A Geologist’s Guide to Broome’s Pindan Cliffs
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An angular unconformity is a surface in rock that signifies a break in time, or more specifically between older rock units below and younger ones above it. You can recognize it by the absence of sediment layers between old and new rocks.
Geologists tend to identify angular unconformities more easily, as they’re more common than disconformities and usually form when an underlying rock unit has been uplifted and tilted before depositing itself. Unfortunately, disconformities are harder to spot, particularly if their surrounding rocks have been eroded away over time.
Geologists use angular unconformities as an aid in understanding Earth history. These unconformities provide evidence of plate tectonic movements and changes to sea level; they also indicate when deposition stopped before erosion took its course and resumed, providing vital clues as to the sequencing of rock layers when reading geologic maps.
At its core, angular unconformities form in four steps: 1. Sediments weathered from land and carried to sea accumulate on lake or ocean floors over time to form rock layers; 2. As the Earth’s crust shifts and collides with each other, lifting or tilting these layers up or tilting them tilted layers erode away over millions of years; eventually these tilted layers are submerged beneath a fresh deposit of sediments that covers them completely;
An angular unconformity marks the boundary between these layers of sediment. They can often be found on geological maps where younger rocks overlie older tilted rocks layers.
Geologists use angular unconformities to establish the relative age of rock layers. They can help locate fossils and track river or mountain stream courses; and can be used to detect oil, mineral and underground water deposits. Furthermore, angular unconformities are an invaluable way to study Earth’s evolution; for instance the famous Siccar Point unconformity in Scotland shows younger red sandstone and breccia resting upon near vertical Silurian-aged sandstones, mudstones and graywacke which shows evidence that younger rock sequence was laid over tilted, eroded and submerged layers of older rock – telling geologists this site that this sequence was laid above tilted, eroded and submerged layers.
Geologic sediments are typically deposited as flat layers known as bedding that are compressed, tilted, and folded by plate tectonic forces into rock layers known as synclines or anticlines (pictured here and here respectively). Tilted bedding provides great information to geologists; it shows which environment existed at the time when rock was deposited as well as time of deposition and helps verify events within rock formation.
This tilted bedding consists of carbonate sandstones deposited in shallow seas or lakes. As can be seen by their thinness, deposition occurred quickly due to river currents moving sediment from left to right and rapidly depositing sediments on both sides. If these sediments had been laid down deeper they might have thickened further and their structure might not have been as clearly defined.
Tilted bedding comes in many forms and patterns that can be described using terms like lamina, bed set and lamina set. The photograph below depicts an extremely intricate lamina with plane parallel laminae contrasting with convoluted laminae; this type of sedimentary rock is known as diatomite.
Bed thickness can provide us with valuable insight into a rock. In most sandstone formations, bedding tends to be thin; in conglomerates and other rocks however, its thickness provides us with an indication of their age.
Geologists are constantly scanning for features that breach geological laws. One such law is cross-cutting relationships, which states that any feature that cuts across bedding must be younger than either side; examples include faults and igneous intrusions.
This image depicts a fault, a planar fracture across which significant earth movement has taken place. Rocks on either side of this fault tend to be older due to tilted bedding (rock layers) being compressed and folded by this movement; thus making the left (west) side appear older than its right (east) counterpart.
Folds, like waves in an ocean, can undulate and tilt layers of rock that were originally laid horizontally. In general, folds’ layers usually align parallel with one another with older strata at its center and younger strata near its right flank – this type of fold is known as symmetrical fold. However, some situations call for additional warping of strata past this point, creating recumbent folds; their recumbence may cause their contents to tilt past vertical and this condition can even result in overturned anticlines or synclines.
Folds form when rocks compressed under pressure by tectonic stress become warped and tilted by compression exerted upon them by compression exerted on rocks by folding. When this occurs, beds which were originally horizontal have become warped and tilted by compression exerted on rocks by folding, warping them from being horizontal into tilting warps that have now developed as folds. A fold may take the form of either an upwarp or downwarp but can also include anticlines and synclines – see image below! A plane drawn through its crest is called its axial plane while its limbs on either side are known as its limbs; when its angles between its limbs with its axial plane are approximately equal symmetrically balanced then its considered symmetrically balanced otherwise its considered as being asymmetrically.
Folds that are non-symmetrical are sometimes described as neutral (not synform or antiform). Recumbent folds with nearly horizontal axial planes and plunge angles between 10deg to 30deg are commonly referred to as recumbent. Unfortunately, this term lacks clarity and could potentially cause miscommunication and confusion.
As its name implies, a recumbent fold is a type of fold that has been bent back into itself like a Lazy Boy recliner. When horizontal beds are folded this way, the strata that make up the fold are truncated at its top by planar-bedded zones; as well as losing their distinctive laminations to form flat sheets–typical features of parabolic recumbent folds. Capitol Butte provides an example of such a recumbent fold upslope from cross-section H, CB-1–its deformation extends across large-scale cross-beds at its left–with current flow coming from right in this view.
As seen during Kilauea’s 2018 eruption, fault slippage causes rock deformation along the faultlines, manifested as low cliffs or scarps similar to those shown in Figure 42. Faults serve as highly efficient “earth movers,” as they don’t need to create new cracks; rather they use existing ones that widen and heighten upon movement along a faultline.
Gullies, vegetation and sea caves may support the theory that an area has been faulted, though these features could also have been caused by other processes. To be certain that evidence of faulting exists in an area, look out for signs such as truncated bedding (rock layers), cataclasites and smooth rock surfaces known as “slickensides,” which show polish marks caused by friction during faulting; these tell us when rocks have been moved during an earthquake.