青藏高原五道梁地区中新世湖相叠层石特征、分类及古气候意义

通过对青藏高原五道梁地区中新世湖相叠层石样品岩石薄片、扫描电镜的观察,发现叠层石纹层主要有层带状、波状和柱状3种形态,且明暗纹层的结构极不相同,富屑层颗粒结构明显,富藻纹层由致密状含有机质泥晶方解石构成,呈泥晶结构;在此基础上,对叠层石进行了成因分类,认为研究区发育骨架叠层石、凝集叠层石和细粒叠层石。综合叠层石的沉积学、矿物学和稳定C,O同位素值特征,认为叠层石沉积期气候湿润,而纹层层偶则形成于呈季节性变化的气候条件下,亮层代表湿润期,暗层形成于相对干旱期。     

青藏高原沱沱河盆地渐新世湖相叠层石:韵律纹层记录的古气候条件

沱沱河盆地通天河剖面渐新世雅西措群发现一套产于湖泛过程中的柱状叠层石。通过对叠层石纹层形态和显微组构的研究,发现这些叠层石具有典型的富藻纹层和富屑纹层交替的显微结构特征,发育三级生长节律,纹层对之间渐变接触,纹层带之间间断明显。采用激光微区取样的方法,针对4块样品的8个纹层对,系统采集了16对碳氧同位素数据,结果是富藻纹层同位素值整体上均高于富屑纹层,正相关性明显。这表明富藻纹层生长时期气候炎热湿润,富屑纹层生长时期相对寒冷干燥。认为湖相叠层石生长受周期性气候变化的控制,其韵律纹层记录了丰富的古气候信息。     

磷块岩形成过程中的生物作用

 对我国震旦纪、寒武纪磷块岩矿床的研究表明,参与成磷作用的生物主要是菌、藻类微生物。生物作用的标志表现为:P₂O₅主要富集在叠层石磷块岩中,富集在叠层石的柱体上,富集在柱体内部的富藻纹层中;而与磷块岩共生的白云石和黄铁矿的δ¹³C值和δ³⁴S值相对均较低,则是生物参与成磷作用的同位素标志特点。生物的成磷作用贯穿成磷过程的始终,但它随作用的相带、环境和阶段不同,生物的种属和作用方式也不同,因而矿石和矿床的质量也随之而异。     

新疆西天山喇嘛萨依铜矿床地质和C、O、S、Pb同位素地球化学

喇嘛萨依铜矿是新疆西天山赛里木微地块内的一处典型铜矿床,关于其成因类型尚存争议。总结了该铜矿床的地质特征,测试围岩、脉石碳酸盐的C、O同位素和硫化物的S、Pb同位素组成,探讨其成因类型。研究表明,喇嘛萨依铜矿床具有后生矿床特征,发育矽卡岩化蚀变,脉石方解石的δ13C值变化范围为-1.04‰~-0.87‰,低于围岩灰岩的δ13C值(变化范围为3.51‰~5.47‰),δ18O值变化范围为9.33‰~9.61‰,明显低于正常的海相碳酸盐岩的O同位素(δ18O=20‰~26‰),C、O同位素组成反映喇嘛萨依铜矿成矿晚阶段流体来自岩浆水和地下水的混合水;硫化物的δ34S值主要变化范围为3.75‰~8.64‰,与区域上海西期斑岩的硫同位素组成(如达巴特斑岩铜钼矿床硫化物的δ34S变化范围为4.9‰~7.9‰)接近,反映硫来源于斑岩;黄铜矿的铅同位素为206Pb/204Pb=18.264~19.544,207Pb/204Pb=15.575~15.656,208Pb/204Pb=38.103~38.705,具有富含放射成因铅、两阶段异常铅特征,与区域上海西期斑岩(达巴特流纹斑岩)的铅同位素组成特征相似,反映成矿金属物质部分来源于斑岩。通过综合分析认为,喇嘛萨依铜矿是与斑岩有关的矽卡岩型矿床。     

滇西北衙金矿方解石的碳氧同位素特征及其成因

北衙金矿床是金沙江-哀牢山富碱侵入岩带上迄今发现的与富碱侵入岩有关的最重要的金矿床.方解石是北衙金矿体中的主要脉石矿物,其形成与金矿化关系密切.本文在划分方解石期次基础上,定量模拟了北衙金矿方解石的碳、氧同位素成因模式.该模式暗示:北衙金矿方解石为碳酸盐蚀变(重结晶)成因,与Ⅱ期方解石形成有关的主成矿期流体的C、O同位素组成分别为δ13C=-5.5‰,δ18 O=10.5‰,具典型的岩浆流体特征;而与Ⅲ期方解石形成有关的晚期流体的C、O同位素组成相对较低,分别为δ13C=-12‰,δ18 O=7‰,说明部分地加入了大气降水.     

