are deposited in low energy environments. Rounding can be related to length of transport as longer transport distances lead to more rounded sediments.
d. High-energy water flows sometimes will erode the layers below.
e. High-energy water flows typically give way over time to lower-energy water flows (e.g., floods, storms, spring runoff). A decline in flow energy results in a decrease in grain size from coarse at the bottom of a bed to fine at the top leading to a graded bed that is sometimes called a fining upwards sequence (FUS).
Lab 2: How Sedimentary Environments (and Hence Sedimentary Facies) Relate to Each Other in Space
a. The body of sediment deposited in a given sedimentary environment (a sedimentary facies) relates to the character of the sedimentary processes occurring within that environment.
b. Transport from uplifted sourcelands to the site of deposition leads to a predictable progression of sedimentary facies.
c. Clastic sediment composition and maturity—particularly in sandstones—often indicates the mineralogy of the sourcelands and the length of sediment transport system (e.g., long= quartz only; short= abundant feldspars).
d. Grain size of clastic sediments will generally be coarsest near to shore (proximal) and finest furthest (distal) from shore.
e. Clastic-dominated sequences usually indicate tectonically active or unstable (uplifted) sourcelands. Grain size will generally be coarsest near to shore (proximal) and finest furthest from shore (distal). The thickness of a clastic rock unit can be used as a general indication of proximity to the sourceland and the rate of deposition in the sedimentary depositional basin. Thick immature clastic sediments imply rapid uplift of a nearby sourceland, closer proximity to the sourceland, or a rapidly deepening sedimentary basin. Thin mature clastics imply tectonic stability and a distal sourceland.
f. Shallow-water carbonate-dominated sequences usually indicate a warm, shallow water environment that is has little clastic input. Thick shallow-water carbonate sequences imply tectonic stability, slowly rising sea levels or slow subsidence of the sea-floor.
g. Deep water carbonate sequences are developed where the location is far from shore beyond the range of clastic input.
h. The sedimentary rocks within a facies may exhibit a degree of cyclicity due to repeated sedimentary events. For example, repeated turbidity flows lead to repeated fining upwards sequences; meandering river channels have repeating channel centre and point bar deposits; and floodplain deposits have repeating layers of silts and sands from seasonal flood events.
Lab 3: Stratigraphy and Changes in Sea Level
a. Long term fluctuations in sea level relative to land are due to either eustatic (global) changes in sea level, or to local tectonic movement which moves the relative elevation of the land in relation to an unchanged global sea level.
b. Major tectonic upheavals and long term cycles of eustatic (global) sea level fluctuations span many millions of years.
c. Short-term cycles of eustatic (global) sea level fluctuations may be caused by glaciations, and span 10s or 100s of thousands of years.
d. Transgression (or an on-lap sequence) is the landward migration of the shoreline in response to a rise in global sea level or to a lowering of the basin floor relative to a static global seal level due to local tectonics. Sediments at a given location progress from shallow water to deep water facies.
e. Regression (or an off-lap sequence) is a result of a relative fall in global sea level or a rise in the sedimentary basin floor in response to local tectonics. Sediments at a given location progress from deep water to shallow water sediment facies.
Plate Tectonic Margins and Sedimentary Basins
Different types of tectonic plate margins influence the formation of sourcelands and sedimentary basins.
a. Diverging continent-continent plate margins create new sedimentary basins in which the basin floor drops relative to a static global sea level.
b. Existing continental shelf margins will gradually accumulate sediment from the exposed continent nearby. The mass of the accumulated sediments on the continental shelf will cause the shelf to sink lower and this further lowers the basement of the shelf relative to global sea level.
c. Converging continent-to-continent plate margins, converging ocean-to-continent plate margins and continent-to-continent collisions are all tectonic settings in which the sedimentary basin floor may rise relative to a static global sea level.
d. Convergent margins are regions of on-land volcanism, uplift of new mountains, and increased weathering of bedrock.