Beachrock Cementation Research Articles

The petrological characteristics and formation mechanism of Lake Beachrock in Lake Qinghai

以青海湖的湖滩岩为研究对象，通过显微镜观察、扫描电镜、矿物阴极发光性观察和电子探针等分析方法研究了湖滩岩的岩石学特征和形成机制.湖滩岩胶结物主要由暗色纹层和文石针组成，识别出五类胶结物晶体和三类胶结物组构.在暗色纹层中，有发现包覆式和缠绕式生长的微生物.微生物对泥晶方解石和黏土矿物起着生物稳定化作用（障积作用、捕获作用和粘结作用）.微生物生长过程中的新陈代谢影响了碳酸盐的溶解度，从而导致产生碳酸盐矿物在微生物席内的丝状体之间或沿着丝状体的沉淀作用.从研究湖滩岩胶结物垂向上和横向上的变化规律入手，发现胶结物的类型和组构与水深、温度、盐度、水动力条件有一定关系.破浪带和碎浪带的水深、温度、盐度适宜，为微生物作用最强的水动力带.通过对青海湖湖滩岩中的微生物作用特征及其发育模式的研究，认为湖滩岩中胶结物的形成受到微生物作用的影响较大，从而支持了微生物作用对碎屑岩早期成岩具有重要意义的假设.;The lake beachrock in Lake Qinghai as the research object, through microscope, SEM, mineral cathode luminance observation and electron probe analysis method to study the characteristics and precocious mechanism of lake beachrock. The beachrock cement is mainly composed of dark grain layers and aragonite needles. There are five kinds of cement crystals and three kinds of cement fabric. There is coating, voluble and conglutinating growth of microbes in the lake beachrock cement. Microorganisms for micritic calcite and clay mineral have a biological stabilization effect(barrier, and the roles of capture and bonding effect). The metabolism in the microbial growth will change the carbonate solubility and resulting in carbonate minerals is produced by the filaments in microbial mat or between precipitations along the filaments. From the lake beach rock cement on the lateral and vertical changes, it is found that the type and texture of the cement are related to water depth, temperature, salinity and hydrodynamic conditions. Water depth, temperature, and salinity are suitable in swash zone and breaker zone where microbial activities arethe strongest. Through the research of microbial characteristics and developmental pattern of beachrock cement in Lake Qinghai, we concluded that cement in the lake beachrock is affected by the microbial action, which support the biological role of clastic rock in early diagenetic hypothesis that it is of great significance.

Beachrock Cementation Patterns Along the Gulf of Aqaba Coast, Saudi Arabia

Beachrocks crop out along the Saudi coast of the Gulf of Aqaba with beds lying at elevations up to 0.8 m above the present sea level and with seaward extents that repeatedly submerge and emerge during high and low water levels. This study discusses the cement composition, petrography and bedding features of beachrocks at 24 different locations, of which a total of eight sites were sampled. The study focuses on the petrographic and microtextural characteristics of the beachrocks to elucidate their cementation environment and diagenetic evolution. The results revealed the predominance of three main cement types: (1) fibrous aragonite cement with morphologies including isopachous rims around grains with or without micritic substrates, pore-filling radial aggregates, pseudospherulite and randomly oriented interlocking aragonite fibers; (2) microcrystalline low-Mg calcite (LMC) cement binding grains and filling inter- and intra-granular pore spaces; and (3) micritic cement in the form of envelopes and/or pore-filling micrite in some samples with meniscus bridges. The last two types also contain infiltrated silt-sized clasts and skeletal remains. The cements indicate precipitation mainly in pore-filled marine phreatic diagenetic zone and, rarely, in the marine vadose zone with the aid of microbial and algal activity. Petrographic investigations and energy-dispersive X-ray spectroscopy of selected samples showed the occurrences of aragonite and LMC.