石笋氧碳同位素古气候代用指标研究进展

洞穴石笋以其独立的绝对年龄和稳定的古气候代用指标而成为很好的陆地古气候研究材料。应用U 系定年,可以获得独立精确的日历年时间序列,结合年纹层统计,可以获得年际甚至是季节性变化古气候记录。作为古气候代用指标,石笋氧同位素具有全球可对比性,现已建立了氧同位素的全球气候变化曲线,显示了比深海氧同位素及极地冰芯氧同位素记录更多的优势。石笋δ18O 的影响因素很多,在东亚季风控制区和其它季风控制区,石笋的δ18O 反映了夏季风的强度或者是季风降雨的变化。这些全球可对比的具有高精度绝对定年的石笋δ18O 记录,突破了早期的单一温度控制机理,不仅为古气候学家提供了坚实的古气候变化时间序列,而且使古气候研究能从机理上探讨其变化规律。δ13C不如δ18O 具有全球可对比性,在同位素平衡的开放体系,石笋δ13C 主要反映了洞穴上覆土壤CO2 的同位素特征。因此,一般认为石笋δ13C可直接反映当地植被的特征,包括植被类型C3/C4 比率的变化及植被的茂盛与衰退,在一些地区也可作为大气降水和温度变化的指标。石笋δ13C 很易受到蒸发作用、滴水的快速去气、动力分馏、碳酸盐的先期沉积等影响而使得其数值偏正,应该予以重视。通过介绍石笋氧碳同位素的研究现状,使得读者对石笋的稳定同位素指标有一个初步认识。同时,对今后的研究方向给与一定的探讨。     

磷块岩形成过程中的生物作用

对我国震旦纪、寒武纪磷块岩矿床的研究表明,参与成磷作用的生物主要是菌、藻类微生物。生物作用的标志表现为:P₂O₅主要富集在叠层石磷块岩中,富集在叠层石的柱体上,富集在柱体内部的富藻纹层中;而与磷块岩共生的白云石和黄铁矿的δ¹³C值和δ³⁴S值相对均较低,则是生物参与成磷作用的同位素标志特点。生物的成磷作用贯穿成磷过程的始终,但它随作用的相带、环境和阶段不同,生物的种属和作用方式也不同,因而矿石和矿床的质量也随之而异。     

准噶尔盆地玛湖凹陷下二叠统风城组含碱层段韵律特征及成因

准噶尔盆地玛湖凹陷早二叠世风城组沉积时期为典型的碳酸盐型咸化湖盆,湖盆咸化过程中析出大量的碱矿(钠碳酸盐类矿物)。通过含碱层段主量元素、碳氧同位素分析,结合岩心宏观及微观矿物岩石特征,探讨了含碱层段韵律特征及形成机理。研究层段碱矿与沉凝灰岩互层,形成明暗相间的咸化韵律。浅色碱矿层质地较纯,主要为苏打和小苏打,主量元素表现为Na_2O含量高,CaO、MgO、SiO_2和Al_2O_3含量低;暗色层段主要为含斑点状碳钠钙石的沉凝灰岩,主量元素表现为Na_2O含量低,CaO、MgO、SiO_2和Al_2O_3含量高。浅色碱矿层和暗色沉凝灰岩段碳氧同位素值均为正,表明其沉积水体具高盐度特征。浅色含碱层段具有更高的δ~(18)O值,反映了碱矿层沉积时期强烈蒸发的气候条件。根据δ~(18)O/δ~(13)C值的波动将研究层段划分为炎热干旱和相对湿润的气候条件,并据此解释不同韵律层盐类矿物形成机理。暗色的沉凝灰岩沉积代表地表流水注入量大于蒸发量,沉凝灰岩中盐类矿物含量少或者呈斑点状从颗粒间的卤水中析出;随着蒸发量的增高以及古气候由温暖潮湿转变为干旱炎热,地表流水注入量急剧减少,强烈蒸发控制了碱矿的沉积,形成浅色质地较纯的碱矿层。气候的频繁交替形成了风二段碱矿与沉凝灰岩频繁互层的特征。     

