Parisian sedimentary minerals Mines of Paris

french cross-section diagram of parisian left bank rock layers; minerals useful construction coded yellow.

paris lies within paris basin, geological bowl-like shape created millennia of sea immersion , erosion. of north-western france spent of geological history submerged sea water coastline, towards our era, , formation of our continents know them, relatively flat area become paris region became increasingly elevated. region alternately invaded , sculpted both sea water, inland sea water lagoons , fresh water, in addition above-water air , river erosion. these cycles produced rich , varied geological strata containing many minerals become source of growth , wealth paris region.

formation

the region of paris has spent of geologic history under water, why has such varied , important accumulations of sedimentary minerals, notably lutetian limestone.

the paris area relatively flat sea-bottom during cretaceous period: first in deep-sea environment, under more agitated near-shoreline sea towards end of same period, paris largely silica-based sedimentary deposits became, under action of pressure , carbonic acid content of seawater, thick deposit of clay. invasion of calcium-rich seas covered more important layer of chalk. paris emerged sea towards end of cretaceous period, , later palaeozoic-era continental shifts, particularly variscan orogeny geological upheavals, created series of hills , valleys throughout parisian basin, creating conditions ideal mineral deposits appear during next eras.

after long period above sea level ended towards cenozoic era, paris began period of alternation between sea , land environments. paris middle of shoreline of bays , lagoons of still seawater, environment perfect silica-based sea life abundant then. sea creatures died , settled lagoon bottom, shells mixed deposits present; pressure additional sea-life sedimentation , chemical action of water transformed result sedimentary stone quite particular paris area, calcaire grossier (calcaire lutécien in more modern publications). paris important deposits of stone occurred during eocene epoch s lutetian age; in fact, age named sedimentary activity in paris region, lutetia city s name during roman times.

paris next important mineral deposit came bartonian age. after period of land-sea alternation brought layers of sand , low-quality calcaire grossier, sea regressed again return refill lagoons seawater. result stagnating pools of evaporating seawater; salts of these, mixed other organic matter , mineral deposits, crystallised calcium sulphate composition gypsum. evaporation cycle occurred several times during age, creating several layers of gypsum divided layers of mineral left sea s brief return. in all, parisian gypsum deposits divided 4 masses , last appearing, haute masse, being important , exploited in parisian history. gypsum, evaporite mineral, known fragility against freshwater invasion, re-dissolving quite readily.

the sea returned 1 last time parisian basin towards end of paleogene period, leaving several layers of varied sediment capped thick layer of clay. last deposit important when paris basin rose sea, time definitively, during neogene, topmost layer protect soluble gypsum layers erosion air , weathering.

erosion

paris began take form know presently, huge rivers resulting melting of successive ice ages cut through millions of years of sediment, leaving formations elevated or resistant river erosion. paris hills of montmartre , belleville places gypsum remained, ancestor of river seine once flowed wide, along present path, half city, many arms , tributaries.