Rapid beachrock cementation on a South African beach: Linking morphodynamics and cement style

Rapid cementation (<5 years) of beachrock is reported from the wave-dominated, microtidal coastline of Durban, South Africa. The beachrock is developing at mean sea level on a steep (10°–30°), reflective beach prone to periodic erosion by high-energy wave action; the most recent erosional event resulted in retrogradation of the beach by up to 10 m landward of the beachrock outcrop discussed in this study. Transmitted light microscopy and scanning electron micrographs show the precipitation of two dominant cement types which are linked to the beachface morphodynamics. During the accreted beach phase, when the locus of beachrock formation lay well landward of the beachface in the undersaturated upper intertidal to supra tidal backbeach, meniscus cements were precipitated. Following a period of erosion, associated with a landward shift of the beachface, the incipient beachrock was then exposed to saturated conditions which promoted growth of acicular cements. Contemporary beachrock cementation occurred very rapidly (<5 years) between significant erosional events. The observed cement textures record short-term beach progradation and retrogradation but are similar to those typically associated with longer-term regression and transgression.

Origin of holocene beachrock cements in northeastern Brazil: Evidence from carbon and oxygen isotopes

This study has utilized carbon and oxygen isotopic data to unravel the origin of Holocene beachrock cements of Rio Grande do Norte State coast, northeastern Brazil. The cements are exclusively constituted of Mg-calcite, with isopachous prismatic rim, equant spar, cryptocrystalline coating or pore-filling, pseudo-peloidal, radial and scalenohedral habits. The δ18OVPDB values of most (93%) of the samples range from −1.8‰ to +0.5‰, which are compatible with precipitation from marine waters. The δ13CVPDB values of these cements are +1.7 to +3.6‰ (average + 3.05‰) suggesting derivation from marine dissolved carbon. In a few cases, the Mg-calcite cement shows low δ13CVPDB (−7.3‰ and −7.8‰) and δ18OVPDB (−4.4‰ and −4.3) values, which may indicate precipitation or recrystallization of marine cements under the influence of meteoric waters.

Trace Fossils in Clastic Beachrocks at the Yallahs Salt Ponds, Jamaica: Implications for Beachrock Cementation

ABSTRACT Edwards, T.C.P. and Mitchell, S.F., 2018. Trace fossils in clastic beachrocks at the Yallahs Salt Ponds, Jamaica: Implications for beachrock cementation. The extent and importance of trace fossils preserved in clastic beachrock from the southern section of the Yallahs Salt Ponds on the SE coast of Jamaica are described. The beachrock contains a seaward-dipping facies of cemented sandstones and conglomerates and a landward facies consisting of cemented sandstones packed with the trace fossil Psilonichnus, which is produced by the ghost crab Ocypode quadrata (Fabricius). The seaward-dipping bedded facies represents more than 95% of the beachrock at the Yallahs Salt Ponds and has an exposed height ranging from 6 to 50 cm and lateral continuity of 1 to 200 m. The bioturbated facies, which represents less than 5% of the beachrock at the Yallahs Salt Ponds, was only found at three localities. It has an exposed height of 40 to 60 cm, with outcrops traceable for 6 to 30 m along the beach. These bioturbat...

A multianalytical X-ray based micro-spectroscopic approach for the analysis of carbonates in beachrock cements

The molecular micro-characterization and the estimation of the Mg content of CaCO3cements are crucial to define the environmental biophysicochemical conditions in which unusual temperate latitude beachrock formations materialize.

A STUDY OF THE FORMATION MECHANISM OF BEACHROCK IN OKINAWA, JAPAN: TOWARD MAKING ARTIFICIAL ROCK

Beachrock is coastal sediment that has been cemented mainly by calcium carbonate within the intertidal zone in tropical and subtropical regions. Man-made beachrock has the potential to inhibit coastal erosion. Considering this important application, we performed field investigations and laboratory tests to understand the formation mechanism of beachrock in Nago, Okinawa, Japan. We performed a needle penetration test, microbial population count and urease activity test, and conducted elemental and mineral analyses of the beachrock and sand. Some microorganisms at the site showed urease activity and the beachrock cement comprised high Mg calcite (HMC). Our investigation showed that evaporation of seawater and/or urease activity of bacteria may have resulted in precipitation of HMC, leading to formation of the beachrock, with partial solidification of some sandy specimens.