辽西凌源雾迷山组碳酸盐岩碳氧同位素特征研究

辽西凌源雾迷山组碳酸盐岩碳氧同位素和Mn/Sr比值数据反映了碳酸盐岩的原始沉积特征.δ13CPDB数值范围-2‰～2‰,δ18OPDB数值范围为-10‰～-4‰,其平均值分别为0.15‰,-6.2‰,碳同位素组成具有旋回性变化特征.在雾迷山组一段中上部,δ13C表现出明显的正漂移,可能为藻类的大量繁殖、有机碳增加所引起.δ18O的增加为气候变冷和冰川作用的结果.在δ13C为负值的层段,δ18O数值较高.碳氧同位素组成的这种特征可能与海平面变化有关.     

新疆柯坪地区寒武纪-奥陶纪碳酸盐岩沉积旋回的碳氧同位素研究

野外观测发现,在新疆柯坪地区蓬莱坝剖面,中寒武统阿瓦塔格组(C2a)至下奥陶统蓬莱坝组(O1p)的碳酸盐岩在剖面上存在不同尺度的沉积旋回:体系域级、米级旋回级、藻纹层级.对49件样品进行微区取样,共得到98件样品,对其进行了精细的不同尺度沉积旋回碳氧同位素组成的分析研究.结果表明,碳酸盐岩样品的δ13C值(PDB)分布在-1.6‰～1.6‰之间,平均为-0.5‰,δ18O值(PDB)为-12.7‰～-4.3‰,平均-6.2‰,因此,除了明显受到后期成岩蚀变作用(退白云岩化)的藻碳酸盐岩外,其余样品的碳氧同位素组成总体位于正常的海相碳酸盐岩沉积范围内.在体系域尺度上,对于Ⅲ2旋回层序高水位体系域(C2a1),海平面变化是控制碳酸盐岩碳氧同位素组成的主要因素;对于Ⅲ3旋回层序(C2a2-O1p),伴随海平面的升降变化,岩石沉积微环境复杂多变.在米级旋回尺度上,米级旋回的形成除了受高频海平面变化影响外,还受到古构造、古气候的叠加影响.在藻纹层旋回尺度上,δ13C值比δ18O值更能反映藻纹层的旋回性变化特征,藻纹层的形成受控于有机质沉积速率的季节性变化.     

Lacustrine stromatolites: Useful structures for environmental interpretation – an example from the Miocene Ebro Basin

The significance of stromatolites as depositional environmental indicators and the underlying causes of lamination in the lacustrine realm are poorly understood. Stromatolites in a ca 600 m thick Miocene succession in the Ebro Basin are good candidates to shed light on these issues because they are intimately related to other lacustrine carbonate and sulphate facies, grew under variable environmental conditions and show distinct lamination patterns. These stromatolites are associated with wave‐related, clastic‐carbonate laminated limestones. Both facies consist of calcite and variable amounts of dolomite. Thin planar stromatolites (up to 10 cm thick and less than 6 m long) occurred in very shallow water. These stromatolites represented first biological colonization after: (i) subaerial exposure in the palustrine environment (i.e. at the beginning of deepening cycles); or (ii) erosion due to surge action, then coating very irregular surfaces on laminated limestones (i.e. through shallowing or deepening cycles). Sometimes they are associated with evaporative pumping. Stratiform stromatolites (10 to 30 cm high and tens of metres long) and domed stromatolites (10 to 30 cm high and long) developed in deeper settings, between the surge periods that produced hummocky cross‐stratification and horizontal lamination offshore. Changes in stromatolite lamina shape, and thus in the growth forms through time, can be attributed to changes in water depth, whereas variations in lamina continuity are linked to water energy and sediment supply. Growth of the stromatolites resulted from in situ calcite precipitation and capture of minor amounts of fine‐grained carbonate particles. Based on texture, four types of simple laminae are distinguished. The simple micrite and microsparite laminae can be grouped into light and dark composite laminae, which represent, respectively, high and low Precipitation/Evaporation ratio periods. Different lamination patterns provide new ideas for the interpretation of microbial laminations as a function of variations in climate‐dependent parameters (primarily the Precipitation/Evaporation ratio) over variable timescales.     

