Geol 33 Environmental Geomorphology

J Bret Bennington

Topographic Expression of Geologic Structures

The relationship between surface topography and underlying geologic structure is dependent on three sets of variables:

• 3-dimensional form of subsurface geologic structures and rock types
• rate of crustal deformation and uplift
• rate of erosion and climate type

Major types of geologic structures

Tilting and Folding:

Horizontal, dipping, and vertical strata

Synclines and anticlines (plunging and non-plunging)

Domes and basins

Jointing

Faulting

Normal and reverse

Strike-slip

Thrust faulting

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Relationship of erosion to rock type

Different types of rock erode at greatly different rates under different climate conditions. If two rock types with different weathering characteristics are exposed in close proximity at the surface, the more resistant rock type will erode to form an upland and the susceptible rock type will form a valley.

Igneous and Metamorphic rocks - these rocks are crystalline and composed of minerals with high hardness. They are generally erosion resistant relative to many types of sedimentary rocks. Igneous sills, dikes, and necks often form prominent features in landscapes when they are eroding among sedimentary country rock. Shiprock, NM, Devil's Tower, WY, and the Palisades cliffs of NY are well-known examples.

Regional metamorphic rock, which can be highly heterogeneous, often exhibits a dendritic drainage pattern, as if streams were downcutting into a large expanse of homogeneous rock. This occurs because the differences in lithology in complex metamorphic rock are expressed at too small a scale to control drainage. The rock is so heterogeneous that it becomes a homogeneous patchwork on a regional scale.

Because of their relative resistance to erosion, large regions of metamorphic and plutonic igneous rock can form mountains in the presence of moderate rates of uplift. For example, the Sierra Nevada mts. of eastern CA are eroded into an extensive granite batholith, as are the White Mts of New Hampshire. Most of the mountains of New England are carved into crystalline metamorphic and igneous rock.

However, in the absence of sufficient uplift, erosion and weathering will attack many of the silicate minerals in crystalline rocks and create a subdued topography of rolling hills or flatlands. The shield areas of continental interiors are broad, flat regions developed on deeply weathered metaigneous and metamorphic rock.

Sedimentary Rock - For the sake of simplicity we can lump sedimentary rocks into three categories - sandstones (quartzose), limestones (carbonates), and shales (claystones). These three types of rock often alternate in close stratigraphic proximity and can create abrupt differences in relief as they weather at different rates.

shales - clay minerals are soft and have good cleavage, resulting in rock that is easily eroded under all conditions. Shale tends to form valleys and lowlands where it is exposed at the surface.

sandstones - pure quartz sandstones and conglomerates that are well-cemented are very resistant to weathering and erosion. These rocks are usually ridge-forming.

carbonates - the erodibility of carbonates varies greatly according to climate. In humid climates such as are present in the eastern US, limestone erodes quickly as meteoric water and groundwater dissolve the carbonate mineral and carry it away in solution. This can produce a distinctive karst topography revealed by caves, sinkholes, and disappearing streams.

In arid climates, such as are common in the western US, the crystalline texture and moderate hardness of carbonate minerals make limestones relatively erosion resistant ridge-forming units.

Once erosion begins to differentially weather different types of sedimentary rocks the process becomes self-perpetuating as drainage is progressively concentrated in valleys underlain by weaker strata.

Horizontal Strata

Sedimentary strata with little or no dip present the same surface to erosion throughout a region. This results in the dendritic pattern of streamflow being etched into the landscape with randomly distributed drainage divides. Differences in the erodibility of horizontal strata will result in differences in slope, with the weaker beds forming shallow slopes and resistant beds forming steep slopes. In arid regions this pattern is amplified into cliff and bench topography. Basal sapping of the weak beds beneath the strong beds results in a shear face as the resistant beds breaks away along vertical joints. Retreat of cliff faces from sapping results in the formation of prominent benches called esplanades.

If the line of erosional retreat is relatively uniform across a landscape it can produce an extensive cliff face called an escarpment. Portions of the landscape that become separated from the main line of erosional retreat by headward eroding streams form mesas, which weather away until small summit areas remain called buttes.