First note on marine-like cementation of Late Holocene beachrock, Iznik Lake (Turkey)

Abstract Micro-fabric characteristics and optically stimulated luminescence (OSL) dating results are presented to discuss the controlling agents and timing of beachrock cementation on the fresh-water Iznik Lake shoreline. The beachrocks are made up of grain-supported polygenic conglomerate containing 20.42% carbonate with encrusted grains, basically as micrite coatings, isopachous aragonite rims, cryptocrystalline void fills and meniscus bridges. The optical ages of twelve samples yielded ages that range from 4.226±0.569 ka on the lowermost beds to 0.706±0.081 on the uppermost. This is the first report of precipitation of marine-like cements in Iznik Lake. The abundant aragonite-dominated cement is likely indicative of precipitation-prone dry evaporative conditions from the climatic optimum to the last millennia.

Geomorphic and Sedimentologic Evidence of Extreme Wave Events Recorded by Beachrocks: A Case Study from the Island of St. Bartholomew (Lesser Antilles)

Abstract Caron, V., 2012. Geomorphic and sedimentologic evidence of extreme wave events recorded by beachrocks: a case study from the Island of St. Bartholomew (Lesser Antilles). In spite of their preferential location inside tropical belts where extreme wave events occur, and in spite of their potential for rapidly lithifying coastal sediments and by doing so promoting their preservation, beachrocks have so far been poorly investigated for the record of tsunamis or storms. This contribution focuses on beachrock occurrences on the island of St. Bartholomew (French West Indies), which is located in a region at risk from such natural hazards. Geomorphic and sedimentologic criteria were used to characterize beachrocks from shores frequently exposed to extreme wave events. Besides known effects of waves on beachrocks, including exhumation and landward transport of broken slabs, this article reports less well-documented perturbations such as the immersion of beachrock layers and cementation of event deposits. ...

Beachrock as Sea-Level Indicator? Remarks from a Geomorphological Point of View

Abstract In the catalogue of sea-level indicators, beachrock plays a significant role because of its wide distribution and extension on long stretches of coastlines. Most publications, however, deal with beachrock cements rather than their incorporation into the coastal environment and its evolution. The relic character and age of most beachrocks has been overlooked, and beachrocks are called “modern” because of their position at or close to sea level. The question arises as to why we have so many beachrocks in a state of destruction, but nearly none in statu nascendi today. An examination of the extension and thickness of cementation, and of the independent indicators of the sea-level history in beachrock occurrences suggests that the relation of beachrock cementation to the tidal levels or to a water table in the beach sediments is very doubtful. There are many more arguments and proofs that place beachrock cementation within the supratidal environment. Only by taking into consideration all aspects of c...

Cementation patterns and genetic implications of Holocene beachrocks from northeastern Brazil

This paper describes the complex patterns of diagenesis in Holocene beachrock ledges located along both the eastern and northern coasts of the state of Rio Grande do Norte, northeastern Brazil. These beachrocks show highly variable diagenetic processes, fabrics and textures. Carbonate cements are exclusively composed by high-Mg calcite and occur in seven main morphologies: (1) cryptocrystalline coatings; (2) isopachous prismatic rims; (3) equant spar; (4) cryptocrystalline or micritic pore-filling; (5) pseudo-peloidal aggregates; (6) radial aggregates; (7) isolated and randomly oriented clumps of scalenohedral crystals. Other diagenetic features include: (a) carbonate-siliciclastic mixed sediment; (b) infiltrated micrite; (c) authigenic smectitic clay minerals; (d) organic films; (e) chalcedony pore-filling cement; (f) vadose silty internal sediment. The texture and composition of the cements are indicative of precipitation in the active marine phreatic zone. The presence of vadose features and planktonic foraminifers within the infiltrated materials suggest that they were introduced in the backshore sands by muddy waters dislocated from the shelf during storms. A general succession of diagenetic phases can be recognized, comprising the precipitation of authigenic smectite, micritic coatings, radial aggregates, isopachous rims of prismatic crystals or equant spar, cryptocrystalline and pseudo-peloidal pore-filling cements, and vadose infiltration of micritic, carbonate-siliciclastic mixed sediments or silty materials. The absence of organic structures, such as microbial (bacteria or fungi) filaments and bodies within the cements suggests that the mechanism behind beachrock cementation is essentially inorganic, most probably the evaporation of entrapped seawater in response to the dry climatic conditions.