宜昌峡东地区的现代叠层石

1978年9月,笔者陪同加拿大地质学家霍夫曼(H. Hoffman)博士考察峡东区震旦系剖面时,于石板滩一石牌虎井滩沿江地段,找到陆相现代叠层石。随后,于1978-1979年间,又数次进行野外观察并采集标本,进行室内人工培养和镜下观察。 关于叠层石的成因、生长机理及古老叠层石的地层学意义等问题,长期存在争论。本文试图通过我国资料,对上述问题进行讨论。     

Recent analog of gypsified microbial laminites and stromatolites in solar salt works and the Miocene gypsum deposits of Saudi Arabia and Egypt

Accumulation of microbial mats and stromatolites dominate in the crystallization ponds of solar salt works west of Alexandria, Egypt. These microbial mats are laminar in the permanent submerged part of the ponds. The microbial mats commonly form sites for growth of gypsum crystals during periods having higher salinity. In the dominant submerged part of the pond, domal stromatolites are common around groundwater seepage holes. In the shallow, intermittent margin of the ponds, the laminated microbial structure forms laterally close-linked hemispheroidal stromatolite type, with unidirectional and multidirectional ripple mark-like morphology on their surface. The microbial laminite and stromatolite types in the modern solar salt works are similar to the organic-rich Miocene gypsum beds of El-Barqan (west Alexandria, Egypt) and Rabigh (north Jeddah, Saudi Arabia). The Miocene organic-rich beds consist of interlayered dark-colored microbial laminae and light-colored gypsum laminae. These beds may have three different variations: regular even lamination, laterally closed-linked hemispheroidal stromatolites, and/or discrete hemispheroidal stromatolites. Petrographic examination of the microbial laminites and stromatolites in the solar salt works and the Miocene gypsum beds indicate that the dark-colored, organic-rich laminae are composed of micritized microbial laminae and/or brown organic filaments. In El-Barqan area, the light-colored gypsum-rich laminae are composed of either gypsum crystal fragments, or lenticular and prismatic gypsum. These gypsum crystals are either entrapped within the microbial filaments or are nucleated at the surface of the microbial laminae to form a radial pattern, whereas in Rabigh area, the light-colored gypsum-rich laminae are composed of secondary porphyrotopic, poikilotopic, or granular gypsum crystals. By comparison of the microbial structure in the Miocene gypsum beds with the recent occurrence of the microbial laminites and stromatolites in the solar salt works, it is demonstrated that the organic-rich Miocene gypsum beds were formed in a very shallow salina with slightly fluctuating brine levels.     

Mississippian (Early Carboniferous) stromatolite mounds in a fore-reef slope setting, Laibin, Guangxi, South China

The Mississippian (Early Carboniferous) is generally a period of scarce carbonate buildups in South China. This study documents outcrops of stromatolite mounds at Mengcun and Helv villages, in Laibin City, Guangxi Province, South China. The stromatolite mounds contain various stromatolite morphologies including laminar, wavy-laminar, domal or hemispheroidal, bulbous, and flabellate-growth columns. Intramound rocks are brachiopod floatstone and dark thin-bedded laminated micrite limestone. Individual stromatolites at Mengcun village are generally 3–6 cm thick and morphologically represent relatively shallow-water laminar (planar and wavy-undulated stromatolites) and deeper-water domal, bulbous and columnar forms. Where mounds were formed, the stromatolites continued growing upward up to 60 cm thick. Thrombolitic fabrics also occur but are not common. Stromatolite microscopic structure shows the bulk of the lamination to consist of wavy microbialite and discrete thin micritic laminae. These mounds are intercalated in deep-water fore-reef talus breccia, packstone formed as a bioclastic debris flow and thin-bedded limestone containing common chert layers of the Tatang Formation (late Viséan). Further evidence supporting the deep-water setting of the stromatolite mounds are: (1) a laterally thinning horizon of brachiopod floatstone containing deep-water, small, thin-shelled brachiopods, peloidal micritic sediments and low-diversity, mixed fauna (e.g., thin-shelled brachiopods, tube-like worms and algae) that have been interpreted as storm deposits, (2) common fore-reef talus breccias, (3) lack of sedimentary structures indicating current action, (4) preservation of lamination with sponge spicules, and (5) lack of bioturbation suggesting that the stromatolites grew in a relatively low energy, deep-water setting. The stromatolite mounds are the first described stromatolite mounds in Mississippian strata of South China and contain evidence that supports interpretations of (1) growth history of Mississippian microbial buildups and (2) environmental controls on stromatolite growth and lithification.     

