The video covers the initial 40 minutes of the" Introduction to Evaporites" workshop.
It shows how and why various sets of hydrological processes exert fundamental control over the textures we see preserved in modern and ancient saline sediments
The video discusses the causes and occurrences of layered brines as an introduction to the significance of brine hydrology as the central control on primary or depositional textures in an evaporite bed.
The video discusses why primary evaporite textures reflect the brine hydrology's layered state at the time various salts were precipitating. It defines primary textures in a subaqueous evaporite bed and how they are responses to brine layering, especially holomictic versus meromictic hydrologies. It documents the how and where of deepwater evaporite textures and how to distinguish this type of salt fabric from the more widely documented shallow to ephemeral water primary evaporite textures.
Atmospheric conditions in the Early Precambrian were reducing and much hotter than today, so seawater was anoxic, more saline and warmer, with much higher levels of dissolved calcium and bicarbonate compared to Phanerozoic seawater. Gypsum (CaSO4.2H2O), which requires free sulphate, was a rare precipitate during concentration of Archean marine brine. Changing atmospheric proportions of CO2, CH4 and O2 meant sodium carbonate salts were significant lower-salinity early Archean marine-brine precipitates, in association with halite at higher salinities. Yet today, sodium carbonate salts, such as trona (NaHCO3.Na2CO3), nahcolite (NaHCO3) and shortite (2CaCO3.Na2CO3) cannot precipitate from brine with the ionic proportions of modern seawater. As the early Proterozoic atmosphere became increasing oxygenated, due to the spread of photosynthetic cyanobacteria, sulphate not sulphide, became the dominant form of sulphur in oceanic brines. By 2.4 Ga, gypsum followed by halite became the dominant marine evaporite salts formed in a concentrating seawater brine, as they do today. Today sodium bicarbonate salts only characterise nonmarine Type 1 brines and never precipitate as primary marine brine salts.
The video focuses on the worlds mesohaline source rocks focuses on hypersaline settings where organic enrichment occurs today in modern and ancient saline lacustrine and supra-salt allochthon brine sump (DHAL) settings. It emphasises the various physical and biological properties and associations that facilitate the organic enrichment and deposition of source rocks. Much of the organic matter preserved in evaporitic carbonates, and the resulting source rocks, originated as planktonic blooms (pelagic “rain from heaven”) or from the benthic biomass (“in situ” accumulations). Such organics typically settled out as seriate pulses of organic matter (often pelleted) that sank to the bottom of a layered brine column. Each pulse was tied to a short period when surface brines were diluted and halotolerant producers (mostly cyanobacteria and algae) flourished in the freshened lit zone. That is to say, the laminated mesohaline mudstones that constitute evaporitic source rocks reflect biological responses to conditions of “feast or famine” in variably layered bodies of brine.
To clarify the geological usage of the term sabkha so it can be more easily applied to a particular vertical sequence in ancient evaporitic sediments, I define a sabkha as "...a saline groundwater-driven sedimentary system indicated by syndepositional intrasediment capillary evaporites, which can precipitate in both marine and continental saline mudflats.". The hydrology of precipitation, via capillary evaporation, is the same in both marine and continental settings.
Modern sabkhas in the Arabian Gulf are occasionally covered by ephemeral marine and continental floodwaters that, until recently, were thought to be the major suppliers of ions to underlying salts growing displacively in the sabkha matrix (per descendum - ions come from above). We know now that this is not the case. Rather, deeply circulated resurging continental groundwater moving into the strandzone is the major supplier (per ascendum - ions come from below) of salts to most sabkhas and many saline pans. The mudflat surface is dry and subaerial for most of the time. It is held in place by capillary moisture, fed from a shallow saline watertable lying a few centimetres to a metre or two below the sediment surface.