Cryptic intertidal microbialites in beachrock, Heron Island, Great Barrier Reef: implications for the origin of microcrystalline beachrock cement

Microbialites with microcolumnar growth morphology occur on the surfaces of solution-enlarged crypts in modern beachrock an Heron Island, Great Barrier Reef, Australia. The microbialites consist predominantly of felted aragonite, but blocky Mg-calcite and fringes of acicular aragonite also occur. A variety of microbial bodies and filaments occur within, and on the surface of, the microbialites. Although most Heron beachrock is cemented by isopachous fringes of acicular aragonite, irregular micritic cement is common adjacent to the microbialites. The micritic cement consists predominantly of felted aragonite and contains microbial filaments and organic matter similar to those in the microbialites. Both the microbialite and the micritic cement are interpreted to be biologically induced. Although physico-chemical models, including CO2 degassing and evaporation, are commonly held to be sufficient to promote the formation of beachrock, the similarity of Heron cryptic beachrock microbialite with micritic beachrock cements supports the contention that some beachrock cements are biologically induced. Additionally, the primary difference between fringes of acicular aragonite and the 'felts' of aragonite in microbialites appears to be that the former nucleated on a well defined surface of organic matter, whereas the latter nucleated within a framework of organic matter that contained irregularly disposed nucleation sites. Hence aragonite fringe cements may also be localized and/or induced by films of organic matter. (C) 1999 Elsevier Science B.V. All rights reserved.

Experimental modelling of beachrock cementation under microbial influence

A laboratory model has been developed, which reproduces natural conditions of beachrock formation. These conditions include the porosity of beach sand, tidal movements (alternation of phreatic and vadose conditions), temperature, evaporation, illumination, and water composition (seawater, with a control of the following parameters: salinity, Ca, Mg, ΣCO2, pH, CaCO3 saturation). The conditions of two beachrocks from the Red Sea (Egypt) and Shark Bay (Australia) have been simulated, and the influence of the micro-organism population (undetermined) sampled in these beachrocks was observed for 3–4 months. In the experiments inoculated with micro-organisms, biomineralization produced characteristic crystals of various forms from 1 to 20 μm in size. The precipitates resemble the micritic cements of natural beachrocks. On the contrary, precipitation was much less developed in the control experiments under nearly sterile conditions. These results confirm the importance of microbial activity during beachrock formation. The first micritic cement, which seems to control cementation and produce the initial consolidation, is formed under a strong microbial influence in or near a biofilm covering the grain surface. The presence of this micritic cement is necessary for the development of the subsequent prismatic cement, which probably results from an abiotic precipitation during higher fluid circulation.

Beachrock in St. Martin's Island, Bangladesh: Implications of sea level changes on beachrock cementation