Fine‐grained agglutinated elongate columnar stromatolites: Tieling Formation,ca 1420 Ma,North China

The Mesoproterozoic Tieling Formation, near Jixian, northern China, contains thick beds of vertically branched, laterally elongate, columnar stromatolites. Carbonate mud is the primary component of both the stromatolites and their intervening matrix. Mud abundance is attributed to water column ‘whiting’ precipitation stimulated by cyanobacterial photosynthesis. Neomorphic microspar gives the stromatolites a ‘streaky’ microfabric and small mud flakes are common in the matrix. The columns consist of low‐relief, mainly non‐enveloping, laminae that show erosive truncation and well‐defined repetitive lamination. In plan view, the columns form disjunct elongate ridges <10 cm wide separated by narrow matrix‐filled runnels. The stromatolite surfaces were initially cohesive, rather than rigid, and prone to scour, and are interpreted as current aligned microbial mats that trapped carbonate mud. The pervasive ridge–runnel system suggests scale‐dependent biophysical feedback between: (i) carbonate mud supply; (ii) current duration, strength and direction; and (iii) growth and trapping by prolific mat growth. Together, these factors determined the size, morphology and arrangement of the stromatolite columns and their laminae, as well as their branching patterns, alignment and ridge–runnel spacing. Ridge–runnel surfaces resemble ripple mark patterns, but whether currents were parallel and/or normal to stromatolite alignment remains unclear. The formation and preservation of Tieling columns required plentiful supply of carbonate mud, mat‐building microbes well‐adapted to cope with this abundant sediment, and absence of both significant early lithification and bioturbation. These factors were time limited, and Tieling stromatolites closely resemble coeval examples in the Belt‐Purcell Supergroup of Laurentia. The dynamic interactions between mat growth, currents and sediment supply that determined the shape of Tieling columns contributed to the morphotypical diversity that characterizes mid–late Proterozoic branched stromatolites.     

A note on the significance of lamination in stromatolites

The lamination found in recent stromatolites, an intrinsic feature of many of them, comprises a number of sediment-rich and organic-rich couplets. The significance of this lamination has figured in at least two avenues of research. In one is its possible influential role upon geochemical and diagenetic trends, in the other its possible calendar role. Algal growth follows a diurnal pattern but sedimentation, which is largely marine, is far less regular or predictable. The monitoring of modern algal mat surfaces has yielded much useful and accurate information concerning rates of mat accretion. Millimetre-scale lamination may be produced on a number of interacting processes but as yet no one has been able to determine with any great assurance whether it represents a daily, monthly, annual or some other regular periodic growth. The evidence from the Trucial Coast tends to show, rather, that burial, subsequent compaction, distortion and, in the case of sabkha-type sections, destruction owing to evaporite mineral growth, all tend to obliterate or at best mask any regular patterns of growth. In conclusion, one is forced to acknowledge that any computations concerning the calendar role of stromatolites are to be regarded with the utmost suspicion.     

柴达木盆地新生代湖相叠层石沉积序列及古环境意义

气候、沉积环境和湖平面的演变直接或间接地影响湖相叠层石的生长,因而叠层石的出现可以作为此三者变化的判别标志。通过对比研究柴达木盆地西部地区西岔沟、长尾台两条新生代陆相地层剖面中近10个湖相叠层石的发育层位、垂向沉积序列,表明叠层石主要发育于新生代两个气候温暖湿润的高湖面时期,而且叠层石沉积亦同时出现在青藏高原其他邻近陆相新生代盆地,显示区域性的大规模异常湿润性气候,以强蒸发和强降水的交替为特征;划分出的4种含叠层石基本沉积序列中,叠层石可在泥岩或砂砾岩硬底上生长,含叠层石灰岩层之上通常是细粒碎屑沉积或间断面,这暗示了叠层石的生长出现在滨湖、浅湖相次级湖泛面附近,或指示湖水的突然加深。     

