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SEDIMENTARY ROCKS TERMS • Weathering: breakdown of pre-existing rock – Physical or chemical breakdown • Erosion: transport of sediment on Earth’s surface SEDIMENTARY ROCKS • • • • Introduction Detrital Rocks Chemical and Biochemical Rocks Lithification Sedimentary Rocks Introduction PSCI 131: Sedimentary Rocks INTRODUCTION • Origin – Only form at Earth’s surface – Weathering of pre-existing rock into soluble and insoluble sediments – Erosion of sediments – Deposition of solid sediment – Precipitation of dissolved sediment PSCI 131: Sedimentary Rocks INTRODUCTION • Textures – Clastic: made of grains held together by cement or compaction – Nonclastic: usually made of interlocking mineral crystals Sedimentary Rocks Detrital Rocks PSCI 131: Sedimentary Rocks Detrital Rocks • “Detrital” from “detritus” - debris • Composed of solid sediment grains; distinguished by size of grains • All detrital rocks have a clastic texture Detrital rock with smaller grains Detrital rock with large grains PSCI 131: Sedimentary Rocks - Detrital Shale • Most abundant sedimentary rock – 2/3 of all sedimentary rock is shale • Made of clay grains – Microscopic sheets – From chemical weathering of feldspar PSCI 131: Sedimentary Rocks - Detrital Shale Scanning electron microscope image of shale showing flat clay grains. Scale bar is 0.005 cm. From: claysandminerals.com PSCI 131: Sedimentary Rocks - Detrital Sandstone • Cemented sand-size grains • Sand: 1/16mm – 2mm diameter • Quartz is predominant mineral PSCI 131: Sedimentary Rocks - Detrital CONGLOMERATE/BRECCIA • Cemented gravel-size grains • Gravel: greater than 2mm CONGLOMERATE has rounded gravel BRECCIA has angular gravel Sedimentary Rocks Chemical Rocks PSCI 131: Sedimentary Rocks Chemical and Biochemical Sedimentary Rocks • Dissolved sediment precipitates out of solution as crystals • Chemical rocks: evaporation, chemical activity • Biochemical rocks: growth of shells PSCI 131: Sedimentary Rocks – Chemical rocks Chemical Rocks: Crystalline limestone - Nonclastic texture - Interlocking calcite crystals - Precipitated from seawater PSCI 131: Sedimentary Rocks – Chemical rocks Chemical Rocks: Microcrystalline limestone - Nonclastic texture - Interlocking microscopic calcite crystals - Precipitated from seawater PSCI 131: Sedimentary Rocks – Chemical rocks Chemical Rocks: Travertine - Nonclastic texture - Porous - Precipitated from groundwater in caves PSCI 131: Sedimentary Rocks – Other chemical rocks Other chemical sedimentary rocks • Rock gypsum and rock salt – Nonclastic texture – Interlocking mineral crystals – Evaporation of salt water – “Evaporites” Bonneville Salt Flats, the ancient floor of Lake Bonneville, now mostly evaporated PSCI 131: Sedimentary Rocks – Biochemical rocks Biochemical rocks: Coquina - Clastic texture - Shell fragments & cement - Fragile - Shorelines PSCI 131: Sedimentary Rocks – Biochemical rocks Biochemical rocks: Fossiliferous limestone - Clastic texture - Shell fragments, whole shells, mud matrix, & cement - Hard - Shallow ocean PSCI 131: Sedimentary Rocks – Biochemical rocks Biochemical rocks: Chalk - Clastic texture - Microscopic shells (plankton) - Very soft - Deep ocean PSCI 131: Sedimentary Rocks-Other biochemical rocks Chert and flint • Composed mainly of microcrystalline quartz FLINT • Nonclastic texture CHERT • Can be chemical or biochemical • Very hard Sedimentary Rocks Lithification PSCI 131: Sedimentary Rocks - Lithification Lithification • How sediment becomes rock • “Lith” = stone PSCI 131: Sedimentary Rocks - Lithification Lithification Processes • Compaction • Cementation PSCI 131: Sedimentary Rocks - Lithification Lithification • Begins with deposition & burial of sediment under more sediment PSCI 131: Sedimentary Rocks - Lithification Compaction • Decreases porosity (empty space) • Drives out water • Can make flat grains stick together PSCI 131: Sedimentary Rocks - Lithification Cementation • New crystals form between grains • Precipitation out of groundwater • Fills in remaining porosity Original sand grain From: northstonematerials.com Cement crystals From: unibe.ch End of Sedimentary Rocks
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