St. Martin's Island has been experiencing local sea level fluctuations and subsequent shoreline changes for the last 2 million years, which produced Pleistocene and Holocene beachrock along the island. The shoreline changes along the island are evidenced by the occurrence and distribution of a few conglomeratic beachrocks containing abundant fossil fragments. This study documents three separate stages of beachrock cementation that mainly depict the late Holocene local sea level fluctuations. Beachrock exposed along the northwestern, eastern, and southeastern coastlines of the island can be grouped into three categories: conglomeratic, fossiliferous conglomeratic, and calcareous sandy. The first two categories are characterized by aragonite needle cement, micritized grains, and micritic envelopes, which are features of early marine phreatic diagenesis. This cementation phase has been formed by local sea level rise. On the other hand, petrographie features in the calcareous sandy beachrock include low Mg‐calcite cement, evidence of neomorphism of aragonite cement and micritic rims to low Mg‐calcite, random needle‐fiber calcite, calcified organic remains, and micritic networks. The diagenetic features suggest that this beachrock passed through marine‐phreatic to meteoric phreatic and meteoric‐vadose diagenetic environments. This diagenetic sequence commonly forms in response to sea level lowering. Therefore, evidence of initial deposition in shoreline environment and later residence in meteoric phreatic and meteoric‐vadose conditions suggests that the changes of beachrock cement in the study area have a direct relationship to the corresponding changes in the sea level. Following Umitsu's (1987) relative sea level curve of the Bengal lowland, it is roughly estimated that the sea level rise at St. Martin's Island look place after ca. 10,000 yBP and before ca. 6,000 yBP, and sea level lowering occurred after ca. 5,000 yBP. However, the oldest beachrock in the study area, which forms a 6‐m‐high cliff, belongs to the late Pleistocene (Chowdhury et al. 1994). Their presence at sea level over large parts of the island suggests that cementation is currently active.

Beachrock cementation in Alexandria, Egypt

Beachrock is found in the intertidal and sublittoral zones along the Mediterranean coast of Alexandria, Egypt. It occurs as beds or patches and shows several stages of cementation and hardness. The seaward dip of the beachrock is similar to that of the slope of the foreshore. The fabric shows micrite and fringe cement types, aragonite was not identified in the beachrock at Alexandria. The average concentrations of Ca, Mg, Sr, Fe and Mn in the beachrock are similar to those of the beach deposits. The process of beachrock cementation in Alexandria is rather recent and typically marine.

Marine Vadose Beachrock Cementation by Cryptocrystalline Magnesian Calcite--Maui, Hawaii

ABSTRACT Active precipitation of high-magnesian cryptocrystalline calcite cement (15.5 mole % MgCO3) in the intertidal zone on the north shore of Maui, Hawaii, results in formation of an extensive beachrock exposure at Hookipa Park. Framework grains in the beachrock are coarse algal, molluscan, and intraclast-bearing sands. Meniscus fabric is dominant in the cement, indicating that most lithification occurs in the vadose zone. The cryptocrystalline cement is generally brownish gold in color, but very thin, clear isopachous fringes of microcrystalline calcite surround darker cryptocrystalline cement, especially in upper horizons of the beachrock. Peloidal texture is scarce in the cement but is present in a few samples. Silt and fine-sand-sized skeletal fragments have been trapped in he evolving cement, resulting in cement-supported fabric. Hollow rims of microcrystalline calcite occupy some pore spaces. These hollow rims may have precipitated at the air-water interface of bubbles that formed in pores, and they may therefore be additional evidence of cementation in the vadose zone. Interstitial water samples from wells that penetrate portions of the beachrock and unconsolidated beach sands reveal temporal chemical variations that suggest the importance of CO2 degassing in the formation of beachrock cement. When chemical and tidal cycles fluctuate with the same period, PCO2 values in pore water of the intertidal zone generally bear a direct relationship to tidal stand. Increasing values of PCO2 most often occur during semi-daily rising tides and decreasing values of PCO2 occur during falling tides. Saturation ratios with respect to calcite vary in direct proportion to PCO2. Saturation levels generally increase during rising tides and decrease during falling tides. T ese data suggest that CO2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach.