柴达木盆地古近纪—新近纪湖相叠层石与藻礁的沉积组合特征与意义

在柴达木盆地发育了各种类型的渐新世—中新世湖相叠层石，其中丘状叠层石具有重要的地质意义，丘状叠层石直接生长在砾岩上，并向上逐渐演化为藻礁，藻礁向上又重新转化为叠层石沉积。通过对内部纹层特征和沉积组分的研究并结合叠层石的同位素、地球化学以及古生物资料分析表明，微生物自身的钙化作用以及微生物引发的沉淀作用形成了这种具有典型内部纹层的丘状叠层石。较高的温度、炎热和干燥的气候条件、坚硬的基底以及较少的藻类干扰作用是叠层石生成的主要因素。总体上，叠层石形成于较为低能的环境中，与高能环境中所形成的藻礁完全不同，而且两者具有明显的相互竞争作用。因此，与气候和水体条件密切相关的后生生物的生长和真核藻类的发育是叠层石在显生宙迅速衰落的主要因素。     

Influence of microbial photosynthesis on tufa stromatolite formation and ambient water chemistry, SW Japan

Photosynthetic influences on tufa stromatolite formation and ambient water chemistry were investigated at two well-studied streams depositing tufa in Southwestern Japan (Shirokawa and Shimokuraida). The tufa stromatolites in both streams are composed of fine-grained calcite crystals showing annual lamination, and colonized by a number of filamentous cyanobacteria as well as non-phototrophic bacteria. Microelectrode measurements of pH, O₂, and Ca²⁺ near the stromatolite surface (the diffusive boundary layer; DBL) revealed that the investigated tufa stromatolites are formed by photosynthesis-induced CaCO₃ precipitation (PCP): cyanobacterial photosynthesis induces calcite precipitation under light conditions, while respiration of cyanobacteria and non-phototrophic bacteria inhibits precipitation in the dark. The bulk water chemistry at the lower sites of the investigated streams showed the daytime decreases of Ca²⁺ concentration and alkalinity that was expected for significant influence of PCP, while the other expected change, increased pH, was not observed. In order to examine this discrepancy, a novel approach using semi-in situ microelectrode measurements was applied to perform precise quantitative calculations. The calculation results demonstrated that the observed Ca²⁺ concentration and alkalinity decreases were caused by PCP, and that the concomitant pH increase was expected to be under the detection level of a conventional pH meter. Although the amount of PCP is supposed to be significantly affected by light intensity, observations in Shimokuraida revealed that the amount of PCP on cloudy day nonetheless accounted for about 80% of that on sunny day. Despite the significant role of PCP for tufa stromatolite formation, PCP accounted for only about 10% of the precipitated calcite in the investigated streams, which indicates that tufa stromatolites, the characteristic deposits in the streams, are responsible for only a small portion of calcite precipitation, and the rest is considered to precipitate inorganically at biofilm-free substrates.     

Temporal and environmental significance of microbial lamination: Insights from Recent fluvial stromatolites in the River Piedra,Spain

Despite extensive research, the environmental and temporal significance of microbial lamination is still ambiguous because of the complexity of the parameters that control its development. A 13 year monitored record of modern fast‐accreting calcite stromatolites (mean 14 mm year^?1) from artificial substrates installed in rapid flow in the River Piedra (north‐east Spain) allows comparison of the sedimentological attributes of successive six‐month depositional packages with the known climatic, hydrophysical and hydrochemical parameters of the depositional system. The stromatolites are formed of dense, porous and macrocrystalline composite laminae. The dense and porous composite laminae, which are composed of two to eight laminae consisting largely of calcified cyanobacteria, are characterized by: (i) dense composite laminae, up to 15 mm thick, mostly with successive dense laminae and minor alternating dense and porous laminae; and (ii) porous composite laminae, up to 12 mm thick, consisting mainly of porous laminae alternating with thinner dense laminae. Most of the dense composite laminae formed during the warm periods (April to September), whereas most of the porous composite laminae developed in the cool periods (October to March). Each dense and porous composite lamina represents up to or slightly longer than six months. The alternation of these two types of composite laminae parallels seasonal changes in temperature. The dense and porous laminae result from shorter (for example, intraseasonal) variations in temperature, insolation and hydrological conditions. The macrocrystalline laminae, with crystals >100 μm long, occur isolated and grouped into composite laminae up to 1·7 mm thick. Their occurrence suggests the absence or poor development of microbial mats over periods of weeks to several months. Thus, stromatolite lamination can record different‐order, periodic and non‐periodic changes in the magnitude of environmental parameters over a single year. These results hold important implications for the temporal and environmental interpretation of lamination in microbial structures.