Geological Terms Beginning With "D"
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A reference location or elevation which is used as a starting point for subsequent measurements. Sea level is a datum for elevation measurements. Datums can also be arbitrary such as the starting point for stream stage measurements or based upon a physical feature such as the base of a rock unit. Shown in the photo is a "bench mark". A location of accurately measured elevation, latituded and longitude that is labeled with a carefully placed marker.
The element produced through the radioactive decay of a parent element.
The very sudden downslope movement of rock and soil on a steep slope. A debris avalanche differs from a debris flow mainly on the basis of its speed.
A type of mass movement in which a combination of rocks, soil, organic matter, air and water flow downslope as a slurry. Debris flows differ from mud flows because they are mainly rock material instead of mud. They can be triggered by heavy precipitation, rapid snow melt or vibrations. Fires that remove vegetation often make a slope vulnerable to debris flows. Many debris flows are rapid, moving at over 60 miles per hour. Others are very slow, moving less than a few feet per year, mainly during times of rain or snow melt.
A horizontal to subhorizntal fault or shear zone with a very large displacement. The rocks above the fault might have been moved thousands of meters or more relative to the rocks below the fault. This often produces a situation where the rocks above the fault have entirely different structures than the rocks below the fault.
The removal of clay- and silt-size particles from a soil by wind erosion. The term can also be used in reference to the removal by wind of any unconsolidated material.
A payment to a mineral rights owner by a lessee if commercial production does not begin according to the terms of the mineral rights contract. Failure to pay the delay rental will result in termination of the lease.
A deposit of sediment that forms where a stream enters a standing body of water such as a lake or ocean. The name is derived from the Greek letter "delta" because these deposits typically have a triangular shape in map view. At a delta, streams often branch into "distributaries" that distribute their water and sediment load in multiple directions. Shown in the photo is the Nile Delta, where the Nile River enters Mediterranean Sea. The green vegetation of the well-watered delta contrasts sharply with the light brown sands of adjacent lands.
A stream drainage pattern that resembles the veins of a leaf in map view. Occurs mainly where the rocks below have a uniform resistance to erosion.
A gravity-driven flow of dense water down an underwater slope. The increased density of the water is a result of a temperature difference, increased salinity or suspended sediment load.
The settling from suspension of transported sediments. Also, the precipitation of chemical sediments from mineral rich waters.
A ground cover of granule-size and larger particles that is typically found in arid areas. This ground cover of coarse particles is a residual deposit - formed when the wind selectively removes the sand-, silt- and clay-sized materials. The persistent action of wind will eventually remove all of the small particles leaving a rocky surface known as a "desert pavement".
A thin coating of dark material, often iron or manganese oxides, that forms on the surface of rocks and pebbles exposed at Earth's surface in a desert region. If these rocks are picked up and turned over the bottoms of the rocks often do not have this coating (as shown by one rock in the photo that has been turned over). The rate of development varies from one location to another depending upon conditions.
A word used in reference to sediments or sedimentary rocks that are composed of particles that were transported and deposited by wind, water or ice.
The work done on a mineral property before mineral production begins on a commercial scale.
A well drilled within the proven area of an oil or gas reservoir to the depth of the productive stratigraphic horizon. These wells are expected to be productive.
Drilling done to delineate the boundaries of a known mineral deposit or to evaluate the deposit in advance of production.
A one celled plant that lives in the shallow waters of lakes, streams or oceans. Many of these secrete a shell or internal parts composed of silica. Diatoms can occur in very large numbers and can make significant contributions to sea-floor or lake sediment.
A light colored, fine-grained siliceous sedimentary rock that forms from a sediment rich in diatom remains.
A seafloor sediment that consists of at least 30% diatom remains.
A planet that has layers composed of elements and minerals of different densities. As an example, Earth is a differentiated planet because it has a metal-rich core, surrounded by a rocky mantle, and covered by a crust of low-density minerals.
A coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz.
The angle that a rock unit, fault or other rock structure makes with a horizontal plane. Expressed as the angular difference between the horizontal plane and the structure. The angle is measured in a plane perpendicular to the strike of the rock structure.
Drilling wells that are deliberately deviated from the vertical to hit a target that is not directly beneath the well site or to penetrate a greater thickness of rock within a productive zone.
The volume of water in a flowing stream that passes a given location in a unit of time. Frequently expressed in cubic feet per second or cubic meters per second. Calculated by the formula Q = A x V where Q is the discharge, A is the cross sectional area of the channel and V is the average velocity of the stream.
A surface separating rock layers of differing properties or compositions. (See seismic discontinuity.)
A pipeline that carries natural gas between a main transmission line and a consumer.
A ridge that separates two adjacent drainage basins.
An uplift that is round or elliptical in map view with beds dipping away in all directions from a central point.
Domestic operations are activities located in the United States, including the offshore territorial waters, U.S. commonwealth territories, and protectorates.
The geographic area that contributes runoff to a stream. It can be outlined on a topographic map by tracing the points of highest elevation (usually ridge crests) between two adjacent stream valleys. Also referred to as a "watershed".
The boundary between two adjacent drainage basins. Drainage divides are ridge crests (or less obvious locations where slope of the landscape changes direction). Runoff produced on one side of the ridge flows into stream "A" and runoff on the other side of the ridge flows into stream "B". The image shows the continental drainage divides of the United States.
A lowering of the water table around a producing well. The drawdown at any given location will be the vertical change between the original water table and the level of the water table reduced by pumping.
A general term for all sedimentary materials deposited directly from the ice or melt water of a glacier.
A contractual agreement under which a mineral rights owner or lessee assigns a fractional interest in a property to another party. This assignment might be done for financial support of exploration and development. It could also be done in exchange for the recipient doing the development work on that property.
A low, smoothly rounded, elongate hill. Drumlins are deposits of compacted till that are sculpted beneath the ice of a flowing glacier. The long axis of a drumlin parallels the flow direction of the ice.
A well drilled in hopes of finding oil or natural gas that fails to make commercial production rates.
A payment made to the owner of an unsuccessful well in exchange for a log of the well and evaluation data.
A mound or ridge of wind-blown sand. Typically found in deserts or inland from a beach. Most dunes slowly move in a down-wind direction as sand is blown up the windward side of the dune, moves over the crest and tumbles down the leeward side.
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