Origin and Diagenesis of Beachrock, Discovery Bay, Jamaica: ABSTRACT

From in-situ pore water and rock analyses of a lithified Holocene beach deposit in Jamaica (1,240 ± 50 yr B. P. at 22 cm depth, 670 ± 50 yr B. P. at 14 cm depth, 0 yr B. P. at surface), we propose a geochemical model for intertidal carbonate cementation. The beachrock unit is laterally and vertically discontinuous with unconsolidated beach sands surrounding it. The unit dips seaward at an angle of 10° and contains localized open orthogonal fractures that are oriented parallel and normal to the shore line. Three distinct cement types are found in Jamaican beachrock: (1) equant and bladed high-Mg calcite (12-29 mole % MgCO3), (2) low-strontium fibrous aragonite (1,700-3,100 ppm SrO), and (3) micritic high-Mg calcite envelopes. These cements vary bo h laterally and with depth in the unit, and accurately reflect the changes in time and space of the chemistry of the interstitial water; the cements are produced in stoichiometric equilibrium with the pore-water chemistry. The high-Mg calcite cements are precipitated when CO2 degases (PCO2 = 10-5.1) through agitation in the surf and consequently raises the pH to a maximum of 8.4 during the higher tides. During these times, the pore waters are saturated with respect to the precipitating Mg calcite containing 15-29 mole % MgCO3. During low tide, when the agitation of the surf is minimal, the CO2 does not degas, increasing the PCO2 to a maximum of 10-4.0. Continued precipitation aids in the in rease in CO2 levels, the decrease in pH to a minimum of 7.9 and the lowering of saturation states of Mg calcites. Phreatic fresh water flows seaward during low tide, preferentially through the open fractures, lowering strontium levels and saturation states in the pore waters. Thus, at low tide, lower Mg calcites of 12-15 mole % MgCO3 are precipitated where fresh water has not invaded (maximum Cl = 22^pmil). Models of Sr partitioning show low-strontium aragonite is produced from the neomorphism of high-Mg calcites near the open fractures in mixed meteoric and marine interstitial waters (Cl = 11.05-13.48^pmil). Our data suggest that PCO2 is the master variable and that beachrock cements are not static but ever-changing in mineralogy and chemistry End_of_Article - Last_Page 312------------

Intertidal Cementation: Some Geochemical, Mineralogical, and Petrographic Considerations: ABSTRACT

The chemistry, mineralogy, and petrography of intertidal cements are dependent on the chemical and physical characteristics of the waters from which they were precipitated--as well as on the biota this water supports. The study of Holocene cementation in Boiler Bay, St. Croix, Virgin Islands, is well suited to illustrate the basic constraints that the chemistry, mineralogy, and petrography of the cements place on any model of beach rock origin. Boiler Bay contains a well-developed beach rock pavement cemented on its western extremity by rhombohedral-bladed, circumgranular magnesian calcite cement, and on its eastern end by acicular, circumgranular aragonite cement. These facts indicate that, although the precipitational fluid was dominantly marine and cementation occurred under phreatic conditions, western beach rock cements were precipitated under a different geochemical regime than were those cements in the east. Pelleted magnesian calcite micrite cements that are characteristic of the seawardmost parts of the beach rock pavements (as well as submarine cements occurring in offshore reefs) were found by scanning electron microscopy to be biologic in origin. Backbeach cemented zones are characterized by low to high magnesian calcite, rhombohedral, equant-to-bladed circumgranular crust cements. This combination of cement mineralogy-petrography indicates fresh to marine, phreatic precipitational conditions in the backbeach area. Cement chemistry, particularly magnesian carbonate content in the calcites and strontium contents of aragonite and calcite, indica es that the beach rocks of western Boiler Bay were precipitated from mixed marine-fresh meteoric waters, and that the eastern Boiler Bay beach rocks were precipitated from normal marine waters with no freshwater influence. Backbeach cements of western Boiler Bay show a complete gradation of cement fabric, mineralogy, and chemistry that would indicate a classic fresh-marine water mixing zone. Hydrogeochemical studies of beach and near-beach interstitial waters confirm that western Boiler Bay is a locus of freshwater influx into the marine system. It would seem, then, that the mineralogy of Boiler Bay beach rock is controlled by fresh-marine water mixing. Artificial substrate experiments, using the western Boiler Bay observation wells, pinpoint the locus of cementation in this area as being within the fresh-marine water mixing zone near the upper surface of the water table. In addition, these experiments indicate that cement growth can be independent of substrate mineralogy and that organic coatings on cement substrates may ultimately be one of the major controls over initial cementation patterns. Finally, the experimental approach in the intertidal environment may well enhance our understanding of controls over carbonate cementation. End_of_Article - Last_Page 518------------

