Historical Geology - Part I

Introduction

We begin by reviewing:

• the rock cycle, sediments, sedimentary rocks, basic stratigraphy, laws of correlation and geologic time,
• the actions of running water and ground water,
• the geologic column, and
• standard map symbols for sediments and sedimentary rocks.

Historical Geology can be divided into:

• the history of the science itself
• the tectonic history of Earth, and
• all the life on Earth.

Exercise:
Learn your sedimentary rocks from the standard rock boxes and review sediments in a teaching collection or from a field guide to sedimentary material.

Environments

A study of modern sedimentary environments reveals features common to all - and differences between every kind. Particle sorting, roundness and sedimentary structures may reveal ancient sedimentary environments:

1. Marine
   • shallow marine, water less than 200 meters, continental shelf and reefs
   • deep marine, 200 to 10,000 meters, rise, abyssal plains, trenches
     
   • Additional resources:
     • Links about the Oceans
     • Oceans -- outline of Earth Science - IV
2. Non-marine
   • fluvial -- floodplain, channel, alluvial fan
   • aeolian -- desert, dunes, loess
   • lacustrine -- lakes, lake beaches, seiches, turbidity, organics, playas and evaporites
   • glacial -- subglacial, englacial, superglacial, proglacial and postglacial
   • paludal -- marsh and coal swamps
   • subterranean -- caves and caverns
     
   • Additional resources:
     • Freshwater -- outline of Earth Science - III
3. Transitional
   • delta -- river enters standing water
   • beach -- between high and low tide lines
   • bar -- follows longshore currents
   • tide flat -- covered and uncovered in each tidal cycle
   • lagoon -- protected area behind bar, deeper than flats
   • Additional resources:
     • Oceans -- outline of Earth Science - IV

Tectonic environments include:

1. Passive marginal environments (cratonic) -- shelf, slope and rise, sometimes covered by epiric seas. The Atlantic is presently a passive marginal environment.
2. Active marginal environments (orogenic) have trenches, accretionary wedges, and coast ranges
3. Subsiding sedimentary basins (like the Illinois Basin and the Gulf of Mexico) which receive terrigenous sediment from surrounding areas. Continually gaining weight, these sediments compress older materials and preserve much Earth history.
4. Mid-ocean ridges can be associated with either marginal environment.
   
   Additional resources:
   • Tectonics -- outline of Earth Science - II
   • Plate Tectonic Links
   • Volcano Links for views of mid-ocean ridges

Exercise:
Locate each type of environment on a modern map. Describe the type of primary sediment, contemporary cross-cutting and truncation, and redeposition cycles. Describe Illinois history from the bottom up by copying major rock time-units to your geological column. Use standard symbols.

Basic Stratigraphy

• Geological eras are defined by characteristic life forms and named by age (Paleozoic Era, Mesozoic Era and Cenozoic Era are examples).
  
  
• Geological periods are subdivisions within eras. The Mesozoic is divided into the Triassic, Jurassic and Cretaceous periods.
  
  
• Geological epochs are divisions of periods. Print out or view the geologic column for an illustration of these divisions and their approximate ages.
  
  
• Formations are mappable, distinct rocks which have contacts with adjacent rock units. They may be combined into groups or subdivided into members. Only part of a formation may be exposed as an outcrop. An isopach is a line drawn through points of equal thickness of a unit or formation. An isopach map shows the thicknesses of beds, formations or any other body in a particular area. As always, read the caption to see what is being mapped.
  
  
• Stratigraphic units are mappable collections of beds which are divided based on a mineral or fossil characteristic. They are either biofacies, clastfacies or some combination of the two. Each could be considered as separable lithofacies.
  
  
• Beds are the thinnest lithostratigraphic unit. Casually, "stratum" or "layer" is sometimes used for bed. Beds are separated by partings.
  
  
• Rock units are identified by composition, texture, color, or some other structural feature without considering fossils or relative time.
  
  
• Time-rock units are formed in a particular geological time (aka chronostratigraphic units).
  
  
• A type section is that rock group which is most representative for worldwide comparative purposes, should be nearly total thickness and totally exposed. Many type sections are only known from core.
  
  
• At the same time, in different places, different rock types are being deposited. Right now, every different type of sediment is being laid down; all are contemporary (of the same age). Contemporary ancient rocks may also have different fascies, depending on their environment of formation and the sediment type available. A facies map shows the distribution of different rock types in a particular unit. On the Illinois map, these are shown by color changes within the periods.
  
  
• Regression of the sea from the land results in the deposition of terrestrial sediments over marine sediments while transgression of the sea over the land deposits marine sediments on top of terrestrial materials. Paleosols buried in other sediments are often indicators of rising water levels. Laminations are fine scale intertonguing units, often found as lacustrine varves or mud/sand laminae on active margins.

Exercise:
Using the U.S. Geological Map, draw a cross-section of the U.S. from the western edge of Iowa, through Chicago, across the Michigan Basin, to Toronto and Niagara Falls. Label horizontal scale and vertical scale which they should be different to properly represent landforms and use different colors to show time periods and standard geological symbols to show generalized lithology.

Words to review:

• arkose,
• basin, bed, bioclastic, boulders, breccia,
• cement, cementation, chemical sedimentary rocks, chalk, chert, clast, clastic, clay, coal, cobbles, compaction, concretion, conglomerate, coquina, craton, crystalline,
• detrital, dewatering, diatomite, dolomite, dome
• epiric sea, era, evaporites,
• facies, facies map, fissile, flint, flysch, formation, fossil,
• geode, gravel, graded bedding, graywacke, gypsum,
• halite,
• immature sediments, ions, isopach, isotope
• jasper,
• laminations, limestone, lithifaction, lithofacies, loess,
• matrix, mature sediments, micrite, molasse, mudstone,
• ooids, oolitic limestone, ophiolite, orogenic
• paleogeography, peat, period, pore space, porosity, precipitation (of ions),
• recrystalization, regression, rock unit, roundness,
• sand, shale, siderite, sieves, silt, siltstone, sinter, sorting, sparite, stratigraphy, stratification, stromatolites
• taphonomy, texture, thin section, time-rock unit, transgression, travertine, trace fossil, turbedites
• varves

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