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Titled Strata

When sedimentary strata are tilted beds weather into parallel high and low relief ridges and valleys. Drainage developed prior to uplift is called consequent drainage. As streams etch into the rising landscape they are deflected by more resistant beds, causing the subsequent drainage to erode valleys in the resistant layers.

Resistant beds form homoclinal ridges that generally have a steep scarp face and a more shallow dip slope. If the dip slope is gentle (<30¡) the ridge is called a cuesta. Steeply dipping beds form sharp ridges called hogbacks. Streams flowing down the side of a homoclinal ridge cut v-shaped notches in the resistant beds. If two notches in a resistant bed enlarge to connect, the triangular remnant of rock between the notches is called a flatiron.

Folds

Initially, anticlinal folds will produce topographic highs and synclines will produce valleys if the rate of uplift is greater than the rate of erosion. However, there is a tendency for resistant beds to be breached by erosion as the anticlinal ridge grows. This occurs because the bed is in tension along the axis of the anticline and becomes fractured and prone to erosion. Once breached the weaker beds below the resistant layers become prone to erosion. Synclines are much more difficult to breach because the resistant beds are put in compression in the axis of the syncline, making them more difficult to erode. Thus, where synclines and anticlines are being eroded rapidly relative to uplift topographic inversion can occur and anticlines will tend to host broad valleys surrounded by ridges and synclines will form broad ridges or elevated valleys. Topgraphic inversion is very common in the Valley and Ridge province of the eastern US.

Plunging folds - if anticlines and synclines have a regional plunge (meaning that their axes are not parallel to the the surface) they will form a distinctive outcrop pattern of sinusoidal Vs. This produces zig-zag ridges that are broadly U or V shaped.

Plunging anticlines will be surrounded by ridges with inward-facing scarp faces and will have sharper, V-shaped noses in the direction of plunge. Plunging synclines will be surrounded by ridges with outward-facing scarp faces and will show blunter, U-shaped noses away from the direction of plunge.

Sometimes folds will plunge in two directions. Double-plunging anticlines are common, creating create canoe-shaped structures.

Domes - tectonic warping or igneous intrusion can lead to the formation of a dome. As erosion removes the overlying beds in the higher center of the dome, older beds are exposed in the interior. Eventually the ingeous intrusive or upwarped metamorphic basement will be exposed in the interior of the dome, surrounded by upturned beds of sedimentary rock in a radial pattern around the dome. Famous examples of domes include the Adirondack region of New York State and the Black Hills of South Dakota. Both of these domes expose beds of Proterozoic rock in their centers, surrounded by younger upturned beds.

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Unconformities

Angular unconformities may have a surface topographic expression due to the discordance in orientation of the beds across the unconformity.

Relation of Streams to Geologic Structures

Streams that exist prior to the uplift of topography are called antecedent streams. During uplift an antecedent stream may cut rapidly into the landscape, becoming incised and confined by the steep sides of its valley. If a stream is large enough and is flowing at a high angle to the trend of the strike it may be able to erode a water gap in a rising ridge. Water gaps that eventually lose their streams due to capture or uplift are called wind gaps.

Water gaps can also be created by superposed streams. These streams begin flowing over material covering the structurally deformed layers. As uplift and/or downcutting progresses the streams erode into the underlying structures and are prevented by being controlled by the structure due to confinement within the overlying material.

Thrust Faults

In addition to synclines and anticlines, the southern valley and ridge province is characterized by large, low angle thrust faults. The principal effect of thrust faults is to ramp large blocks of crust on top of one-another. This creates regions of topography governed by repeated underlying stratigraphy. By convention, a faulted block of crust is named for the thrust fault beneath it (which it traveled along). Thrust faults develop as folding in anticlines becomes increasingly recumbent (bent over to one side) until the anticline is sheared off along a low angle thrust. For this reason, thrust faulted regions tend to be composed of repeated synclines and homoclines, separated by thrust faults and lacking in anticlines.

Where an overlying block is eroded completely through to reveal the strata of the lower block a window or fenester results.