Cementation of Lime-Mud and Pellet Mud Beneath Tidal Flats of Southwest Andros Island, Bahamas: ABSTRACT

Scanning electron microscopy delineates several diagenetic products currently forming beneath the prograding Holocene wedge of carbonate sediments along southwest Andros Island. Various diagenetic reactions are associated with distinct chemical pore-water environments in the affected sediments. Pore fluids of near normal marine concentrations occur in sediments over widespread areas of the tidal flats and, in these, small amounts of non-lithifying aragonite have been precipitated at isolated locations. Meteoric water infiltrates and is stored beneath topographic high areas (hammocks), while a halo of brackish groundwater surrounds the hammocks. Non-lithifying protodolomite cement occurs in sediments saturated with brackish water beneath the hammocks and adjacent areas. An unusual Mg-calcite (7 to 8 mole % MgCO3) occurs as a beachrock cement where brackish hammock-water mixes with more normal-marine (bank) water. Sediments saturated with fresh water beneath hammocks are being calcitized. Groundwater near the surface of low positive topographic areas with a high exp sure index, such as flanks of hammocks, modern beach ridges, and tidal-creek levees, become hypersaline due to evaporation of capillary waters. The well-known protodolomitic crusts form in these environments. Most carbonate precipitation beneath the tidal flats is nonlithifying; some nodule formation, nodular beachrock, and, of course, the crusts of varying mineralogy are exceptions. End_of_Article - Last_Page 788------------

Vadose Beachrock Cementation along A Southeastern Michigan Marl Lake

ABSTRACT Beachrock, consisting of carbonate-cemented algal pisoliths and gastropod shells, forms a resistant unit 15 to 25 cm thick along the eastern shore of Ore Lake in southeastern Michigan. The cement occurs as thin crusts of highly acicular, low-magnesium calcite crystals. These crystals are actually aggregates of many small fibrous crystallites, each having the form of an elongate trigonal prism, An orderly arrangement of these crystallites forms a prismatic crystal with many small elongate triangular intercrystallite spaces. Crystals lining large pores consist of crystallites exhibiting either rhombohedral or flat terminations, whereas terminations in small voids are entirely rhombohedral, Scanning electron microscopy shows that in large pores the ends of the flat crystals define smooth curved surfaces around the void margins. These surfaces record the position of a meniscus surrounding air bubbles trapped within the larger pores, demonstrating cementation with the vadose zone. Crystal growth by elongation of the c-axis ceased upon coming in contact with the air phase. Smaller voids were completely filled with fluid, and in these areas uninhibited growth of calcite occurred. The acicular prismatic crystal morphology resembles that found in low-magnesian calcites from speleothems, suggesting that it may be the result of calcite precipitation from extremely low-salinity water. Mg/Ca ratios of beachrock pore waters are low, demonstrating that even though these crystals exhibit an anomalous habit for low-magnesian calcite, concentration of ions was not responsible for the lateral inhibition of crystal growth, and that magnesium poisoning of lateral crystal faces did not play a significant role in the formation of these acicular cements. Ore Lake beachrock resembles its marine counterparts in occurrence, rapidity of cementation, and general fabric, suggesting a similar origin. Lake waters and inflowing meteoric ground waters at Ore Lake are chemically similar and evaporation rates are negligible. Thus, the occurrence of these cements and an observed CO2 loss from the groundwater favor degassing of groundwater as a general mechanism for beachrock formation, rather than the mixing of meteoric and saline waters or evaporative concentration.