Geo lab 9: Geologic Maps

What is a geologic formation?

Discrete, mappable rock unit.

Horizontal beds - rule of V's

For horizontal beds, "V" points upstream

Vertical beds - rule of V's

For vertical beds, contacts are straight, regardless of the topography

Beds dip downstream less steeply - rule of V's

If beds dip downstream less steeply than the channel, "V" points upstream. "V" is narrower than the contour lines.

Beds dip downstream more steeply - rule of V's

If beds dip downstream more steeply than the channel, "V" points downstream.

Beds dip into the hill slope - rule of V's

If beds dip into the hill slope, "V" points upstream. "V" is wider than the contour lines.

In an eroded syncline where is the youngest stratum exposed?

In the middle/center

Map patterns: anticline, syncline

Mirrored repetition of beds may indicate the presence of a fold. 3,2,1,2,3 = anticline 1,2,3,2,1 = syncline Can tell if they are symmetrical or asymmetrical by size of beds.

Is the thickness of a geological formation on a map always equivalent to the thickness of the stratigraphic unit as observed in the field?

No

Map patterns: outcrop width

The angle at which a bed intersects the land surface affects the width of its outcrop pattern on the map. 1. Variation related to slope of the land surface. -Steep slope = narrower outcrop pattern. -Gentle slope = broader outcrop pattern. 2. Variation related to dip of strata -For any dip shallower than 90°, outcrop width > true bed thickness -For a vertical bed, outcrop width = true thickness -Steep dip = narrower outcrop pattern -Shallow dip = broader outcrop pattern

Map patterns: rule of V's

The pattern created where a bed crops out in an stream channel allows us to determine the dip of the bed. 5 cases:

Formation

a discrete, mappable rock unit typically indicated on a geological map by: a unique color or fill pattern on the map. A map notation indicating its name and age. ex. Buda Limestone (bu) Cretaceous (K) = Kbu

Depositional contact

between adjacent sedimentary strata, lava flows, etc. or their metamorphic equivalents

Igneous intrusive contact

between host rocks and an invading igneous body

Fault contact

between rock units that have strike-slip or dip-slip displacement

Geologic contacts

interfaces between adjacent rock units (formations) shown as lines on geologic maps. Geologic contacts between unfolded horizontal strata follow contour lines.

Interpreting structural forms on geologic maps

look for omission or repetition of beds. Omission of beds may indicate an unconformity or fault.

Geologic maps

show the outcrop patterns of rock bodies (formations) and geologic structures (faults, folds) on the land surface. Often drawn on a topographic base map and have a stratigraphic key showing rock units in a column arranged by geologic age.

Each rock units or geologic formation is distinguished by unique ____ and ____ on a map.

symbol, color